How to Choose an Air Conditioner Buying a room air conditioner: How big? One Size Doesn't Fit All Matching the air conditioner to the room you want to cool is your first consideration. If it's too small, it simply won't cool the space. If it's too large, it will cool the space so quickly that the thermostat will shut the unit down before it can lower the humidity level in the space. The result is a cool room that feels damp and clammy. Room air conditioners are "sized" according to their cooling capacity, expressed in Btu per hour. You'll find the rating on packaging or in product literature; it typically ranges from 5,000 to 25,000 Btu. Units below 12,000 Btu start at $250 to $300. Those in the 12,000- to 15,000-Btu range cost from $450 to $600, while large units go up to $1,200. Air conditioners rated at 15,000 Btu should be enough to cool most rooms up to about 875 sq. ft. How do you get more precise than that? There are several different formulas. Most of the ones you'll see in stores simply compare the square footage of the room to Btu capacity.

The Association of Home Appliance Manufacturers (AHAM), a Chicago-based industry trade group, offers a more complete formula that includes window area, sun exposure, insulation levels, how the room is used and other variables. How Much Do You Need? For a calculation of the cooling capacity your space requires, use our Air Conditioning Calculator. You can get a free copy of the formula from the AHAM fax-on-demand service or through the organization's site on the Web. Although the form looks daunting, it takes only a few minutes with a calculator and tape measure to figure out the square footage of the room and measure a few other variables. Whichever way you determine the size you need, remember that for the 115V circuits typical of bedrooms and living rooms, you'll have to stay below 15,000 Btu; units above 15,000 Btu usually require a 230V circuit. And if you want to cool a large, open L- or T-shaped space, you're better off with two smaller units rather than one large one.

"The goal is to keep the cold air from pooling on the floor," explains Hank Rutkowski, director of technical affairs for the Washington, D.C.-based Air Conditioning Contractors of America. "Two units prevent cold spots in front of the air conditioner and warm spots around the corner." Where to Find It Amana Home AppliancesAmana, IA 52204800-843-0304 The Association of Home Appliance Manufacturers 20 N.Wacker Dr.Chicago, IL 60606312-984-5800 Box 4808Syracuse, NY 13221 Emerson415 Walbash Ave.Effingham, IL 62401800-332-6658 Fedders North America415 Walbash Ave.Effingham, IL 62401800-332-6658 Friedrich Air Conditioning Co.Box 1540San Antonio, TX 78295-1540800-541-6645 Whirlpool Corp.Consumer Assistance CenterBenton Harbor, MI 49022800-253-1301Danby 15000 BTU Window Air Conditioner 15,000 BTU air conditioner cools up to 700 sq. ft. 4 way air direction Sleep Mode: Prevents the room from becoming too cold at night Auto on/off: Have the unit start or stop to meet your schedule

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3,500 BTU heating capacity Perfect Aire® 12,000 BTU Window AC w/ Electric Heat 12,000/11,600 BTU cooling capacity Perfect Aire® 18,000 BTU Window AC w/ Electric Heat 16,000/13,000 BTU heating capacity Perfect Aire® 25,000 BTU Window AC w/ Electric Heat 25,000/24,700 BTU cooling capacity DAC15009EE 15,000 btu window air conditioner - Euro Grey 25.5 x 23.6 x 17.2 inches #2,320,502 in Home and Kitchen (See Top 100 in Home and Kitchen) #488 in Home & Kitchen > Heating, Cooling & Air Quality > Air Conditioners & Accessories > Air Conditioners > WindowCentral AC units are normally hardwired into the electrical system. Home air conditioners are offered in two styles: a whole home central AC unit that is often combined with a central heating system, and window units designed to cool off individual rooms. The two types of appliances require different electrical voltages to work properly. Even a small air conditioner can put a strain on a home electrical system.

Central Air Conditioners Central air conditioners require a 220-volt or 240-volt, dedicated circuit for operation. When a central air conditioner starts up, it may require up to 5,000 watts of electricity, making it one of the largest consumers of electric power in the home. An air conditioner's cooling capacity is measured either in British thermal units (BTUs) or in "tons." The greater the number of BTUs or tons, the greater the cooling power of the unit. A larger unit also requires more electrical power. Buying a unit too large for your home wastes electricity, and causes the unit to wear out faster. Window Air Conditioners Window air conditioners are offered in either 110/120-volt or 220/240-volt models, depending upon their cooling capacity. A unit designed for cooling one small room such as a bedroom normally provides 15,000 BTUs and often works on a smaller, standard 110/120-volt wall outlet. Because of its large power consumption, you should not have any other appliance on the circuit to avoid an overload.

A window unit larger than 15,000 BTUs may require a 220/240 dedicated circuit. Adding Circuits Many homes do not have 220/240-volt circuits as standard equipment in living spaces, and they must be added by a qualified electrician. This can cost between $500 to $1,500 as of the date of publication, depending on the type of home and its location and the work that must be done, according to Bill Ferrera of Ferrera Electric in San Francisco. All window air conditioners provide the necessary information about the electrical requirements on the unit packaging. As with a central unit, buying a window unit that is too large wastes energy and money. Sizing a Unit The specifications listed for individual air conditioner units state the size of the area they cool. For example, an AC unit that cools a 120-square-foot area would work best in rooms that are 10 feet wide and 12 feet across. By matching your room or home size with the unit's specifications you can select the appropriate size air conditioner.