Consumer Reports' guide to window air conditioners

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Although more and more homes have central air conditioning, about 6.5 million window units are sold each year. Here's how to find the right one for you.

BUYING

Before going to the store, determine the size of the space you need to cool and where you'll place the unit. An air conditioner that's too small won't cool the room. One that's too big will cool so quickly that it won't have time to remove enough moisture, leaving your room cold and clammy. (Click here for Consumer Reports' Ratings of window air conditioners.)

Calculate the optimum machine size. Take into account not only the size of the room to be cooled but whether the unit will be placed in a window that gets shade or direct sunlight, the height of the ceilings, and even the part of the country where you live. Click here for a worksheet from the Association of Home Appliance Manufacturers that will help you make the right determination. All you need to get started is a tape measure, a scratch pad, and a calculator.

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Assess the airflow. Air conditioners generally do a better job blowing air in one direction than the other. To uniformly cool a room, you'll need to ensure that air is distributed throughout. When the window air conditioner is located near a corner, it must be able to direct air to the center, so check whether your air conditioner needs to blow air to the right or to the left.

How quiet? If the unit is going to be placed in a bedroom or another quiet area, consider a model quiet enough that the only sound you might hear is the fan running. Too much noise could disturb light sleepers when set on low and are distracting on high. (Click here for Consumer Reports' Ratings for window air conditioners. Our latest tests of almost three dozen room air conditioners include an $550 model that cools superbly and quietly, and even comes in colors that match the drapes. Other top performers start at $175.)

INSTALLATION

If you're planning to install the air conditioner yourself, consider buying one with a slide-out chassis. That way you can attach the cabinet and adjustable side curtains to the window before sliding in the heavy working parts of the machine. One person can do it, but it's easier with two.

Check the electricity. Before installing an air conditioner, be sure that the electrical circuit to the room can handle the electrical load of the unit. Read the owner's manual; larger models usually need a dedicated circuit. Never use an extension cord with an air conditioner.

Secure the unit. Always use the manufacturer's safety hardware, such as sash locks and mounting brackets. Unless the manufacturer's directions say otherwise, the window air conditioner should be level from left to right and pitched slightly toward the outdoors so water that condenses on the evaporator drains properly to the rear of the unit and doesn't leak into the home. Seal around the perimeter of the unit with new weatherstripping.

MAINTENANCE

A clean machine will keep you cool and cost less to run. Plan on a thorough cleaning before and after the cooling season and regular filter checks during the season.

Clean or replace dirty filters. You'll need to clean the filter regularly. Depending on how much time the unit is actually operating and how clean the air is, cleaning may be needed every few weeks to monthly during the cooling season. With that in mind, make sure you determine how easy it is to remove the filter when selecting a new unit—some are trickier than others. Remove debris with a vacuum then wash the filter in warm, soapy water; be sure filters are dry before you reinstall them. Replace damaged filters.

Vacuum coils and fins. When the filter is removed for cleaning, it's also a good time to check the surface of the evaporator coil, which will now be visible. If there is dust or debris on the surface, gently remove it. Taking care not to deform the soft fins, use an upholstery-brush attachment to vacuum the coils. If your unit has a slide-out chassis, you will usually have good access to the condenser coil when the chassis is removed from the cabinet. That's a good time to inspect and clean any debris off that coil.

Seal the perimeter. Be sure to seal any air leaks around the unit.

Avoid "short cycling." Though most models with electronic controls now have built-in timers to prevent the unit from restarting immediately after shut-down, those with the "old-style" mechanical controls may not. Wait 5 minutes after shutting off the unit to restart it. That allows pressure in the refrigeration system to equalize, avoiding stress on the compressor.

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