The HVAC Buyer’s Guide

Are you buying a new heating and air conditioning system, or some part thereof? The well-worn practice of checking reviews and getting bids is not enough. Most botched installations are performed by contractors who will pass that test. Most of those same contractors won’t pass the test of an informed customer. Read this page and you’ll be the informed customer.

You need not understand everything on this page. You need only to familiarize yourself with the concepts. Even if you just read “The Basics” you’ll still know more than most. Your newfound knowledge will help you to distinguish the tradesmen from the badmen. A tradesman will echo the trade-truths found on this page. A badman will echo the half-truths found on this this page*.

* Opens a new page to my new site dedicated to education.

The Basics

How NOT to Buy a Heating and Air Conditioning System 99% of homeowners get it wrong. It’s your turn to be a one-percenter.	Learn More↓
How to Choose a Contractor Be shrewd as serpents and innocent as doves. I’ll help you with the shrewd part.	Learn More↓
How to Choose a Brand Did you find a good contractor? Then let them choose.	Learn More↓
How to Choose Features The more you buy, the more ~~you save~~ trouble you’re going to get.	Learn More↓
How to Negotiate the Price You probably want to. You probably shouldn’t.	Learn More↓
How to Keep It Legal Or… “How to avoid premature death and unnecessary taxes.”	Learn More↓
How to Design the System Your system should be designed on a computer, not scribbled on scrap paper.	Learn More↓
How to Get What You Paid For Most homeowners don’t get what they’ve paid for and don’t even know it.	Learn More↓

Furnaces / Air Handlers

Furnaces & Air Handlers Blow, Man The furnace’s or air handler’s blower is important to both heating and cooling.	Learn More↓
Bigger Is Not Better, Unless It Is Too much capacity is inefficient, uncomfortable, and may shorten the system’s life.	Learn More↓
It’s the Little Things The hundred dollars’ worth they don’t replace now may cost you thousands later.	Learn More↓
Clearances Count The repairman needs room to work and the equipment needs room to breathe.	Learn More↓
High Efficiency, Low Return? A super high efficiency furnace or heat pump may cost you more than it saves.	Learn More↓

Air Conditioners / Heat Pumps

Bigger Is Not Better Most outdoor units are oversized but underdeliver.	Learn More ↓
Extraordinarily few systems deliver the 450 to 500 CFM per ton that is ideal for dry climates. Even systems in humid climates often lack proper flow. Learn why.	Learn More ↓
Oversized and/or high efficiency cooling coils (AKA evaporator coils) are almost always an excellent choice in dry climates.	Learn More ↓
Air conditioners can lose 5% or more of their capacity to undersized refrigerant lines. Proper line size is especially important on marginally sized systems.	Learn More ↓
Acid in an air conditioner? It happens more often than you think and will slowly kill a compressor over a period of years. Don’t let it happen to you.	Learn More ↓
A huge percentage of air conditioners get scale (burned metal flakes) in their lines the day they’re installed. Learn why and prevent it.	Learn More ↓
Checking refrigerant pressure is not enough. After installation the air conditioner’s refrigerant charge should be fine tuned by the superheat and/or subcooling methods.	Learn More ↓
Don’t be fooled by bogus formulas. The payback on equipment with super high SEER ratings is sometimes never realized. Demand a real calculation.	Learn More ↓

Ducts

Wire flex ducts are infamous for how poorly they’re usually installed. A visual inspection will reveal a lot if you know what to look for.	Learn More ↓
Return ducts are routinely undersized. In many cases they represent the single greatest potential for improving the airflow of existing duct systems.	Learn More ↓
Most homes waste energy and their comfort is uneven because of pressurization issues. Extra return ducts and/or jumper ducts can be the solution.	Learn More ↓
Severe air leakage in duct systems is common. Duct sealing may represent a good opportunity for improving energy efficiency.	Learn More ↓
If you have a two story home then a zone system is a must. Both manual and automated systems can work well. Unfortunately they are rarely done right.	Learn More ↓
If just one contractor mentions the need to upgrade ducts and the rest don’t, that one may not be the crook. Consider the use of existing ducts carefully.	Learn More ↓
The cheap diffusers (vents, grilles, registers) installed by most contractors are poor performers. The difference between high and low quality diffusers can be enormous.	Learn More ↓

Indoor Air Quality

Bigger Filters Are Better Most filters I see in the field don’t perform as intended because they’re too small.	Learn More ↓
Duct cleaning is rarely useful and often fraudulent. It not only has virtually no potential to improve air quality, It’ll likely worsen it.	Learn More ↓
UV purifiers stand little chance of purifying a duct’s air stream. However, in the right application they can sterilize a cooling coil. Learn to separate myth from reality.	Learn More ↓
Duct sealing is one of the few effective methods of improving air quality. But it’s a messy business worth keeping your eye on.	Learn More ↓
Addressing whole house performance is a potentially expensive but often very effective way to improve air quality.	Learn More ↓
To actually improve air quality you must make major and potentially expensive changes in your home and in your behavior. You’re kidding yourself otherwise.	Learn More ↓

The Basics

How NOT to Buy a Furnace and Air Conditioner

Buying central HVAC the same way you buy a car is a huge mistake. Unlike a car dealer, an HVAC contractor both designs and assembles your HVAC system. As a result, HVAC equipment installed by one contractor may perform nothing like the exact same equipment installed by another contractor. In other words, selecting the right HVAC contractor is more important than selecting the right HVAC brand.

Would you trust the fine folks at McDonald’s with filet mignon? Would you buy the best replacement knee and hire the cheapest surgeon? Would you employ these guys to hang wallpaper? Of course not. But if you’re trying to get a low price on what you think is the best equipment, then you may as well find out if Dr. Nick accepts your insurance. This essay expounds further. (That last link opens Internet Archive, which may be slow to load.)

As a smart consumer, you might respond by saying, “I am searching for a good price on the best equipment, but only from those contractors who are the cream of the crop.” That’s a great idea, but how do you know who’s the cream of the crop? Did you check their license, bond, insurance, references, ratings, and so on? Those checks are fine as far as they go. However, as you’ll soon learn, they don’t go very far at all…

Any California contractor performing work worth $500 or more must be licensed. Check the license here. And then keep checking. As this story illustrates, you need not prove HVAC competence to get an HVAC license.
All licensed California contractors are bonded. That sounds good, but the required bond is nothing more than surprisingly limited insurance. It just isn’t worth much. The state seems to agree because they forbid contractors from advertising that they’re bonded.
A bond is not liability insurance. California doesn’t require HVAC contractors to have liability insurance. You have to require it. If you want proof a contractor has liability insurance, ask the contractor for a certificate of liability insurance.
As the state’s 10 Tips pamphlet suggests, check for references. And then keep checking. A contractor’s handpicked references prove nothing. Those happy “customers” could be his friends and those pretty pictures could be of someone else’s work.
An A+ rating with 3B is no different than a clean license or good referrals. In all cases, an absence of the negative doesn’t prove the positive. The same goes for Yowl, CZ Certified, and Angela’s Lineup. Check those things, of course. And then keep checking!

A contractor who fails the traditional checks is probably a bad choice. However, a contractor who passes the traditional checks is not necessarily a good choice. There are dozens of local contractors who are 3B Accredited, CZ Certified, and so on that rip people off every day of the week and twice on Sunday. This blog chronicles as much. It’s no surprise when you realize that the so-called watchdogs are funded by the businesses they’re supposed to be watching.

So what do you do? I still like your idea of accepting a competitive bid from the cream of the crop. However, you don’t yet know who that is. That’s what this page is for. A hospital can interview doctors because they understand medicine. I’m going to help you interview HVAC contractors by helping you understand HVAC. Only then will you know what to listen for during the interview (in-home estimate). Only then will you know who’s the cream of the crop.

How to Choose a Contractor

Now that your confidence in the system is shaken, let’s build confidence in yourself. I suggest the following steps.

Learn how a great heating and air conditioning system is installed by reading this page. You needn’t understand everything. You just need to get the gist of it. Your newfound knowledge will help you separate the tradesmen from the tradesboys.
Make a list of contractors and perform the traditional checks. Personal referrals aren’t bad. Yowl and Angela’s Lineup are the next best thing. While those two sites are flawed by virtue of grade inflation, at least they’re not fatally flawed.
Schedule the interview (in-home estimate) and prepare the home. Trim the bushes that have swallowed the old air conditioner. Clear the closet that has furnace, attic, and/or crawlspace access. If Fido won’t stop barking, send him to the sitter.
Be gracious and shrewd by allowing the contractor to steer the conversation at first. The more comfortable he is, the more he’ll reveal about his values. If he values irrelevancies like this and these, you’ll hear that. If he values his craft, you’ll hear that.
(That last link opens a new page to my new site dedicated to education.
Save your informed questions for the end. Show the contractor your hand (your unusual level of knowledge) too soon and he may win by bluffing. For example…
1. Ask the contractor upfront about a building permit (explained below) and he may promise to pull one even if he normally doesn’t.
2. Tell the contractor upfront that Brand A is as good as Brand B (explained below) and he may agree to be agreeable.
3. Tell the contractor upfront that you want a heat load calculation (explained below) and he may promise to perform one even if he doesn’t know how.
Unless it’s an emergency, don’t sign the contract immediately. If the contractor pressures you with a “today-only special” then he’s a greaseball in my opinion. Neither should you waive your three-day right to cancel or let them start work for those same three days.
Discuss it with your trusted advisors and pick the contractor that stands head and shoulders above the rest. If they all stand about the same then…
1. You may have yet to interview a real tradesman. Sometimes it takes a lot of digging to find that one precious gem.
2. Or maybe you really haven’t performed step one and, therefore, lack the ability to distinguish the fake gems from the real.
3. Or perhaps you’re influenced by turnaround time and money. The highly skilled contractor may take longer and cost more.
4. Or maybe you’ve actually interviewed more than one real tradesman. While anything is possible, that’s improbable.

How to Choose a Brand

Everyone wants the highest quality product. So which brand offers that? No one knows!

For the sake of discussion, let’s equate quality with longevity. Everything else, like quiet operation and energy efficiency, we’ll call a feature (to be discussed below). After decades of looking, I’ve found no solid proof that one brand lasts longer or runs more reliably than another. Generating that kind of data would require buying dozens of HVAC systems, installing them identically into dozens of identical homes, and using them identically for years. Such a study would cost millions. And the results of that study would be of no use to you because that study’s installing contractor would not be your installing contractor.

Consumer Reports’ 2014 repair data gives unwitting proof that there is no proof that one brand is better than another. Trane and American Standard are made by the same company. Their equipment is virtually identical. Yet Trane air conditioners had a 20% higher frequency of repair than American Standard. Why the difference? Trane ACs were sold to new home builders. The typical home builder cuts as many corners as possible, so many of those Trane ACs were installed poorly. A higher failure rate was the inevitable result. Similarly, and for similar reasons, Carrier furnaces had a 25% higher frequency of repair than the virtually identical Bryant furnaces.

Those statistical contradictions may be why Consumer Reports stopped giving us raw data. Now they use mathematical models to estimate reliability. Even so, contradictions remain. As of May 2021 their predicted reliability rating for Payne, Carrier, Bryant, Tempstar, and Day & Night furnaces is 5, 4, 4, 3, and 3 respectively. All five brands are made by Carrier. Most of their product lines are very similar, if not identical, and Payne is their bargain brand! One possible explanation for Payne’s “win” is that Carrier reserves their high-end features for their high-end brands. In other words, a basic Carrier furnace is indeed virtually identical to a basic Payne furnace. However, Carrier also offers incredibly complex furnaces that Payne does not. The added complexity almost certainly reduces Carrier’s overall reliability.

So how do you choose a brand? Find a good contractor. Install one of the brands he recommends as long as long as it shouldn’t be disqualified (see below). And then make sure he’s going to back it.

How to Disqualify Certain Products or Brands

While I’ve never seen solid proof that one brand is best, there are certain products and brands I would avoid based on their undesirable features.

I don’t trust microchannel tubing. It seems to be more fragile and prone to leakage than traditional tubing. What’s worse, fixing leaks in microchannel tubing is extremely difficult. A leak in microchannel tubing may result in replacement of the entire machine. Nortek and York make heavy use of microchannel coils, but I’ve also seen it in Carrier, Trane, and others. I would avoid microchannel tubing if at all possible.
I don’t like anything made by Nortek, whether it has microchannel tubing or not. I’ve repaired thousands of furnaces and air conditioners. I dislike their equipment the most. Like most manufacturers, Nortek sells under many different names. So make sure you find out who actually makes the product you’re buying.
I’d avoid anything that does not have a local wholesaler. If your equipment fails on a Friday and the part is sitting in a nearby warehouse, you might get it fixed on Friday. If the part is a hundred miles away, you’ll be waiting till next week. I’d avoid any contractor who’s a hundred or even just fifty miles away for the same reason. Many contractors will drive two hours round trip for a big job, but many won’t for a little repair.
I would avoid anything with an LG compressor inside of it. Their early compressors had major problems. Apparently LG has recently come out with their third generation compressor. Is it any good? I’ll let you know in 20 years. Carrier makes heavy use of LG compressors. Apparently even some of the cheaper Trane and Lennox air conditioners use LG compressors. So, if you can, find out who makes the compressor that’s inside your new air conditioner. Copeland is best in my opinion. Alliance may be good too because, though I can’t prove it yet, it appears that Alliance is really just Copeland by another name.

So which brands do I like? Right now it’s Trane and Rheem. But you’ve got to realize that, while my opinion is informed by extensive experience, it is just my opinion. I can’t back my opinion with proof. If the only good contractor you can find insists on installing a Carrier product with an LG compressor inside of it, then I guess he’s convinced that LG has worked out the kinks. That’s his opinion. It’s unlikely he’ll be able to back his opinion with proof either.

One final and extremely important comment on brand:

Most people don’t realize that, when they’re buying a typical residential split system that has a gas furnace (typical for this local market), they’re actually buying three major pieces of equipment. They’re buying a furnace, a condenser (the thing outside that most people incorrectly call an air conditioner), and an evaporator coil (a cooling coil that attaches to the furnace). Do NOT allow anyone to install a third-party evaporator coil unless there is absolutely no other choice. What’s a third-party coil? It’s a coil that’s not made by the brand you thought you were buying. From what I can tell, third-party coils exist because of a loophole in the federal efficiency laws that allowed them to claim efficiencies that aren’t true. That allowed them to undercut the name brand coils.

Name brand coils don’t cost that much more than third-party coils. The wholesale cost difference may be somewhere between $50 and $300. Shaving a few percentage points off your system’s efficiency to save such a paltry amount is ridiculous in my opinion. Even so, many contractors are in the habit of installing third-party coils without even telling you. When confronted, some contractors reply with half-truths like, “Lennox owns ADP, so really ADP coils are Lennox coils.” Lennox may own ADP, but ADP is not held to the same standard as Lennox. Or sometimes they flat-out lie with statements like, “They’re all the same.” Take it from someone who has serviced hundreds of evaporator coils. They are not all the same.

The bottom line? With rare exception, every major piece of new equipment that you’re buying should be the same brand. That’s true whether it’s an air conditioning system with gas heat, a heat pump system, or whatever. If you buy a Trane system, make sure they put in a Trane evaporator coil. If you buy a Rheem system, make sure they install a Rheem evaporator coil. One obvious exception is if the differing brands really and truly are the same. As previously noted, Lennox and ADP coils are not the same at all. However, Carrier and Bryant coils are indeed identical. There are many examples like that. You have to dig to come up with the truth.

How to Choose Features

As explained above, anything that doesn’t add to the equipment’s longevity is what I call a feature. Before being upsold on a seemingly attractive feature, consider this: In many cases the core components of the high-end and entry-level models are the same.* In some cases the high-end model is the entry-level model with a bunch of bells and whistles piled on top.

(* The compressor inside of the condenser or heat pump can be a notable exception. I have seen several instances where a manufacturer will put an LG compressor in their entry-level machine and a Copeland compressor in their high-end machine. As discussed above, I really dislike LG compressors. There may be times when you have to buy a mid-level or high-end unit if you want a guarantee from your contractor that your new unit will not have an LG compressor.)

Since many mid-level and high-end features increase complexity and reduce reliability, it’s wise to choose only those features you truly need or want. Don’t be upsold on something that sounds good, but that you never wanted until the contractor said you wanted it. If the entry-level model suits your needs, buy it. On the flip side, if you…

Want less noise? You may have to buy a mid-level or high-end model.
Want higher energy efficiency? You may have to buy a mid-level or high-end model.
Want more aesthetically pleasing equipment? You may have to switch brands.

Certain mid-level and high-end features like reduced noise, two-stage operation, and so on are available from virtually all brands. But not all brands offer the highest of the high-end features. And not all brands offer certain unique features that might not be high-end, but might be desirable for some homes:

If you live on the coast then you might consider a brand that offers all aluminum coils in their outdoor unit. They seem to fare better in salty environments than copper coils.
If you want to install a zone system, you might consider a brand that offers a two-stage compressor and furnace.
If your old AC is in tight quarters, then you’ll want a new one that’s similarly compact. Some brands have found ways to reduce their footprint.

Not all fancy or unique features lower reliability, but many do. Choose wisely.

How to Negotiate the Price

In one word: Don’t.
In ten words: If you’ve found a good contractor, then don’t negotiate aggressively.
I know those words are like a knife to the penny pincher’s heart, but hear me out:

The best contractors need and appreciate their customers. On the flip side, the best contractors are usually in such high demand that they don’t need any one customer in particular. Therefore they need not negotiate. Barring unusual circumstances, a contractor who has to drop his price significantly may not be the best after all.
The market determines what a fair price is. Most good contractors have worked their market for quite some time so they will, by virtue of being good, charge that fair price. If you believe a good contractor’s price is unfair, then perhaps that contractor isn’t actually good. Or perhaps you don’t yet know your market.
If by some fluke a good contractor drops his fair price significantly, hasn’t that become an unfair price for the contractor? That he agreed to it doesn’t mean he’s going to let it happen. Whether consciously or unconsciously, the odds are good that he’s going to cut corners to make the price fair.
There are rare occasions when a good contractor makes a conscious decision to work both cheap and good. As you could’ve guessed, that doesn’t happen often. What does happen often is for a contractor to raise his price artificially high so he can then “discount” it to make you think you’re getting a deal.

While I don’t recommend aggressive negotiation, there’s nothing wrong with finding out why things cost so much. I explain the pricing backstory here. (That link opens a new page to my new site dedicated to education.) If you want to understand why there can be such a disparity between high and low bids, see the table below. It shows the approximate wholesale costs some California contractors incur when replacing an entry-level gas furnace.

	real tradesman	real hacker
furnace	$1000 to $1500	$1000 to $1500
building permit	$150 to $450 – required by virtually all localities	$0 – illegally skipped
Title 24 duct testing	$400 to $500 – 3rd party test for air leakage	$0 – illegally skipped
duct sealing labor	a few to many hours – needed to pass duct test	0 hours
sheet metal	$100 to $500 – custom transitions = better airflow	$30 – blocky transitions made from scrap
filter holding device	$100 to $300 – easy access to the filter	$0 – filter jammed somewhere
SSU, connector, etc.	$50 to $150 – parts near the furnace replaced	$0 – old parts reused = breakdowns later
installer’s wage	$60 per hour tradesperson, plus benefits	$30 per hour day laborer
installation labor	6 to 12 hours not including duct sealing	3 to 6 hours
taxes paid	many hundreds	$0… unless you 1099 him
overhead	more	less

The time, money, and effort that goes into a good installation is considerable. Years ago (back when I still did installations) I quoted $3,000 for installations that hackers quoted for $2,000. Despite being a thousand dollars less, the hacker’s “per hour take home pay” was more. More importantly the furnace’s installation, and ultimately the homeowner, would have suffered for it. The real danger in buying HVAC equipment isn’t paying too much. It’s paying too little.

How to Keep It Legal

Whether you’re replacing an existing HVAC system or installing one for the first time, a building permit is required in virtually all jurisdictions. The same goes for wall heaters, wall air conditioners, PTACs, ductless mini-splits, or anything else that’s a fixed installation. Though many contractors act like permits are optional, they are the law. That legal obligation is as much yours as it is the contractor’s. The permit serves a few useful purposes:

A permit may help protect you from sloppy work. Installers who know their work is going to be inspected tend to do a better job.
A permit helps to ensure that both your homeowners insurance and the contractor’s bond and liability insurance are in force. A botched installation can cause significant damage. If it was an illegal installation, the bond and insurance companies might have legal right to refuse coverage.
A permit now means less trouble later when you sell your home. It’s a home inspector’s job to find code violations. Some home inspectors even pull a home’s permit history, or the lack thereof, and include that in their report. Getting a contractor to fix code violations years after he installed the equipment is next to impossible.

A typical HVAC system uses electricity, burns fuel, makes exhaust gas, and creates condensation. In other words, it can damage your home and kill your loved ones. The building department is there to make sure it doesn’t. And though this rarely happens, they’re also there to fine you for not pulling a permit. Pull that building permit and you just might avoid premature death and unnecessary taxes. The process typically goes like this:

The permit should be mentioned on the bid and/or home improvement contract. It may be listed as a line item (so that you know how much the government is costing you), but in most cases it shouldn’t be an optional line item.
Once you’ve signed the contract, the contractor usually pulls the permit on your behalf. You can pull the permit yourself, but the contractor should be a lot more familiar with the process.
The permit expires after a certain number of days, often 180. If the permit has expired, some building departments will perform the inspection without requiring payment to reopen it. Others will require additional fees.
After the installation is complete, duct and refrigerant testing will probably need to be performed by a HERS rater. (That’s a California requirement. I don’t know if other states have similar programs.) The contractor usually hires the HERS rater. However, if you want assurance that the test is legit, hire the rater yourself. Ask the contractor for the name of the HERS raters he normally uses. Then hire a different rater.
After HERS testing is complete, the building inspector will come out to inspect the work. The contractor doesn’t always need to be there. That’s especially true of simple jobs. The inspector may just need an adult there to let ’em in.

As alluded to, a home improvement contract is required for any home improvement project valued at $500 or more. I’ve seen a lot of what contractors try to pass for contracts. Most of them are much too brief to be legal. A legal contract has a detailed description of the work. It informs you of your three or five-day right to cancel, the maximum down payment a contractor can ask for, and much more.

How to Design the System

Most new HVAC systems are designed with nothing more than outdated rules of thumb and bad tradition. Simply copying the old system’s design qualifies as bad tradition. Many old systems, even those that seem to work well, were not actually designed and installed very well. Ideally your new system should be designed with Manual J and Manual D calculations or something like them. Those manuals are published by the trade organization ACCA. Most contractors who perform Manual J & D calculations use software based on the manuals.

Manual J

A Manual J heat load calculation determines what size your heating and air conditioning system should be. If you’re just installing equipment, the contractor may just take the home’s exterior measurements along with the estimated R-values of the exterior doors, windows, and insulation and enter that data into the computer. Don’t let your contractor add in fudge factors. Manual J has been shown to oversize equipment by some amount already. I’d expect this process to take between one and a few hours, depending on the size of the home.

If you’re installing a new duct system then a room by room Manual J calculation is needed. Since every room has a different airflow requirement, it follows that every room must be measured. That’s in addition to the exterior measurements discussed in the paragraph above. I’d expect a room by room Manual J calculation to take at least a few hours to complete. I once designed multizone HVAC system for a rather large home. That particular Manual J calculation took an entire day to complete.

There are potentially valid alternatives to Manual J software. If your prospective contractor proposes to use an alternative method, he needs to convince you of its validity. A single page form, fill-in-the-blank website, or any other goofball method just doesn’t cut it. A real heat load calc almost invariably requires software and takes time. If your contractor says he’s using a valid method but won’t explain it, can’t produce a somewhat impressive printed report, and/or did it a little too quickly then there’s a pretty good chance it’s not valid at all.

Other Calculations

A Manual D calculation uses the results of a Manual J calculation to determine how big each room’s ducts should be. Manual D takes into account how powerful the blower is, how far each room is from the blower, the pressure drop created by various components like the air filter, and so on. A Manual D calculation doesn’t usually take as long to perform as the Manual J unless there are complications. If the contractor has Manual J software, he almost certainly has Manual D software. He should be able to produce that report as well.

Manuals J & D are the most important calculations, but there are others. There’s Manual S for equipment selection. Manual T is for diffuser selection. Manual Zr covers zoning. The list goes on.

Pragmatism

At the time of writing, I only know of one local residential HVAC contractor who probably performs heat load calculations with some regularity. (It’s only a probability. I can’t prove it.) If you find a contractor who claims they will perform a heat load calculation, ask them what brand of software they use. Also ask them if that software is ACCA approved and if you can get a copy of the full report that the software produces. They should answer the first two questions without hesitation. As for the report, you’re really only entitled to that after you’ve paid them something.

Don’t get too hung up on the calculations. You’re unlikely to find a contractor who’s well versed in all of them anyway. Just finding one that performs J & D calculations is hard enough. If they don’t use ACCA approved software, then let them make the case for their methodology. And if the contractor you’re leaning towards doesn’t perform any calculations at all, that doesn’t have to be a deal breaker. If he can convince you that he’s going to do a good job without the calculations, then so be it. Believe it or not, there are good arguments both for and against heat load calcs.

The previous paragraph doesn’t sit well perfectionists, but the reality is you can’t always find a contractor who does everything by the book. I sometimes refer a contractor who doesn’t perform J & D calculations. Even so, I know they adhere to most of the practices found on this page. And just as importantly, I know they back their work. That makes them a rare find indeed. While I created this website to teach the textbook ideal, no contractor is 100% ideal. My goal is to teach you the ideal so you can choose the contractor who comes the closest.

How to Get What You Paid For

Most new central heating and air conditioning systems do not deliver their rated capacity or efficiency. That’s because a new HVAC system’s capacity and efficiency depend heavily on how well the system is installed. In my experience at least, most new HVAC installations are subpar. While it’s true that most new HVAC systems lower a customer’s utility bill, in most cases those bills could have been even lower still if the system had been designed and installed better. Most homeowners are unaware of these failings because most contractors do very little testing of the new systems they’ve installed.

You might be able to avoid those failings by adding a line to the contract that says the equipment’s performance will be measured and documented before final payment is made. A test of AC, furnace, duct, and overall system capacity and efficiency looks something like this, this, this, and this respectively. Exactly how much of that testing should be performed on your system depends partly on how much work you’ve had done. It also depends on how capable the contractor is. From what I’ve seen, most local contractors either can’t or won’t perform most of those tests.

If you’re getting entirely new equipment and ducts, then not only should the new system deliver its rated capacity and efficiency, the home should feel fairly even as well. While each room’s airflow can be measured and compared to Manual D calculations, nothing beats living with the system for a while. You might want to put something in the contract that says the contractor will amend the ducts as needed even if it takes you months to notice the unevenness. If the new system is installed in mild weather, it can indeed take months to notice balance problems.

Furnaces / Air Handlers

Furnaces & Air Handlers Blow

One of the most important roles a gas furnace or (air handler if you have a heat pump system) plays is that of blower. The furnace’s blower (or the air handler’s blower) blows air through the ducts in both heating and cooling modes. The fan you see on the outside machine has no direct connection to the air inside. That fan just helps to release heat from the refrigerant (Freon, Puron, etc.) in the summer and (if you have a heat pump) absorbs heat into the refrigerant in the winter. To reiterate, the visible fan outside has no direct connection to the inside air!

If your HVAC system has air conditioning, then the furnace’s blower (or air handler’s blower) should push at least 400 CFM (cubic feet per minute) of air through the ducts for every ton (12,000 BTUs) of air conditioning capacity. In particularly dry climates like ours locally, 450 CFM per ton is even better. Unfortunately, most systems deliver nowhere near 450 or even 400 CFM per ton. In a humid climate, 350 CFM per ton is more appropriate, but even that goal is often not reached. One survey found that the average HVAC system blows just 300 CFM per ton. Such low airflow robs the system of capacity and efficiency.

This rampant airflow problem is due primarily to undersized ducts. Restrictive filters, cheap diffusers, dirty coils, etc. are also to blame. Many furnace and air handler manufacturers seem to design their blowers on the assumption that the ducts are sized “by the book”, but in my experience that’s rarely the case. As a result, most furnace and air handler blowers deliver an overall airflow that’s hundreds of CFM less than their rating. So if your home has undersized ducts (it probably does), and if you’re not going to replace them with properly-sized ducts, then you may or may not want to install a furnace or air handler with an oversized blower.

The furnace’s blower (or the air handler’s blower) has a tonnage rating just like the condenser (or the outdoor unit if you have a heat pump) does. Back when I performed installations, I would often install a furnace (or air handler) with a blower that was one ton bigger than the condenser (or outdoor unit). For example, I’d match a 3 ton condenser (or outdoor unit) to a furnace (or air handler) with a 4 ton blower. The 4 ton blower in that situation wouldn’t deliver 4 tons of air (1600 CFM) of course, but in almost every case it would deliver 3 tons of air (1200 CFM) through the existing undersized ducts. That helped the 3 ton condenser (or outdoor unit) to actually work the way it was supposed to.

Upsizing the furnace’s (or air handler’s) blower by one ton relative to the condenser (or outdoor unit) doesn’t usually cost that much extra. However, proceed with caution. An oversized blower will probably increase overall airflow, but it may also increase noise. The good news is that an oversized blower can be turned down if necessary. For example, a 4 ton blower can be turned down to work the same as a 3 ton blower. But a 3 ton blower can never be made to work like a 4 ton blower. Additionally, while an oversized blower may increase airflow, it’s not likely to help room to room balance. Finally, if your ducts are just way too darn small, trying to compensate with blower power is probably a bad idea.

There are other potential disadvantages to oversizing a blower. When it comes to gas furnaces, getting an oversized blower can mean that the furnace’s BTU input is too high. There are ways to deal with the excess BTUs, like getting a two-stage furnace and running it in first stage only. Also, if you get an oversized blower to compensate for undersized ducts, then the system may not pass the fan watt draw test performed by the HERS rater. Of course that’s not a problem if the system is not going to be tested that way.

Obviously properly sized ducts are always better than an oversized blower. However, new ducts will probably cost thousands. If you’re buying a new furnace or air handler anyway, and if you’re not upgrading your ducts, spending an extra couple hundred on a more powerful blower might make sense.

Bigger Is Not Better, Unless It Is

Furnaces
(Air handlers are a few paragraphs down.)

The second most important role a gas furnace plays is the obvious one of heater. A typical furnace has two sections. (There are excellent pictures here as well.*) It has a blower section and it has a heating section. When I’m talking about a furnace’s size in this section of the guide, I’m strictly talking about its heating section. That is to say, I’m talking about its heating capacity. As you read above, I do sometimes advocate for oversizing the furnace’s blower. However, I would never advocate for oversizing the furnace’s heating capacity. An oversized furnace may short cycle, may wear out prematurely, and may blast the occupants in the face with too much hot air.

Oversized furnaces are a common problem locally. We and much of the southwest tend to have hot and dry summers combined with fairly mild winters. So we need furnace blowers that can blow a lot of air in the summer while not blasting us with too much heat in the winter. As one who has installed in the southwest, there have been occasions where I could have used a 5 ton blower in a furnace with a 60,000 BTU input. No such furnace exists, at least not that I have found, because furnace manufacturers seem to design their furnaces with cold climates in mind. That being the case, getting a strong blower often means getting too much heat with it.

The answer to the predicament of needing a strong furnace blower combined with not-too-much heat output can be a multi-stage furnace. The highest output of a two-stage furnace is “full blast” just like a simpler one-stage furnace. The lowest output of a two-stage furnace may be about two-thirds of furnace’s heating capacity. (The lowest output of a modulating furnace is even lower at around 40%, but I discourage most folks from buying modulating furnaces because of their expense and complexity.) That lower output can result in more even heating and better comfort. I have installed quite a few two-stage furnaces with oversized blowers (as recommended above) and set them up so that they run in first stage only during the heating mode.

If you happen to live in an area with really cold winters then furnace oversizing may not be as big of an issue. Even so, don’t let your guard down. Oversized furnaces are installed in all climates.

Air Handlers

Most air handlers in central heat pump systems are rectangular boxes that look something like this. Like a gas furnace, the typical air handler of a heat pump system has two sections*. The air handler’s blower section is like the furnace’s blower section. It has a blower that blows air through the home’s ducts in both heating and cooling modes. As described in the “Furnaces & Air Handlers Blow” section above, oversizing the blower in an hair handler might be a good thing to do.

The heating section of the air handler, on the other hand, is very different from a furnace. Instead of releasing heat by burning fuel, it releases heat from hot refrigerant (Freon, Puron, etc.). The thing releasing the refrigerant within the air handler is usually called a coil. You can see a coil inside of an air handler in the fifth picture on this page.* The larger the coil, the easier it is for the coil to release heat and the more energy efficient it will be. So, in dry climates at least, it’s wise to install the biggest air handler that’s allowed. The biggest air handler with have a strong blower (as recommended above) and a large coil.

The biggest coil that’s officially allowed is determined by AHRI. That’s the organization that the HVAC industry relies on to certify compatibility between air handlers and outdoor units.

As for humid climates, if the coil is too big then the system won’t dehumidify properly in the summer. Your contractor will need to pick wisely.

* I have no opinion about that website, other than they have excellent pictures.

It’s the Little Things

There are a lot of little things that many contractors skip during an installation, but shouldn’t.

Make sure they replace the furnace’s or air handler’s on/off switch. Some of them take the form of a simple light switch. Others take the form of an SSU. If you have an air handler, it may look something like this. I’ve had lots of people pay me hundreds of dollars to replace those items only a few years after someone else installed a mostly new system. Insist they make it entirely new!
It’s always better to connect your furnace or air handler to the on/off switch by way of MC cable, flexible conduit, or something else equally robust. If, for some strange reason, they use a cord then make sure they install a new cord and a new outlet or SSU. I say “strange reason” because everyone should know by now that cords are not a great choice. They’re not even allowed in some jurisdictions.
If the furnace or air handler has fuses, make sure they install new fuses that are properly sized. Getting a spare set is nice too.
Some jurisdictions require excess flow valves like this to be installed on gas furnaces. I often find them missing.
Make sure they replace the furnace’s gas cock. That’s the manual on/off gas valve that’s found next to the furnace. They don’t last forever and they do leak sometimes.
Make sure they replace the gas furnace’s flexible gas connector. It should not penetrate the furnace’s sidewall. The flexible connector should sit outside the furnace and hard pipe should penetrate the sidewall.

These may seem like small issues compared to the larger concepts explored on this page. However, as an HVAC service man, I see the problems caused by neglecting these items all the time. On rare occasion neglect of those items can lead to property damage and even injury.

Clearances Count

All furnace and air handler installation manuals explain what clearances around the equipment are required. For example, page 4 of this manual gives that furnace’s required clearances. Some of those clearances are for safety reasons. The 24″ clearance required in front of that furnace is for servicing purposes. Some jurisdictions require 30″ clearance in front of an air handler or furnace for servicing purposes. The entire front of the furnace or air handler needs that clearance. Partial clearance doesn’t cut it.

The furnace’s vent pipe (exhaust pipe) should also have proper clearance. A single wall vent should have six inches clearance to any combustibles. A double wall vent should have one inch clearance to any combustibles. Your contractor can point out what single wall and double wall pipes are. I don’t know anything about this site, but they do have excellent pictures of what I’m talking about.

High Efficiency, Low Return?

When it comes to heat pumps, the choice of standard, medium, or high efficiency mostly depends on which outdoor unit you choose. We’ll talk about that later in the Air Conditioners / Heat Pump section below.

When it comes to gas furnaces, your two basic options are 80% efficiency or “ninety-something” percent efficient. Typical ninety-somethings are somewhere between 94% and 98%. I’ll just refer to them as “90+”. 90+ furnaces are more complex, are more expensive to work on, and are more likely to fail than their 80% counterparts. That 14 to 18 percent improvement in efficiency may never actually pay you back the added capital, maintenance, and repair costs that come with a 90+ furnace.

I don’t agree with everything this guy says about the pitfalls of buying a high efficiency furnace. For example, it certainly is possible to save money by choosing a high efficiency furnace over a standard furnace. Even so, he makes good points. Going high efficiency is not a slam dunk by any means. If your salesman is pitching energy savings then take a look at the energy efficiency comments below.

Of course there are reasons to buy a 90+ furnaces beyond that of energy savings. 90+ furnaces tend to be a little safer and run a little quieter than their 80% counterparts. Plus some folks just like the idea of burning less fossil fuel. There’s certainly nothing wrong with that.

Air Conditioners / Heat Pumps

(I’m in the process of updating this page. Most of the information about air conditioners applies to heat pumps as well, but the language from this point on doesn’t address heat pumps specifically. Plus I am not a fan of heat pumps for our area. They work well in certain applications, but for most folks in our area HEAT PUMPS ARE HOT GARBAGE. Variable speed heat pumps are even worse. Read the 6th entry on this page to learn more.)

Bigger is Definitely Not Better

1) In a perverse sort of balancing act, a huge percentage of air conditioners and heat pumps are oversized but underdeliver. A study done by Proctor Engineering Group found that an air conditioner sized by Manual J is, on average, 24% too big. Add in the fudge factors that fearful contractors are wont to do and you could end up with an air conditioner that’s 50% too big. Oversizing is bad. More specifically, oversizing a condenser is virtually always bad. (As explained above, oversizing the furnace’s or air handler’s blower blower can be good.) Most oversized condensers run less efficiently and don’t last as long as their properly-sized counterparts.

There are many reasons for the rampant oversizing problem. Some of them are covered below. But one very important reason is customer perception. It’s sometimes difficult to convince a customer that bigger is not better. While an air conditioner that can turn a house into a meat locker inside of ten minutes may be pleasing on a hot day, it’s a bad thing in many respects. In humid climates such rapid cooling will leave you feeling cold and clammy and possibly lead to mold growth. An oversized air conditioner costs more to run and may have a shorter service life. A properly sized air conditioner won’t take the house from 90 to 70 in ten minutes. But when used properly it will keep you comfortable, be more energy efficient, and may last longer.

2) The first step to high performance air conditioning is proper airflow. I’ve mentioned it above already. It’s worth another mention. In a dry climate you should get that airflow up to 400 CFM per ton at an absolute minimum. 450 to 500 CFM per ton is ideal. Even in humid climates where an airflow of 350 CFM can be appropriate, many systems fail to deliver that. I once measured a system that delivered only 200 CFM per ton! That system would do poorly in any climate. Such amazingly low airflow is more common than you might think. Maintaining proper airflow will improve performance for reasons that you probably aren’t aware.

An air conditioner has two functions: First, it lowers the temperature you read on your thermometer. We call that sensible capacity because both you and the thermometer sense it. Roughly 70% of a typical air conditioner’s capacity will be spent on sensible heat removal (temperature reduction), though that number varies considerably. Second, it removes moisture from the air. We call that latent capacity. Roughly 30% (plus or minus) of your air conditioner’s capacity is spent on latent heat removal (moisture removal).

In a very dry climate latent heat removal is a waste of energy. Unfortunately, that waste is often compounded by low airflow. Low airflow can cause that 70/30 ratio to drop down to 60/40. But, with high enough airflow and a big enough coil, you can sometimes get that number up to 85/15. That 25% difference translates into 25% more sensible cooling capacity from the same exact air conditioner for nearly the same cost in electricity. In other words, the thermostat will shut off 25% sooner. For humid climates latent heat removal is desperately needed. But reduce airflow too much (like in the 200 CFM example above) and the reduced airflow is no longer enhancing moisture removal. It’s just wasting energy and capacity.

3) The second step to high performance air conditioning in a dry climate is the use of an oversized and/or high efficiency evaporator coil. High efficiency coils are usually bigger than their cheaper counterparts. Generally speaking their large size allows for better airflow. Their large size also means they have greater surface area. That means the refrigerant is spread out over a larger area allowing for more effective cooling. A larger coil combined with the blower recommendations above means (for example) that a two ton air conditioner could be matched to a three ton high efficiency coil and a three ton furnace. Such a combination is not only approved by the manufacturer in many cases, it’s the best way to design a system for a dry climate.

High efficiency and oversized coils may only cost a few hundred more than cheap coils. Regardless, if you’re in a dry climate, don’t think about it. Just do it as long as there is an approved AHRI match. If there’s not an approved match, well I have to admit going “off label” and doing it anyway a few times. Going off label must be done with extreme caution and I would never do it with multistage or variable speed compressors. In any event, in some instances the results can be unbelievable. I increased one customer’s sensible heat removing capacity by 34% by simply replacing their compact cheap coil with an oversized high efficiency coil. See the pictures here and here.

If you’re in a very humid climate then oversized and / or high efficiency coils can be exactly the wrong thing to do. The bigger the coil the more it will favor sensible heat removal over humidity removal. High SEER air conditioners may come with a high efficiency coil by default. A super high efficiency air conditioner is more likely to have trouble keeping your house dry if you live in a humid area. If you live in a humid area then you must be diligent in picking a contractor that understands how to balance energy efficiency with moisture removal.

4) The next step is to properly size the refrigerant lines. Many are undersized. While the effect isn’t nearly as dramatic as that of undersized ducts or undersized coils, it can become critical when there’s a big distance between the furnace and air conditioner and/or when your system is barely big enough.
The numbers below are for R-410A systems. R-410A is a refrigerant that’s often called “Puron”. We’ll be transitioning to new refrigerants in the coming years. Their recommended refrigerant line sizes may be different.

For example, the recommended suction line (the refrigerant’s return line back to the compressor) size for many 5 ton systems is 1 1/8 inches (outside diameter). Yet most five ton systems I’ve seen have 7/8 inch suction lines. I’ve even seen some 5 ton systems with 3/4 inch suction lines. In that case a 40 foot run of 3/4 inch suction line results in a loss of only 1% of capacity. But on a 100 foot run the 7/8 inch line results in a loss of 5% of capacity. (These numbers are taken from one OEM’s refrigerant piping guide.)

Even if it doesn’t look like your cooling coil (AKA evaporator coil) and air conditioner (AKA condenser) are that far apart, they may be farther apart than you think. That’s because we’re actually talking about total equivalent length, or TEL. For example, every short radius 90 degree copper fitting used to assembly your refrigerant line has a TEL of about 8 feet. A 60 foot refrigerant line with 5 of those 90 degree fittings would have a TEL of 100. That means it would have same resistance to refrigerant flow as a perfectly straight 100 foot line. Given the turns and bends that most refrigerant lines take, a TEL of 100 feet or more is pretty common.

If you’re upgrading from a Freon-based system to a Puron-based system, and if you’re going to use the old refrigerant lines, it’s important to discuss the possibility of contamination entering your new equipment from the old lines. A small percentage of old copper lines have an oily buildup that the new compressor’s POE oil will clean off and bring back to your new compressor.

While we’re on the topic, make sure they insulate the ends of the refrigerant lines. Too many hurried installers leave them exposed. In some cases the condensation that forms on the exposed line will drip down onto the furnace and damage electronic components.

5) Micron gauges are important and they’re not always used. Trade schools teach the use of micron gauges as a matter of course. Doing so can prevent the formation of acids in your new system. Yet some installers don’t even own one.

6) How your refrigerant lines are connected is another area of incredible long term importance. There are at least two methods

The copper lines can be brazed together. Brazing means heating the copper lines to around 1200 degrees and then applying a filler material to the joint. At such high temperatures the copper oxidizes. Scale (burned metal flakes) will form in your copper lines. The scale will float around the refrigerant circuit and contaminate the compressor’s oil sump. It could even clog the metering device as seen on point 16 of this page. To prevent the scale from forming your installer is supposed to purge the lines with nitrogen. Nitrogen is inert and purging with it will keep the inside of the copper lines clean. But quite frequently that step is skipped.

An alternative to brazing is to use a silver solder like Stay-Brite 8. Stay-Brite 8 is one of the few solders available that’s approved for air conditioning. (That’s not to say that it’s approved by a particular OEM. I’ll have to save that story for another time.) Its advantage is that the copper pipe has to be heated to only around 400 degrees. At that temperature virtually no scale is formed. Stay-Brite 8 is more expensive than brazing alloy. In the right application, it’s well worth it.

7) Make sure that the refrigerant charge in your new air conditioner is fine tuned. Too many installers skip this step. The factory puts a base charge of refrigerant in their new air conditioners. On split systems the factory has no way of knowing how long the refrigerant lines will be or what type of evaporator coil will be installed. As such the refrigerant charge should be fine tuned. Checking pressures is not enough. Refrigerant must be adjusted by either the subcooling method, the superheat method, the manufacturer’s specified method, or some combination thereof.

The exception to this rule might be package units. If your heater and air conditioner are combined into a single machine then adjusting the refrigerant charge may not be needed. However, on rare occasions a new package unit will be low on refrigerant due to a refrigerant leak. So even a package unit might need to be checked.

8) Starting in 1992 the minimum efficiency for newly manufactured air conditioners was 10 SEER (seasonal energy efficiency ratio). In 2006 that rose to 13 SEER locally. In 2015 that rose to 14 SEER locally. At the time of writing, it’s now 15 SEER locally. For mild climates like ours in the SF Bay area, it may not make sense to upgrade to anything higher than a 15 SEER model. And I certainly don’t recommend anything higher than 16 SEER for most people. Anything above 16 SEER often means you’re buying complex equipment that costs more up front, breaks down more often, and costs more to fix when it breaks.

Some salesmen use formulas based on heating degree days and/or cooling hours to calculate your energy savings and convince you that a high SEER air conditioner or high AFUE furnace makes financial sense. Those numbers are meteorological in nature. They are based on statistical averages that may bear no resemblance to how you use your system. Pages like this can be equally misleading. The real way to determine how much you will save is to look at your past utility bills, determine baseline and peak usage, and calculate from there. It’s not hard to do. If your salesman is pitching energy savings then demand that s/he prove it by working with your past utility bills and not some trumped up formula.

That does not mean there aren’t good reasons to buy high SEER and high AFUE equipment. High SEER air conditioners are often quieter, sometimes have better warranties, and may look nicer. High AFUE furnaces are often quieter and safer. And of course there are some circumstances where a customer will indeed save big money by buying high SEER and high AFUE equipment. Just don’t assume so. Crunch real numbers to prove it.

Ducts

The greatest deficits found in most systems are not surprisingly found in the part unseen by the customer. Few customers inspect crawl spaces to look at their ducts. And even if they did, few would know what to look for. Read on and you’ll have an idea of what to look for. The first key to a high performance duct system has already been mentioned. Make sure that a Manual D calculation is performed and that the ducts are installed accordingly.

1) Next make sure that the ducts are installed correctly. Most commonly in our area wire flex duct is installed. Imagine a giant slinky with plastic bonded to it. Wrap that with a layer of insulation and wrap it again with an outer plastic shell and you have wire flex duct. The most common problems with wire flex ducts (besides being undersized) are in how they’re attached and how they’re hung.

If the ducts are hung they should be supported every four feet with two inch strap. There should be little up and down sagging. Excessively sharp turns and kinks will reduce airflow. Unnecessarily long runs will also reduce airflow. Some installers will loop a long duct around rather than be bothered with cutting off the extra. If in an attic, laying them on top of the ceiling is acceptable for most local jurisdictions. In fact, per California Title 24, burying them with blown insulation after they’re installed is allowed as a way to make them more energy efficient. If under the house they should not lie on or even touch the ground. That can be quite the challenge in crawl spaces with minimal space. Metal ducts are required to have four inches clearance to the ground.

The ends of each section of wire flex duct will be attached to the metal collar of a boot, wye, plenum, etc. Each end should be secured with a mechanical connector like a Panduit strap and also sealed with tape or mastic. A Panduit strap looks like a giant wire tie that you secure with a tensioning tool. The mechanical connector is what keeps the duct attached. Tape and mastic don’t work well for this purpose. Many tapes will dry out over time and loosen. Tape and/or mastic are only to be used for making connections airtight.

Any exposed metal of the duct system that resides in unconditioned space should be insulated. And in humid climates, the insulation should have an airtight vapor barrier over it.

2) Return ducts are very often undersized. If you are planning on replacing your ducts then size the return duct as big as space allows, within reason of course. You can’t really oversize a return. If you can fit a 20″ return then it may be a good idea to put a 20 inch return in even if conventional wisdom says you shouldn’t. If your air conditioner is rated for 5 tons then even a 20 inch return may not be enough. You may need to add a second return. An oversized return will compensate to some extent for undersized supply ducts.

One thing you should not worry too much about when it comes to your return duct is placement or how many you have. Some contractors believe that large homes must have more than one return. That’s not necessarily the case. Returns can help to alleviate stagnation in areas that lack supply vents. But the primary means by which comfort is achieved is through proper air mixing provided by the strategic selection and placement of supply diffusers. Generally speaking returns act as little more than drains. That’s an important function, but not one that necessarily enhances comfort.

Pressurization issues can be addressed with added returns. However, addressing pressurization does not necessarily require adding returns.

3) Point 2 of this section assumes that you have a typical home (for this area of the country) with just one or perhaps two return ducts. If you can afford it then put a return or jumper duct in every room that has a door which is frequently closed. That usually means every bedroom and possibly other rooms.

Homes are made terribly inefficient and air quality is negatively affected by pressurization issues. If a duct is trying to dump 100 cubic feet of air per minute into a bedroom through a duct that has an area of 50 square inches, what chance does that air have of escaping back to the return duct in the main part of the house through a crack under the door of less than a tenth that size? It has no chance whatsoever.

Your bedroom may look fairly airtight. But on older homes especially there are numerous cracks through which air can escape to the crawl spaces and outdoors. A typical HVAC system running with the bedroom doors closed will put those rooms under a positive pressure and the central portion of the house under a negative pressure. The bedrooms will leak the air you paid to condition to the outdoors. That will cause the center of the house to be negatively pressurized. It will draw in air from outdoors to make up the difference. Some of that unplanned makeup air will come from crawlspaces that may not have very good air quality.

Leaving most of the doors open most of the time will solve the problem. Or a return duct can be installed in each room that has an oft-closed door. Or jumper ducts can be installed. Jumper ducts don’t connect directly to the HVAC system. They’re very short ducts that simply jump the wall separating a closed room from the central portion of the house.

4) Leaky ducts have a similar effect on house pressurization as described in point 3 above. Most systems leak more air on their supply ducts than they gain through leaks on the return duct. As a result they’ll negatively pressurize the house and force outside air in through whatever cracks the air can find. Some of the cracks may lead to a damp crawl space under the house or to an attic full of insulation and rat droppings.

California Title 24 rules require (under certain circumstances) new duct systems to leak less than 6% and old duct systems to leak less than 15%. That may sound like a lot of leakage. But many existing duct systems leak that much and more. Even if your home isn’t affected by those rules, the maximum leakage that they call for is reasonable for any home.

5) Zone Systems

Two story homes need most of the cold air delivered upstairs in the summer and most of the warm air downstairs in the winter. That’s not going to happen without some kind of zone system. There’s simply no way for a single, fixed duct system to adjust itself for the different seasons. Even a large single story home can have similar problems.

There are many potential solutions to uneven comfort. Improved insulation, upgraded windows, and many other factors all play their part. But as it relates to heating and air conditioning, a zone system can be the answer. For a two-story home a zone system is often the only answer. The simplest zone system might be comprised of manually controlled dampers that the homeowner adjusts twice per year. The ultimate zone system would be completely separate HVAC systems serving different parts of the home. Such a setup is easily the most comfortable, the most energy efficient, and also the most expensive to install. The next best solution is to split a single HVAC system into multiple automated zones. A single system with automated zones will be the focus of the rest of this section.

The Problem With Zone Systems

Unfortunately zone systems are notoriously problematic because they are so rarely designed and installed correctly. Before we get into the nuts and bolts of the matter, there are a few fundamentals you might want to know so you’ll get the big picture. Alternatively you may wish to read the rest of this section on zone systems and then refer to the links if you want more information.

HVAC 101: An HVAC System’s Cooling Capacity
HVAC 101: Airflow’s Effect On Compressor Longevity
HVAC 101: Furnace Longevity

While they touch on other issues, all three links above describe why maintaining proper airflow is so incredibly important. Low airflow causes a loss of cooling capacity, increased energy usage, decreased compressor and / or heat exchanger life, short cycling of the equipment, etc. And low airflow is precisely what you get on most zone systems. It is the fundamental problem with zone systems. When only one zone comes on your home needs only a fraction of the system’s cooling or heating capacity. And with only a portion of the ducts open they can only accommodate a fraction of the airflow. Yet most furnaces and air conditioners attempt to run at 100% capacity any time they’re on, even when just one zone calls. Most furnaces and air conditioners are single stage units, meaning that they attempt to deliver a constant output of 100%. They need 100% airflow, or at least something close to it, at all times.

The hypothetical I routinely tell customers is this: Imagine that you have ten ducts supplying air to the house. And imagine they all have the exact same airflow. If you were to take duct tape and seal off five of those ducts what you’d find is that the other five would not have double the airflow. They’d have considerably more to be sure. But the increased static pressure caused by the choked off ducts robs disproportionately from velocity pressure. So the total airflow running through the equipment drops.

The airflow problem with zone systems is enhanced by the fact that most ducts are undersized. Yet a zone system’s ducts should be oversized by 25% or more.

The final nail in the coffin is the fact that even on a two zone system the split is usually not 50/50. Quite often the larger floor (usually downstairs) accounts for perhaps sixty to seventy percent of the ducts. When a two zone system is trying to cool only the smaller floor you often end up asking the blower to push 100% of the air through 35% of the ducts. It isn’t going to happen. You’ll get drastically reduced airflow and the problems that are described in those three links above will manifest. Three and four zone systems are even worse. Trying to maintain anything remotely close to 100% airflow through one zone of a four zone system is a near impossibility.

The Solutions

Obviously the HVAC industry has done its best to come up with solutions to this airflow problem. Some of them work magnificently and some very poorly.

One of the more commonly employed solutions in the old days is no solution at all. Even now some contractors still install a bypass damper and duct. The bypass duct is a short duct that connects the output (supply air) of the equipment to the input (return air). When only one zone of a multi-zone system is operational, static pressure rises. The bypass damper will sense the rise in static pressure and open up the bypass duct. This has the effect of increasing airflow through the equipment, but it also causes return air temperatures to climb radically in winter and drop fast in summer. Normally the air entering the equipment is house temperature. But during the winter a bypass will circulate very hot air directly from the furnace’s output right back into the furnace. As a result the furnace may quickly overheat and shutdown. During summer it’ll circulate ice cold air directly into the cooling coil. The cooling coil will get much too cold and may ice up and/or shut down on its safety. Even when a bypass damper is installed according to instructions, the problems just described often exist.

Another solution is to implement a dump zone. A dump zone might be an area of the house that is generally unoccupied. When the zone system senses the need to increase airflow it will open the damper(s) to the dump zone. This less-than-great (but better than a bypass) solution would most logically be applied to systems with three zones or more. If applied to a two zone system, the dump zone would end up being the zone that’s not calling for conditioned air. With the dump zone open your two zone system would be operating as though it didn’t have zoning at all.

Yet another solution is to employ controlled pressure relief dampers. That’s a fancy way of saying that it turns the entire house into a dump zone. But it does so in a more controlled fashion. For example: If you had a three zone system where only zone A was calling, the controller would probably sense the need to increase airflow. Instead of opening up zone B fully and dumping all the excess there, a controlled pressure relief damper system would open both zones B and C a small amount. Studies have shown that most people don’t notice temperature swings of a degree or two. By dumping just a small amount to all the other zones, the theory is that we’ll get the extra airflow we want without anyone noticing. How it actually works out in practice is hard to predict.

An even better solution is to limit yourself to two fairly even zones and to install two-stage equipment. Two-stage furnaces and air conditioners run at around 2/3rds capacity in first stage. Trying to shove 2/3rds of the system’s capacity down half the ducts isn’t necessarily easy, but it’s a lot easier than trying to shove 100% down them. With properly sized ducts, such a setup can rival the efficiency and longevity of having two entirely separate HVAC systems. The cost may sting a little, but I strongly recommend installing two stage equipment on zoned ducts.

The ultimate solution to the airflow problems associated with zone systems may be to install a modulating furnace and a variable speed air conditioner. Modulating furnaces and variable speed ACs can often put out as little as 40% of their rated capacity. Some go even lower than that. I emphasize “may” because such equipment is extremely complex and expensive. I have it in my home, but that’s because I don’t pay retail. Thus far I have always discouraged customers from buying it. If you really want it that bad, just don’t ask how much. ;^)

The solutions above represent a mere fraction of what you’ll find out there. There are many hybrids of these solutions as well as completely unique ideas.

When The Solutions Fail

In my experience, most zone systems are pretty pathetic. Even so, many customers are happy with their pathetic zone systems. How can that be? The first answer is the fact that most HVAC equipment is fairly tough and takes the abuse for quite some time. The second answer is simply that customers don’t know any better. The lost energy efficiency isn’t usually noticed because customers have no way of knowing what their bill would have been otherwise.

The lost equipment longevity isn’t noticed because often the failures don’t manifest themselves until after the original purchaser of the zone system has moved out. Even if a failure does occur, most homeowners blame the equipment maker before they blame the contractor’s lousy design. The thought that the design of the zone system is causing the problem doesn’t occur to them. They assume it must be poor quality on the part of the manufacturer. In addition, problems with overheated heat exchangers and iced cooling coils can go unnoticed by the customer for a very long time. In the case of a furnace, the safeties will kick in repeatedly to save the day, at least until the safeties breaks. And in some climates the summers are mild enough and/or short enough that any frost or ice formation on the cooling coil quickly melts away between cycles. In addition, the zone system itself often has safeties built in that will short cycle the equipment and mask the airflow problem.

Does all of this mean that you must replace your entire heating and air conditioning system in order to have zone system? Of course not. But what it does mean is that we’re often left with choosing between very expensive upgrades or unpredictable results. Warranties as well as personal assurances that a system will run well assume proper airflow is maintained. If it isn’t then how can any tradesman predict the results? There is no manual that tells us how much we can hack a project and still have happy customers. It’s a reality that most homeowners don’t want to hear. And it puts the ethical tradesman at a disadvantage. There are plenty of contractors that are willing to slap a zone system onto existing ducts while giving you any assurance you ask for. They can get away with it for the reasons I gave in the paragraph above. Make yourself well informed so that they don’t.

What to do?

Get measured results. If you have a zone system installed then insist that at least 275 CFM per ton of airflow be maintained even when just the smallest zone is calling. That number is based on what a couple of OEM’s have been known to allow on their zone systems. Even that’s pushing the limits IMO. I would prefer that it be at least 300 CFM or more. If the contractor balks at that idea then ask him for documentation showing what minimum airflow is allowed for their proposed system. When all zones are calling then at least 350 CFM per ton must be maintained. If you live in a dry area then it should be 400 CFM per ton (as outlined above). The only way you’ll know that they’ve achieved that result is if they pull an airflow hood out and actually measure the airflow. Or, at the very least, they should measure TESP (the static pressure on the furnace) and plot the airflow on the blower data chart that comes with the furnace. Of course, that happens so rarely that making such a request may leave you hard pressed to find a contractor capable of doing so.

6) The sale of new equipment tends to be where a contractor makes his biggest profits. The profit margin on new equipment combined with the relatively low commitment of man hours to put it in makes for a nice P&L statement. The installation of new duct systems, on the other hand, is quite the opposite. New duct systems require comparatively inexpensive materials and a lot of man hours. When business is booming and staffing is short many contractors have little interest in looking at your ducts. Ironically it’s extraordinarily common for an existing HVAC system’s ducts to be its single greatest weakness. So if you get several bids and only one suggests that your ducts need improvement, don’t automatically assume he’s just trying to pad the bill. It may be that he’s the only one trying to do the job right.

7) The diffusers most homes have are the HVAC equivalent of Yugos. They’re made with one goal in mind: cheapness. So few contractors ask for high quality diffusers that most local wholesalers don’t even stock them. The cheap diffuser that Ace Hardware sells is the same thing most local HVAC wholesalers stock.

High quality diffusers will improve airflow, air mixing and do so more quietly. For example, a cheap Shoemaker 350 series 4×14 inch floor diffuser can handle up to 84 cubic feet of air per minute silently. But the high quality 375 series equivalent can handle up to 128 cubic feet of air per minute silently. That’s 52% more air at the same silent noise level. Furthermore, the cheap diffuser has to drop the air velocity down to 400 feet per minute to remain silent. The quality diffuser can deliver its higher volume silently at 500 feet per minute, which is the minimum velocity we want to ensure proper air mixing.

The example above was based on silent air delivery. But the quality 4×14 inch diffuser can deliver up to 230 cubic feet of air per minute at acceptable noise levels. Force 230 CFM of air through the cheap diffuser and the resulting noise would be more suited to a warehouse, not a home. At 230 CFM the air velocity from the high quality diffuser is 900 feet per minute. Such high velocity serves a very useful purpose. Proper air mixing is critical to even comfort. While it’s true that hot air rises, if you deliver that air at a high enough velocity it will tend to wrap around the room and create circular currents. Those circular currents will help to push hot air down and scoop cold air off the floor.

Indoor Air Quality

Bigger Filters Are Better

The overwhelming majority of air filters I see in central HVAC systems are way too small. That means too much air is being forced through them, which in turn means they don’t catch as much dirt as they’re supposed too. Additionally, undersized air filters don’t allow an HVAC system to heat and cool as it should. That’s because the HVAC system’s blower can’t push enough air through the undersized filter(s) or, if it can, the blower is working harder than it should. In short, undersized air filters result in a loss of both filtration efficiency and energy efficiency and may also shorten the HVAC equipment’s life.

Pleated Filters

A 1″ thick air filter, whether it’s fiberglass or pleated or washable or whatever, is the most common air filter I see in central HVAC systems. Most 1″ thick air filters are designed for air to move through them at 300 feet per minute face velocity or less. Using 300 FPM and 400 CFM per ton as our criteria, the following chart shows a rough approximation of how big an area an HVAC system’s 1″ thick filter(s) should have. If you’re in a humid climate, multiply by .875. If you’re buying new equipment, your contractors can tell you what tonnage they’re proposing. If you’re evaluating existing equipment, contact me and I’ll help you figure out what tonnage your system is.

system tonnage	filter area	example filters / area
2	387 in²	1x 16x25x1 / 400
2.5	484 in²	1x 20x25x1 / 500
3	580 in²	1x 20x30x1 / 600
3.5	677 in²	2x 14x24x1 / 672
4	773 in²	2x 20x20x1 / 800
5	967 in²	2x 20x25x1 / 1000

Compared to what’s shown above, most central HVAC systems with 1″ thick air filters have moderately to grossly undersized filters. For example, I’ve seen quite a few 5 ton systems with a single 20x25x1 filter sitting in its furnace or air handler. That’s a little over half of what’s recommended! In situations like that, it’s best to upgrade to thicker filters or an entirely different kind of filter.

The above chart really is a rough approximation. It doesn’t account for other potentially important design criteria. For example, an old rule of thumb says that a central HVAC system’s air filter should not create more than .1″ WC of restriction to airflow (AKA pressure drop). If you don’t know what that means, suffice it to say that the vast majority of filters create a lot more restriction than that. To achieve that .1″ WC goal, you might have to double the chart’s filter size recommendation. In most cases that’s just not practical or even possible.

Electronic Air Cleaners

Undersized filters can be a problem no matter what type of filter you’re talking about. For example, most electronic air filters claim to catch ninety-something percent of the dirt that passes through them. Installed on a 2.5 ton HVAC system, this electronic air filter (stats are on page 3) will do exactly that and without too much restriction. But put that same air filter on a 5 ton system and 95% filtration efficiency drops down to less than 70% and the restriction more than doubles. And that’s only when that air filter has just been cleaned. As that filter collects dirt, all of those numbers get worse.

The bottom line is that for truly effective filtration and proper system performance, most central HVAC systems need much bigger filters than they currently have.

2) Duct cleaners routinely imply or outright state that they can improve air quality and enhance an HVAC system’s performance. The cold hard truth is that they routinely do the exact opposite of both of those things. I spell that out in great detail here. (That link opens a new tab to my new educational website.)

3) So-called air purifiers are all the rage these days. Some say that by putting high intensity ultraviolet light in a duct the air passing through it will be purified. Others focus on its ability to sterilize a cooling coil.

UV light will destroy an airborne bug if exposed long enough. But simple math suggests that that will never happen in a duct system. Let’s be real generous and assume that the lamp of a UV purifier has a two foot effective radius. Air velocity through a duct commonly reaches 600 feet per minute. That’s 10 feet per second. That means that the airborne bug may be dosed for one-fifth of a second. Even under the unrealistic conditions suggested by one manufacturer, the airborne bug will still only get a half second of exposure. That is simply not enough time. The idea that a UV lamp can purify the air stream is totally unproven and defies common sense.

What a UV light can do is sterilize stationary surfaces near it. Since UV purifiers can be installed right above or below the cooling coil they have the potential of killing anything that’s growing there. The number of residential systems that can benefit from that in very dry climates are an extreme minority in my opinion. Things that grow need a consistent source of moisture. If you live in a very humid climate, as the YouTuber linked in the previous paragraph does, then it may be beneficial. Think of it as a potential solution to a real problem, if indeed you have identified a real problem. Install a UV purifier for no particular reason other than it seems like a good idea and you may well regret it for the reasons below.

Strong ultraviolet light will break down oil based products. Wire flex ducts, air filters, blower motors, and cooling coil drain pans all have materials that may be negatively affected by UV light. If those components are not protected then you stand a chance of damaging them in the long term. Plus, some UV lamps create ozone, which is bad for your lungs. In addition, few UV lamps last more than a couple years. Most OEMs recommend replacing them yearly.

If there is a known and specific need that UV can actually help with; and if you protect your equipment properly; and if you replace the bulbs at the recommended interval; and if you can tolerate the ozone, then it is possible to benefit from a UV purifier.

4) Duct sealing is uniquely effective in improving air quality. Ducts are typically located above or below the house in a crawl space. Considerable air leakage is not uncommon. If a furnace is pushing 1,600 cubic feet of air through the ducts every minute; and if the ducts leak at a rather pedestrian rate of 15%; then a house can lose 240 cubic feet of its air every minute to the crawlspaces. That’s the entire air volume of a 2,000 square foot house being flushed out every 67 minutes, which is three times the normal infiltration rate of an average home. That lost air has to be made up by entering the house through cracks in the walls, floor, and ceiling. An awful lot of those cracks lead to areas of the house where you’d never dream of breathing deeply. Yet that’s where much of a house’s air is coming from when the HVAC system runs.

If you elect to have duct sealing performed then choose your contractor carefully. Duct sealing can be labor intensive and difficult. The ducts’ insulation and vapor barrier must be partially or completely removed to apply duct mastic. It’s not uncommon for the mastic to splatter all over the place. And unfortunately it’s also not uncommon for the insulation to be reapplied in shoddy fashion. A simple visual inspection of the crawl space can verify your duct sealer’s professionalism. Taking before and after pictures is not a bad idea either. And the use of a Duct Blaster can measure how much your ducts are leaking after the sealing is performed.

5) A growing industry is that of home performance. Home performance specialists address the house as a system rather than just focusing on a specific trade. Whole house remediation might involve insulation upgrades. Upgrading attic insulation is often a slam dunk in terms of payback. Upgrading wall insulation may or may not be. And, surprisingly, upgrading the windows may makes little real financial sense despite what the propagandists have been advertising. Obviously insulation and windows don’t have huge impacts on air quality. But an area that does is moisture intrusion and air infiltration. Even a seemingly dry crawlspace can add gallons of water to a home’s air every day and degrade air quality. That moisture is carried in by means of air infiltration. As it relates to air quality, air infiltration is perhaps the most important problem a home performance specialist addresses.

A home can be thought of as a big chimney. Warm air rises out of the house through dozens and sometimes hundreds of little holes and cracks in the upper portion of the house. Cooler and sometimes polluted makeup air enters the house through holes and cracks in the lower portion of the house. As a result, a typical home might change out its air with the outdoors eight times a day or more. Old ceiling lights are some of the most notorious sources of leaks. Plumbing penetrations under sinks and baths are also big sources of unwanted air. Even electrical outlets will leak air into and out of a home. A blower door test can determine where these leaks are.

The home performance industry has its share of drawbacks just like any other. First, realize that even though some companies advertise themselves as home performance contractors, the California Contractors State License Board has no such classification. The same is probably true in the rest of the country. In some cases general contractors will perform the work. In doing so they dabble in many different trades. In other cases specialty contractors like insulation or HVAC contractors will do the same. Second, the price can be high. Bills of ten, twenty, and even thirty thousand that don’t include new HVAC components are not uncommon. Throw in new HVAC and a few other things and bills can top out at forty grand or more. Third, you’re still faced with the challenge of weeding out the shysters from the craftsmen. There are plenty of home performance contractors who’ll be more than happy to sell you duct cleaning and other bogus services if you let them.

6) If you’ve read the entire page up to this point then you should either be congratulated or your sanity should be questioned. Maybe both. ;^) Having done so you now realize that there’s a great many forces at work in a home that all intermingle. This is especially true of air quality.

Most homes naturally exchange air with the outside. That exchange rate may be a low as a few air changers per day to as many as eight air changes per day, sometimes more! On the flip side, some newer homes are so airtight that exchange rate becomes too low. In those homes they purposely bring outside air with a fan so that the inside air doesn’t become too contaminate with household chemicals. Whether it’s natural or mechanical infiltration, the “infiltration problem” is a 24/7 problem. With all of that outside air coming in, that $2,000 air cleaner just doesn’t accomplish much when it’s only filtering the air for an hour or two per day as the system heats or cools.

In response to the 24/7 infiltration problem, you may choose to run the furnace blower 24/7 so that your $2K air cleaner works 24/7. However, if the ducts leak badly, then running the blower will force outside air in at a faster rate than natural infiltration does. Will the filter remove airborne dirt faster than the leaky ducts brings it in? It’s anyone’s guess. Also, as you run the blower to clean the air, you may be running that air through ducts that are in a blazing hot attic or in an ice-cold crawl space. Most ducts don’t have that much insulation on them, so filtering the air 24/7 could increase your heating and cooling bills substantially. And to top it all off, all of these measures are made moot by homeowners who like to open the house up during nice weather.

Real improvements in air quality can be elusive. Much of the money spent to that end is more placebo than anything else. A real effort involves sealing leaky ducts, installing improved filtration, cleaning or replacing filters regularly, running the furnace blower 24/7 (or in the circulate mode that some thermostats have), sealing up major leaks in the home’s shell, and keeping the home closed up all the time. If you’re willing to do all of that then you may indeed make a substantial improvement to the home’s air quality. Anything less and you may fall into the placebo category.