How Central Heating and Air Conditioning Works

Preface: I realize this page needs pictures. Getting the written part down was my first priority. My mechanically un-inclined friend says the following is understandable. I hope she’s right!

The most common type of central heating and air conditioning system found in SF Bay Area homes is that of split systems. A split system is comprised of three major pieces of equipment: the furnace, the evaporator coil (often called the cooling coil), and the condenser. The furnace and cooling coil are attached to each other. They’re usually located in a garage, attic, crawlspace or closet. The condenser, which is the thing that most people call an air conditioner, sits on the ground outside or sometimes on the roof. In other words, part of a split system is located in the house and part of it is located outside. That’s why it’s said to be split.

Simply speaking, the furnace has two compartments: the blower compartment and the heating compartment. The blower that’s in the furnace’s blower compartment works like any household fan: It sucks air from one side and pushes it out the other. Likewise, the furnace’s blower sucks air from the house through a large duct that’s connected to a return air grille (often called the return vent). The return vent is usually located in a hallway or other central location. Sometimes there are two return vents and, less commonly, there are several. The furnace’s blower then pushes your home’s air through the furnace’s heating compartment.

After exiting the furnace’s heating compartment, the air continues on through the cooling coil (the cooling compartment, if you will) that’s attached to the furnace. That may seem confusing at first, but the heating compartment is hot only in the heating mode and the cooling coil is cold only in the air conditioning mode. Think of it as a three compartment system. The blower compartment is active any time the system is on, while the heating and cooling compartments activate as necessary. After exiting the cooling coil, the air continues on through the supply ducts and then finally to the diffusers (often called supply vents) located in almost every room of the house.

In the heating mode, the gas company provides the fuel that makes the furnace’s heating compartment hot. In the air conditioning mode, it’s the condenser sitting outside that provides the refrigerant (often called Freon) that makes the cooling coil cold. Even though it’s quite often (and quite incorrectly) called an air conditioner, the condenser doesn’t cool your home’s air. It cools the Freon. The condenser is like your car’s radiator. The radiator cools the water that in turn cools the engine. Likewise, the condenser cools the refrigerant that in turn makes the cooling coil cold. It’s cooling coil that makes your home’s air cold.

In summary, the blower (in the furnace’s blower compartment) sucks air from the house through one, two or sometimes a few return vents. The blower then pushes that air through both the furnace’s heating compartment and the cooling coil (the cooling compartment, so to speak) regardless of the mode. In the heating mode, the heating compartment gets hot while the cooling coil remains dormant. In the air conditioning mode, the cooling coil gets cold while the heating compartment remains dormant. The hot or cold air then continues on to the many supply vents strewn through the house.