Capacity

Air conditioner sizing is crucial

Contractors and government agencies say the biggest mistake consumers make when buying air conditioners is getting a unit with too much capacity. An unscrupulous contractor will sell you an air conditioner with too much capacity because it costs more, but the bigger disadvantage occurs once you own it. It will frequently cycle on and off. This is inefficient and expensive. Veteran contractor Fred Weldin adds, "It's much better for comfort and for the equipment to have the compressor operating relatively continuously in the worst weather."

Experts say a "Manual J" calculation is essential for determining the appropriate capacity. Manual J, published by the Air Conditioning Contractors of America (ACCA), is a method for calculating the design cooling load and correct system size. Do not accept bids over the phone or the Internet. A contractor must measure rooms, windows and insulation levels to perform a Manual J load calculation. A contractor should review the load calculation with you. Note that building codes in many jurisdictions require Manual J calculations.

Capacity is measured in BTUs (British Thermal Units). The specification is most commonly expressed in tons. Sometimes you'll see BTUs per hour (a ton equals 12,000 BTUs per hour). A range of two to five tons is typical for a house.