Heating & Air Conditioning Installation
The Definitive Guide for Homeowners
Buying a new furnace, air conditioner or ducts for your home? The well-worn practice of getting three bids, asking for references and checking 3B ratings is not enough. It never was. Most botched heating and air conditioning systems were installed by contractors that would pass those tests. Most of those same contractors would not pass the test of an informed customer.
Be the informed customer. Read this page. You need not understand all or even most of what’s written here. You need only to familiarize yourself with the concepts. 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 page. While all contractors engage in some marketing, the craftsman’s focus will always be his craft.
Each short description’s number links to a more detailed description below.
1) Good contractors have a clean contractor’s license, an A+ rating on 3B and a five star rating on Yowl.
A lot of bad contractors do too. Check those traditional sources of information. Then keep checking.
2) There’s no proof that one brand is better than another. There’s ample proof that some contractors are better than others. Find a good contractor and the brand will take care of itself.
3) To the surprise of many homeowners and (apparently) some contractors, a building permit is required. A home improvement contract is too. They protect you in a number of ways.
4) Rules of thumb just don’t cut it. The best way to determine equipment and duct size is with Manual J and Manual D calculations.
5) The fact that a new system blows hot and cold air doesn’t mean it’s delivering the capacity and efficiency you paid for. Ask for measured results.
1) The furnace’s most important role is that of blower. Since most systems don’t blow enough air, oversizing the blower is an appropriate thing to do in some homes.
2) Too much heating capacity can cook both you and the furnace. Be weary of having too high a BTU input. A multi-stage furnace can help in this regard.
3) Fires have been started by flexible gas connectors that passed through the furnace sidewall. Don’t let hurried installers do that to you.
4) The exhaust pipe on 80% efficient furnaces gets very hot. Proper clearances must be maintained.
5) The money saved by upgrading to a 90% or higher efficiency furnace isn’t what some claim, especially in areas with mild winters. Consider the pros & cons carefully.
1) In a perverse sort of balancing act, most air conditioners are oversized but underdeliver. Learn why so you can make sure it doesn’t happen to you.
2) 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.
3) TXV’s are best for any type of climate. TXV’s can improve capacity, efficiency and longevity. In dry climates they should be combined with oversized coils.
4) Air conditioners can lose 5% or more of their capacity to undersized refrigerant lines. Proper line size is especially important on marginally sized systems.
5) 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.
6) A huge percentage of air conditioners get scale (burned metal flakes) in their lines the day they’re installed. Learn why and prevent it.
7) Checking Freon pressure is not enough. After installation the air conditioner’s refrigerant charge should be fine tuned by the superheat and/or subcooling methods.
8) Don’t be fooled by bogus formulas. The payback on equipment with super high SEER ratings is sometimes never realized. Demand a real calculation.
1) 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.
2) Return ducts are routinely undersized. In general they represent the single greatest potential for improving the airflow of existing duct systems.
3) Most homes waste energy and their comfort is uneven because of pressurization issues. Extra return ducts and/or jumper ducts can be the solution.
4) Severe air leakage in duct systems is common. Duct sealing (NOT duct cleaning) may represent a good opportunity for improving energy efficiency.
5) 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.
6) 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.
7) The cheap diffusers (vents, grilles, registers) installed by most contractors are poor performers. The difference between high and low quality diffusers can be enormous.
1) Most high quality filters don’t perform as they should. Either they catch less than was advertised, impede system airflow dramatically, or both. Learn why.
2) Duct cleaning is rarely useful and often fraudulent. It not only has virtually no potential to improve air quality, but it will likely worsen it.
3) 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.
4) Duct sealing is one of the few effective methods of improving air quality. But it’s a messy business worth keeping your eye on.
5) Addressing whole house performance is a potentially expensive but often very effective way to improve air quality.
6) 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.
Finding and Signing a Contractor
- Any California contractor performing an HVAC repair or installation worth $500 or more must be licensed by the Contractor’s State License Board. The absence of a license is, quite obviously, a red flag. However, the presence of a license is certainly not a green flag. As this story illustrates, getting a license is a lot easier than you might think. Check the license. And then keep checking.
- All licensed contractors are bonded. While that may sound good, a bond is nothing more than a surprisingly limited form of insurance. The bond company doesn’t do a background check on the contractor. They don’t do much of anything. The bond is, in my opinion, almost worthless. The state seems to agree because they forbid contractors from advertising the fact that they’re bonded.
- California doesn’t require contractors to have liability insurance. It’s up to you to require it. Check to make sure your contractor has liability insurance. And then keep checking. Any contractor can get insurance if he can afford the premiums.
- As the state’s 10 Tips pamphlet suggests, check for references. And then keep checking. I can’t tell you how many times I’ve heard horror stories that started with “A friend recommended this guy to me.” If a friend’s recommendation can go terribly wrong, the recommendations of strangers can too.
- An A+ rating with 3B is no different than a clean license or good referrals. In all cases, an absence of the negative is not proof of the positive. The same is true of Yowl reviews and CZ Certification. Check those things, of course. And then, as I like to say, keep checking!
It’s not my intent to completely discredit the traditional checks, only to put them in proper perspective. Think of yourself as an employer and the contractor as a job applicant. The traditional checks are the contractor’s application, so to speak. It’s no surprise that the application is heavily embellished. You have to check the application against reality with a tough interview. A tough interview starts with informed questions. Read enough of this page and you’ll become a tough, informed interviewer.
There is no proof that one brand is better 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. That process would need to be repeated every time a manufacturer redesigns their equipment. It would cost those sponsoring the study millions. And the results would be of no use to you because the sponsor’s installing contractor would not be your installing contractor. As you’ll learn by reading this page, the installing contractor is far more important than the brand that’s installed.
Consumer Reports gives an underwhelming nod to the importance of the contractor. Meanwhile, their frequency of repair data gives an unwitting nod to the contractor’s importance: Trane and American Standard are made by the same company. Their equipment is virtually identical. Yet Trane air conditioners have a 20% higher frequency of repair than American Standard air conditioners. Why the difference? Trane got into new construction while AS did not. The typical tract home builder cuts as many corners as possible, so many of those Trane AC’s were installed poorly. A higher failure rate was the inevitable result. Similarly, Carrier furnaces have a 25% higher frequency of repair than the Bryant furnaces that Carrier also makes. These statistical curiosities point to this one certainty: Choosing the right contractor is far more important than choosing the brand.
If those statistics haven’t convinced you, consider this: Most of the parts in the brand you believe is superior are probably the same parts in the brand you believe is inferior. Compressors are a great example. The compressor pumps refrigerant (Freon). It’s the most complex and expensive part of the condenser (outdoor portion of the air conditioner). And most compressors come from a company called Copeland. Carrier, Lennox, Rheem, Goodman and many other air conditioner manufacturers get their compressors from Copeland. Circuit boards are another example. Every furnace has one. And almost every furnace manufacturer gets theirs from White Rodgers or Honeywell. You name the part and, with occasional exception, there’s probably just a few third party OEMs supplying the many manufacturers.
Things aren’t quite as simple as I’m suggesting, but almost. The bottom line is this: Find a good contractor. Install the brand he recommends. You can read more about brand here.
The contractor is required to draw up a home improvement contract. He’s also required to pull a building permit with the city or county building department. Though many contractors act like they’re optional, they are the law. The legal obligation is as much yours and it is the contractor’s. The permit in particular serves a few useful purposes…
- It helps to ensure that both your homeowner’s insurance and the contractor’s liability insurance are in force. A botched installation can cause significant damage. If it was an illegal installation, the insurance companies might have legal right to refuse coverage.
- A permit also has the potential of protecting you from sloppy work. Installers who know their work is going to be inspected tend to do a better job.
- A permit now means less headaches later when you sell your home. It’s a home inspector’s job to find code violations. I’ve seen working HVAC systems with a dozen code violations. The homeowners had no idea because permits were not pulled and because the systems seemed to work OK despite the 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. You’re likely to pay for those corrections yourself.
As for the home improvement contract, I’ve seen a lot of what other 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-day right to cancel, the maximum down payment a contractor can ask for and much more.
Your system should be designed using Manual J and Manual D calculations. Those manuals are published by the trade organization ACCA. Most contractors that perform Manual J & D calculations use software based on the manuals. Unfortunately, most contractors don’t perform those calculations at all.
- A Manual J calculation is the gold standard in determining what size your furnace and air conditioner should be. It can be performed as a whole house calculation in order to choose the right size equipment for the home. Or it can be performed as a room by room calculation as the first step in choosing the right size ducts for each room. Don’t be fooled by short forms or any other goofball method a contractor may use. And don’t let your contractor add in fudge factors. Manual J has been shown to oversize equipment by some amount already.
- The results of a room by room Manual J calculation are entered into a Manual D calculation. A Manual D calculation determines what size your ducts need to be to deliver the air each room needs. Have you ever lived in a house that didn’t get enough air to the farthest bedroom? If a true Manual D calculation had been performed, and if the ducts had been installed and balanced accordingly, then that would not have happened.
There are other manuals. There’s Manual S for equipment selection. Manual T is for diffuser selection. Manual Zr covers zoning. Don’t get hung up on the manuals. There aren’t many contractors well versed in all of them anyway. The point is, there’s a science to this profession. Your contractor may not be able to blind you with that science, but he better shine just a little.
It’s an amazing fact of HVAC life that most new air conditioners don’t deliver their rated cooling capacity. Most homeowners assume that if an air conditioner is blowing cold air, it must be delivering the amount of cold air it’s rated to deliver. A few do and some are close, but most fall short by a good amount. I suggest adding a line to the contract that says the air conditioner’s output will be measured before final payment is made. Measuring overall output isn’t hard. Good contractors will embrace a request for measured results. The bad ones might suddenly lose interest in the job.
If you’re getting new ducts, you might want to add another line to the contract that goes something like this: “Each room’s airflow will be balanced to within 10% of the Manual D calculation.” I’m not a lawyer and this is not legal advice. I’m just saying that if you don’t put it in writing prior to the start of work, it probably won’t happen. Most homes have some degree of imbalanced airflow. Most of the time the imbalance stems from poor design and installation practices.
1) The most important role a furnace plays is that of blower. The furnace’s blower blows the air through the ducts for both the heating and air conditioning modes. Some people believe that the fan outside (usually on top of the condenser) is the fan moving the air inside. But, in fact, the fan outside has no direct connection to the air inside. It’s the furnace’s blower that blows the air inside in both heating and cooling modes.
The air conditioning mode requires more airflow than the heating mode. In a dry climate, it’s preferable to blow 450 to 500 cubic feet of air per minute for every ton (12,000 BTUs) of air conditioning capacity. Most systems deliver nothing close to that. In humid climates something in the ballpark of 350 CFM per ton is more appropriate. Even that goal is often not reached. This rampant airflow problem can be blamed in large part on improper duct design and installation. It can also be blamed on restrictive high efficiency furnace filters. And yet another factor is that manufacturers design many of their furnace blowers based on unrealistic assumptions. A furnace supposedly rated for 4 tons of airflow (1,600 CFM) rarely delivers that much through real world ducts. As such most furnaces should have blowers rated one-half to one full ton higher than their air conditioners. For example, if forced to use existing ducts I usually match a 3 ton air conditioner to a furnace with a 4 ton blower. The use of variable speed blower instead of a standard blower usually has a similar effect. Variable speed blowers are smart fans that calculate actual airflow. They’ll speed up to compensate for restrictive ducts up to a point.Be very careful. Oversized and/or VS blowers can’t always cure airflow problems. Even when they can help to increase overall airflow it’s not the preferred solution. Properly sized ducts are always your best bet. Plus, in most cases extra airflow will do nothing to fix uneven comfort. If that far bedroom doesn’t get enough air but the rest of the house does then you need a properly sized duct to that bedroom, not a more powerful blower.
2) The second most important role of a furnace is the obvious one of heating. One key here is to not oversize. Oversized furnaces will deliver too hot an air and too much of it. As a result your furnace may short cycle and the hot air will tend to hug the ceiling (stratify). An oversized furnace tends to wear out more quickly as well.
The west coast is sometimes held hostage to an east coast engineering mentality. Cold east coast and Midwest climates need big furnaces to handle big heating loads. On the other hand, the southwest has fairly hot and dry summers with relatively mild winters. As such they need furnaces that can blow a lot of air in the summer while not blasting too much heat in the winter. Yet (for example) you simply can’t buy certain 5 ton furnaces with a BTU rating of less than 100,000. There have been occasions where I could have used a 5 ton blower on a furnace with a 60,000 BTU input. But at the time no such furnace existed.
The answer to the southwest’s high airflow / low heat predicament can be the multi-stage furnace. The highest stage of a multi-stage furnace is “full blast” just like a regular furnace. But the first stage(s) can start as low as 40% of the furnace’s rated input. The lower output can result in more even heating and better comfort. It’s quieter as well. Just don’t be misled. In terms of energy efficiency a multi-stage furnace has no advantage over a single stage furnace.
If you happen to live in an area with cold winters then furnace oversizing may not be as big an issue. But don’t let your guard down too much. Some hack installers have dazzled with their ability to screw things up. Oversized furnaces are installed in all climates.
3) Make sure that the furnace’s flexible gas connector does not penetrate the furnace’s sidewall. The flexible connector should sit outside the furnace and hard pipe should penetrate the sidewall. In some jurisdictions they’re beginning to require excess gas shutoff valves right at the appliance as well. It may seem like a small issue compared to the larger concepts explored on this page. But locally at least this problem is rampant. Since on rare occasion it can lead to death and destruction, it’s worth a mention.
4) Also make sure that your furnace vent pipe (exhaust pipe) has 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. Just like the flex connector issue, it’s a relatively minor thing to deal with that has potentially dangerous results if it is not.
5) The decision to buy an 80% efficient furnace versus buying a 90% (or higher) efficient furnace should be considered carefully. 90+ furnaces are more complex, are more expensive to work on and are more likely to fail than their 80+ counterparts. The ten to fifteen percent savings on your gas bill may never actually pay you back the added capital, maintenance and repair costs. If your salesman is pitching energy savings then take a look at this related note below.
Of course there are reasons to buy a 90+ beyond that of energy savings. 90+ furnaces tend to be safer, have better warranties and run quieter than their 80+ counterparts. California Title 24 rules give some Californians added incentive to purchase 90+ furnaces in certain climate zones.
1) In a perverse sort of balancing act, a huge percentage of air conditioners 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. That doesn’t mean you’re actually getting 50% too much. Many air conditioners don’t deliver anything near what they’re rated for.
There are many reasons for this. Some of them are covered below. But one very important one 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% of your air conditioner’s capacity is spent on latent heat removal (moisture removal).
In a 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. 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 is the use of a TXV (thermostatic expansion valves) as the metering device. An air conditioning system’s metering device controls the flow of refrigerant to the cooling coil. Though not nearly as common as before, some cheap cooling coils have fixed metering devices that feed refrigerant to the coil at a fairly constant rate. In contrast the TXV varies the flow of refrigerant depending on heat load. The result is better energy efficiency, a potential increase in cooling capacity and potentially longer compressor life. A TXV can be added to most coils. You can see a TXV next to the alternative here.
High efficiency coils typically come with a TXV already installed. High efficiency coils are usually bigger than their cheaper counterparts. Their large size allows for better airflow. And because of their size the refrigerant is spread out over a larger area allowing for more effective cooling. In dry climates we can amplify the effect by oversizing the high efficiency coil. That 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, it’s the best way to design a system for a dry climate.
High efficiency and oversized coils may cost less than $100 more (wholesale) than cheap coils. If you’re in a dry climate, don’t think about it. Just do it. 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, 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.
For example, the recommended suction line (the Freon’s return line back to the compressor) size for most 5 ton R-22 air conditioners is 1 1/8 inches (outside diameter). Yet most five ton systems have 7/8 inch suction lines. The larger suction line is more expensive and much more difficult to work with. On a twenty-five foot run the 7/8 inch line results in a loss of only 1.4% of cooling capacity. But on a hundred foot run the 7/8 inch line results in a loss of 5.5% of cooling capacity. (Those numbers are taken from one OEM’s refrigerant piping guide.)
It may seem unlikely that your furnace and air conditioner are 100 feet apart. But we’re actually talking about total equivalent length or TEL. For example, every short radius 90 degree copper fitting used in your refrigerant line has a TEL of about 8 feet. A 60 foot refrigerant line with 5 of those 90 degree fittings would have the same resistance to refrigerant flow as a perfectly straight 100 foot line. Given the turns and bends that most refrigerant lines take, a 100 foot (equivalent) line is not as uncommon as you might think.
One major OEM recommends the following suction line sizes for R-410A and R-22 systems. The first number is R-410A (Puron), the second is R-22 (Freon) – both in outside diameter.
-For a 1 ton system it’s 1/2 and 1/2.
-For 1.5 ton systems it’s 1/2 and 5/8.
-For 2 and 2.5 ton systems it’s 5/8 and 3/4.
-For 3 and 3.5 ton units it’s 3/4 and 7/8.
-For 4 and 5 ton units it’s 7/8 and 1 1/8 inches.
Smaller sizes are allowed, though as outlined above it results in reduced capacity. Larger sizes are recommended on especially long runs. Since R-410A is more dense, the recommended suction line sizes are smaller. Other manufacturers have similar recommendations.
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 but seldom used. Trade schools teach the use of micron gauges as a matter of course. Doing so can prevent the formation of acids. Yet most installers don’t even own one.
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 or the manufacturer’s otherwise specified method.
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) Energy efficiency is a hot topic that’s steeped heavily in propaganda. Since 1992 the minimum efficiency for newly manufactured air conditioners was 10 SEER (seasonal energy efficiency ratio). The minimum efficiency as of 1/23/06 is 13 SEER. In 2014 higher SEER ratings will take effect for certain regions of the country. For mild climates like ours in the SF Bay area it doesn’t often make sense to upgrade to anything higher than a 13 SEER model. Even in more extreme climates it’s not a given that higher SEER equipment is worth the extra cost.
Some salesmen use formulas based on heating degree days and/or cooling hours to determine your energy savings and convince you that a high SEER or high AFUE model makes 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 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.
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 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 doesn’t work well for this purpose. Many tapes will dry out over time and loosen. Tape is only to be used for making connections airtight.
Old fashioned duct tape is no longer considered proper for air sealing. There is a new type of tape that looks like packing tape that is specifically designed for ducts. And even this new tape is only useful for certain connections. Little gaps and other types of air leaks should be sealed with a duct mastic. Big gaps should first be covered with secured sheet metal before being sealed.
Any exposed metal of the duct system that resides in unconditioned space must be insulated. In some cases the return duct is exempted from this requirement.
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 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 – an important function, but not one that 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 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.
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.
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.
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.
The most commonly employed solution is not much of a solution at all. Most contractors will install a bypass damper. The bypass 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 the static pressure in the ducts will increase. A bypass damper will sense the increase in static pressure and open up. This has the marginally beneficial effect of restoring some of the lost 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 you’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 sort of solution would most logically be applied to systems with three zones or more. If applied to a two zone system your 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 inevitably 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.
The absolute best solution is to limit yourself to two fairly even zones and to install multi-stage equipment. Multi-stage furnaces are rather common place these days. The price premium is not huge. On the other hand multi-stage air conditioners are prohibitively expensive for a lot of people. But if you can afford to install both a multi-stage furnace and air conditioner with properly sized ducts then it becomes a very elegant solution. Airflow problems become much easier to deal with because now your furnace and air conditioner don’t require 100% airflow in first (low) stage. Such a setup can rival the efficiency and longevity of having two entirely separate HVAC systems.
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. For example: The advent of variable speed motors presents us with yet another possible solution. A variable speed blower will speed up and slow down in response to changes in static pressure. But its ability to do so is limited. Slapping in a zone system with a variable speed blower without upgrading the ducts is usually a bad idea. A variable speed blower will most likely try and fail to maintain 100% airflow when only one zone calls. But if the ducts are made big enough then 100% airflow could be maintained at all times without the need for multi-staged equipment, dump zones or bypass dampers. The first challenge in implementing this solution is being able to install big enough ducts. In some cases they’d have to be so big that they wouldn’t fit. The second challenge is picking the right diffusers (vents, grilles, registers). If the diffusers are sized to maintain proper throw at all times then they may be very noisy some of the time.
When The Solutions Fail
With the most commonly utilized method of dealing with poor airflow in zone systems also being the worst method, you may wonder how it is that many customers are happy with their zone systems. The first answer is the fact that the 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 has moved out. Even if a failure does occur most homeowners blame the equipment maker before they blame the contractor. 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 hack 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 is pushing the limits. If the contractor balks then ask him for documentation showing that anything less than that is allowed. When all zones are calling then at least 350 CFM per ton must be maintained. If you live in a dry area it should be more than that (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. 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 my local HVAC 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 our 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
Indoor Air Quality or IAQ has become the darling of HVAC business gurus. Their ship has come in, so to speak. As a result there is an abundance of both excellent products and expensive snake oil vying for your hard earned dollar. Read carefully and make an informed decision.
1) The most common failure of air filtration design has to do with the size of the filters. HVAC system designers tend to specify filters that are much too small. Trying to jam too much air through a filter causes it to become an air restriction which results in less airflow. We’ve covered the importance of maintaining good airflow above. All filter manufacturers have maximum allowed velocities beyond which their filters and/or the HVAC system won’t work correctly. In the case of this standard one inch filter you can see a few paragraphs down that its rated ability to catch dirt is based on a relatively slow 300 feet per minute. Bump that velocity up to a more common 500 FPM and they don’t even publish the results.
Most electronic air cleaners and similar whiz bang filters have an added challenge. Not only do some of them become a restriction when too much air is jammed through them, but they also catch less dirt. Most electronic air cleaners will advertise that they catch ninety-something percent of the dirt that passes through them. Installed on a 2.5 ton air conditioner this electronic air cleaner (page 3) will do exactly that. But put that same air cleaner on a 5 ton air conditioner and 95% arrestance drops down to less than 70%. And that’s only when the air cleaner has just been cleaned. As the plates collect dirt the arrestance drops further.
The bottom line is that for truly effective filtration and proper system performance the air velocity through most filters should be around 300 feet per minute or less. To achieve that might mean installing more than one filter. And to maintain that you must replace or clean the filters regularly.
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.
3) Ultraviolet 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 will 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 humid climate where the cooling coil stays wet for weeks at a time then it may be beneficial. Think of it as a potential solution to a known problem, if there is a known problem such as dirty sock syndrome. 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 are negatively affected by UV light. If those components are not meticulously protected then you stand a very good chance of damaging them in the long term. Plus, many UV lamps create ozone. Some have promoted ozone as beneficial, but it is in fact pollution. 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 often difficult. The duct’s 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 reinstalled 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 sometimes 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 one 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 great sources of unwanted air. Even electrical outlets will leak considerable 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 grand 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 twenty 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 be congratulated. Either that or your sanity should be questioned. ;^) 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.
A home will naturally change out its air with the outdoors as much as eight times per day or more. This process of infiltration goes on 24 hours a day. That $1,500 electronic air cleaner isn’t doing much when you consider that your furnace may run for only one or two hours per day, even less during mild weather. The answer some have come up with is to run the furnace blower 24 hours a day. Variable speed blowers in particular cost little to run and are very quiet. But if your ducts are leaking badly then running the blower forces outside air in at a faster rate than natural infiltration does. Will the filter remove particles faster than the artificially increased infiltration brings them in? It’s anyone’s guess. 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 house shell and keeping the house closed up all the time. If you’re willing to do all of that then indeed you may improve air quality in the home. Anything less and you can easily fall into the placebo category.