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With air conditioners, the term “size” has nothing to do with a unit’s physical dimensions but, rather, its ability to produce cooled air. Both room-size and whole-house air conditioners are manufactured in a variety of capacities, so it’s helpful to understand how they are measured.
An air conditioner’s ability to produce cooled air is measured in BTUs (British Thermal Units) per hour and in tons. A BTU measures heat output. One BTU equals the amount of energy needed to increase 1 pound of water by 1 degree F. One ton is the equivalent of 12,000 BTUs per hour, or the amount of energy it takes to melt 1 ton of ice in a day.
With air conditioners, bigger is not necessarily better. For one thing, the larger the capacity, the more a unit will cost since output is directly related to cost. Also, it’s important to pick a size that is appropriate for your house.
An air conditioner that is too small can’t keep up with load requirements on a particularly hot day. One that is too large will cycle off and on too frequently, doing a poor job of dehumidifying the air, which degrades comfort. In fact, it’s better to slightly undersize an air conditioner than to oversize it. Also, the air flow into and out of rooms must be carefully balanced to ensure efficient operation of the system.
These factors, as well as how well a house is insulated, how it’s used by your family, your climate, and more must be taken into account when selecting and designing your system. Though you should consult a qualified air-conditioning contractor before making any purchase, you can estimate your requirements by figuring you will need about 1 ton of capacity for every 400 square feet of living space in a marginally-insulated house. For example, a 2,000-square-foot house would normally require about a 5-ton air conditioning system; a newer, well insulated house can often be served by a smaller system.
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