Willis Carrier invented the first air conditioning system in 1902—but not to cool the air. Carrier’s system was devised to dehumidify the air in a printing plant so the paper would no longer curl in the moist air. Dehumidification is as critical to comfort as is temperature.
How Does Air Conditioning Work?
Air conditioners use fans to move interior air over a cold surface, usually comprised of grids or “coils” of copper or aluminum tubing. As the coils chill the air, its moisture condenses on the coils and is drained away.
The coils are chilled by a refrigerant that’s pumped by a compressor through insulated lines to a condenser located outside. Refrigerants are liquids with a low boiling point, that is, the temperature at which the refrigerant turns from a liquid to a gas. That phase change from liquid to gas cools the refrigerant. In air conditioners the phase change happens at the coils, cooling them and removing heat from the inside air.
After leaving the coils, the refrigerant moves to the condenser where the pressure from the compressor changes it from a gas back to a liquid.
This second phase change makes the refrigerant give up the heat it gained from the inside air. After passing through the condenser, the refrigerant once again makes its way to the coil. Heat pumps provide winter heat in exactly the same way as air conditioners remove it in the summer, except the refrigerant flow is reversed.
Basic Air Conditioning Systems
There are several types of air conditioning units to choose from, depending on the specific needs of your home.
Central Air
Most new homes are built with central air conditioning. The same ducts used to heat the house in the winter carry cool air in the summer. The cooling coils might be near an air handler (large fans in the ductwork that move air) in the attic or in the basement, or in both places.
If you’re having a new system installed or an old condenser replaced, insist that your contractor do a “Manual J” calculation. A Manual-J calculation evaluates the size of the house, its insulation, solar heat gain from the windows, and air leakage. It’s the only way to be sure your condenser is properly sized. Too small a condenser won’t cool adequately, and too large a one won’t dehumidify well.
Mini-Splits
A mini-split is a small central-air system. Many also function as heat pumps. Depending on the cooling needs of the house, a ductless mini-split will have one or more wall-mounted heads that circulate air. Ducted mini-split systems have small sections of duct to move air between rooms.
With their reputation for high efficiency, mini-splits have become a popular way to retrofit air conditioning in older houses, and as the prime heat and cooling source in well-insulated new homes. As with central air, be sure your contractor does a Manual J calculation to properly size a mini-split.
PTAC Systems
Packaged Terminal Air Conditioning systems are familiar to anyone who’s spent a night in a motel. PTACs fit in an opening in a wall and often function as heat pumps as well. PTACs are popular as moderately priced ways to condition smaller homes.
Window AC Units
Held in place by the upper sash of a double-hung window, the main virtue of a window air conditioner is that it’s cheap to buy. In heating climates, you’ll want to take them in every winter, otherwise, they can leak a tremendous amount of air.
Similar to window units are through-the-wall air conditioners. The main difference is that these units fit inside a sleeve that’s installed in the wall and they stay in place year-round.
Maintaining Your Air Conditioning Unit
The chief maintenance on most air conditioners is to keep the air filters clean so the airflow is not impeded. Central units rely on the same replaceable filters as does the heating system. The other systems’ filters are usually cleanable with a vacuum.
Don’t forget to check the outside part of the air conditioner, as well. Make sure it’s kept clear of leaves and other debris, and prune shrubs well back.
Efficiency and Capacity of Your AC Unit
The capacity of all but the smallest air conditioners is measured in tons. A one-ton air conditioner has the same cooling capacity as one ton of ice. Sometimes, BTUs are used to describe capacity as well. For cooling purposes, 12,000 BTUs are equal to one ton.
It takes electricity to run the compressor and fans that make an air conditioner work. The fewer watts of electricity used to create a ton of cooling, the more efficient the air conditioner. This is referred to as the Seasonal Energy Efficiency Rating, or SEER. The higher the SEER, the more efficient the unit.
New standards will require minimum SEERs of 14 in northern states and 15 in southern states for air conditioners with outputs smaller than 45,000 BTU/hour. For larger units, the standards will be slightly lower in the South, at 14.5 BTU/hour. If the unit is also a heat pump, that SEER will have to be at least 15 everywhere. The most efficient units available today have SEERs of 26 or higher.