How do Central AC Systems work?

"Central Air" is a traditional ducted air conditioning system made up of several components.

Outdoor Unit

The outdoor unit or "condenser" is mounted on a pad outside the building. The "condenser" houses the compressor, the condenser fan and the condenser coil. The compressor is the heart of the system as it pumps refrigerant. The condenser coil rejects heat from the refrigerant. The condenser fan helps the coil reject heat to the outside ambient air.

Indoor Unit

The indoor unit or "air handler" is mounted in the building, usually in the attic. Air handlers can also be located in the basement, in a crawl space, in a closet, or in a mechanical room. The air handler houses the evaporator coil and the indoor fan as well as some type of expansion device. The evaporator coil removes heat and humidity as the fan draws air across it. The air is pulled through the return duct. The expansion device (TXV, Orifice,etc.) meters the amount of refrigerant sent to the evaporator coil from the condenser which is being pumped by the compressor. The heat that is absorbed by the evaporator is rejected outside through the condenser. The humidity in the air collects as water droplets on the evaporator coil and drip down into a drain pan. The water is piped to an external drain or to a condensate pump and pumped to a drain.

Line Set

The "line set" is the refrigerant piping running from the condenser to the air handler. It is made of copper and usually insulated with black or white foam. The line set may run along the building in a "line set cover" or "line duct" which is a plastic cover that improves the appearance of the piping. These are generally white or beige and look similar to a gutter leader. They can be ordered in many different colors and also painted to match existing siding and trim.


The ductwork in the building consists of several items. The supply grills in each conditioned room, sometimes called diffusers or outlets are where the conditioned air is supplied to the room. The return grill or "return" is the large grill usually at the top of the stairs or centrally located in the ceiling. The air completes it's cycle in the room and is returned to the air handler where it passes over the evaporator coil. returns can be mounted in different configurations based on the location of the supply diffusers and other factors. Some systems use two or more smaller returns instead of one large one. In a building that has "forced air" heating, the air handler will have some type of heating coil. This can be a hot water coil remotely supplied by a boiler or hot water heater. It can also be an electric coil. Forced hot air can also be configured as a furnace with an evaporator coil mounted to it. In this case, the furnace houses the supply fan.

"Central Air" systems are great. They provide excellent cooling and humidity reduction. If ductwork can be installed in the building easily and the building is divided into zones, then "Central Air" is a perfect application.

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