Sizing the unit. Using the heat-gain calculation, your contractor will recommend an air conditioner size, expressed either in tonnage or Btu per hour (Btu/h). One ton equals 12,000 Btu per hour. Why is getting the proper size unit so important? An undersize unit won't be able to cool rooms down completely on the hottest days and will cost more to operate because it has to run longer than a correctly sized unit. An oversize compressor also costs more to operate because it simply takes more electricity to run a bigger unit. And an oversize unit doesn't lower humidity effectively. That's because it cools the air so quickly that it shuts off before it has a chance to circulate the proper volume of air past the coils to extract the necessary moisture. The result is a room that doesn't seem as cool as the temperature indicates. In fact, the room can feel clammy and damp.

Unit efficiency. After determining the size of the unit, your contractor should talk to you about efficiency, which is expressed by the seasonal energy-efficiency rating (SEER). The SEER rates how many Btu an air conditioner will remove for each watt of electricity it consumes. The higher the SEER, the less it costs to operate. Federal law requires that new A/C units have a SEER of at least 13. These units have lower operating costs, tend to be higher quality, have more safety features, better sound shields and lower voltage requirements. In general, the higher the SEER, the higher the initial cost for the unit.

Unit types. Next, you must decide what type of system to buy. There are two kinds. A "package system" gangs the condenser, which cools the refrigerant and exhausts warm air, with the fan-and-coil system, which cools and blows the air. The ducting connects directly to the unit. This is essentially a large wall air conditioner with ducts. But package systems are rare.

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