The cooling capacity of residential air conditioners is given in Btu, and is the number of Btu (of heat) the unit can move out of the cooled space in one hour. Central air conditioning is usually installed by specialized contractors who calculate how much cooling capacity is required. The sizing suggestions below are for consumers selecting a room air conditioner.
As a first guess, a room requires about 10,000 Btu for every 500 square feet of floor space. But many other factors can increase that:
Narrow rooms often require more capacity than squarish rooms, since they have more square feet of (possibly warm) wall surface for each square foot of floor space.
Room A and corridor B have the same floor area, but the corridor has 25% more wall surface than the room.
Large windows are poor insulators, compared to walls. If they face south or west, sunlight streaming through them will warm the room. (Consider awnings, trellises, and blinds, before relying on air conditioning.)
Heat sources such as appliances, the kitchen being the obvious example. A kitchen exhaust fan should be installed prior to air conditioning. (In bathrooms, exhaust fans can reduce the load on an air conditioner by removing humid air.) Water heaters are another common heat source; even a large chandelier with many incandescent bulbs may need to be taken into account.
People themselves. The more people in a room the greater the need for cooling. A dining room that is fine for two, for example, may become a sauna during a dinner party. (Allow an extra 500 Btu per occupant.)
Simply buying the largest available unit isn't a good solution to the sizing problem; it may give less satisfaction than a smaller one. An oversize unit typically cycles on and off more frequently, so room air circulates through it for fewer minutes per day than it would through a smaller unit. As a result, the room air may not be satisfactorily dehumidified.
Rooms that are long and narrow, L-shaped, or have other peculiarities that impede the circulation of air may be better served by two small air conditioners than by a single large one.
Ordinary 115-volt house circuits are usually 15-ampere, sometimes 20-ampere. The capacity of the circuits can be determined by checking the fuse box or circuit breaker panel.
The National Electrical Code limits the size of the largest appliance that can be plugged into a 15-amp circuit to 12 amperes. The largest available units that will operate on 115 volts, 12 amps, have cooling capacities between 12,000 and 14,000 Btu. For larger units, an electrician must install a separate circuit. Today many homes are built with 208-volt or 230-volt single-phase circuits for air conditioners already in place; such circuits can be spotted as unusual receptacles near windows, such as the one below.
The National Electrical Code requires room air conditioners that run on voltages above 240 volts to be wired in, not plugged in.
In the United States, the average air conditioner runs 750 hours each year. Since air conditioners use a lot of electricity, their efficiency became a public issue. The Energy Policy and Conservation Act (December 1975, Public Law 94-163) requires all room air conditioners to carry a tag comparing that unit's efficiency with other units of the same general type. The EER (for Energy Efficiency Rating) is the ratio of the Btu's per hour to the number of watts the unit draws. (The EER has been criticized on the grounds that the test conditions don't fairly represent the real world, so the federal government devised a second test giving another figure of merit, dubbed the SEER, for Seasonal Energy Efficiency Rating. Generally it's about 1 point lower than the EER.)
The 1987 National Appliance Energy Conservation Act provided for minimum efficiency standards for various types of appliances, including room air conditioners. (This act preempted minimums previously established by some states, including New York and California.) Regulations under the Act divide room air conditioners into 12 classes and set a minimum EER for each class, typically 8 or 9, which is required of all units built after January 1, 1990.
You may be better off with a central unit instead of a room air conditioner. A useful source of advice on those units is the residential forum at HVAC-Talk.com. Your posted messages must not request do-it-yourself installation advice or price quotations.
Copyright © 2000 Sizes, Inc. All rights reserved.
Last revised: 21 March 2000.