• Don't hire a contractor who wants to size your unit based solely on the floor area of your house. Contractors should calculate how much cooling a home needs according to the guidelines set forth in the Manual J of the Air Conditioner Contractors of America (ACCA). To gather the necessary information, your contractor should spend about an hour poking around your house, taking measurements in each room and asking questions. He or she needs to measure floors, ceilings, and walls--including all the windows--and check insulation in the attic, walls, and crawlspaces. Some other factors that go into the cooling load formula include indoor and outdoor temperatures, number of occupants, shading, and roof color.

• Insist on getting a copy or computer printout of the cooling load calculations. These can be useful in comparing bids from contractors. Question the contractor if the calculations don't include all the items mentioned above, or if you see anything that you don't understand.

• Ask your contractor to size the air conditioner based on the latent cooling load (which considers the humidity of the air) as well as the sensible cooling load (which considers the temperature of the air) for your home.

• Don't be tempted by the lowest bid. Be willing to pay for the time the contractor must spend to do the job right.

• Check for duct leaks and disconnected ducts. Also be sure air flow is not restricted by ducts that are crushed or too small for the system. Ideally, the contractor should use diagnostic equipment to find leaks and then fix them with quality duct sealants (not duct tape). It doesn't make a lot of sense to buy a larger air conditioner to cool your attic or crawlspace!

• Buy a high-efficiency unit. New air conditioners are required by federal law to have a SEER of 10 or higher. Even though it will cost more, it's usually worth it to buy at least a SEER 12 unit, because over time that initial investment will be paid back with energy savings. Look for an ENERGY STAR label when you shop. If you are replacing an existing air conditioner, you must replace the whole unit, including the inside coil and often the blower fan, to achieve the rated efficiency.

• Install for ease of maintenance. Make sure the inside coil can be reached for cleaning. The contractor may have to install an access panel, depending on the model. The coil should be cleaned every two years. The air filter should be located where it is easy to remove. Check it every month during the summer, and clean it or change it whenever it is dirty.

• Place the outside unit on the north or east side of the house, out of direct sunlight, but don't add shade for it. Leave plenty of room for free air flow on all sides, and leave at least 4 feet of clearance at the top. Keep the area free of debris and shrubbery. The air conditioner draws a lot of air through it. It's more important for the unit to have a lot of space than for it to be well shaded.

When you buy a new air conditioner, there are three things to remember. Don't buy an oversized unit. Buy an efficient model. And make sure that the unit you buy is installed properly.

Studies show that one third to one half of home air conditioners don't work the way they should because they are oversized. Contractors generally size air conditioners at least a half ton larger than necessary, and often oversize by a ton or more. (We're not talking about the weight of the air conditioner here, but tons of cooling. One ton of cooling is approximately how much cooling you'd get from melting a ton of ice. One ton of cooling is equivalent to about 12,000 Btu of air conditioner capacity.)

An oversized air conditioner turns on and off more often than it should, even during the hottest weather. You pay more for it, and it uses more energy, raising your utility bills. It won't dehumidify the air as well as a smaller system would. It's noisy, especially if the grilles in your house were designed for a smaller unit, as most are. An oversized air conditioner doesn't mean cool comfort for you. It means higher first costs, higher electricity bills, and a home that's uncomfortable to be in.

So make sure that your contractor sizes your air conditioner properly. This should not be done by rule of thumb. Ask your contractor to use the sizing manuals put out by the Air Conditioning Contractors of America (ACCA). Manuals J and S help the contractor size and select equipment; Manual D gives guidance on designing the duct system. The contractor measures your house, notes the window area and the direction the house faces, and measures insulation levels. These data are entered into formulas that are used to calculate the amount of cooling your house needs.

Another important consideration is the ability of the selected air conditioner to remove moisture from the air. Different models have different abilities to handle moisture. The contractor should understand how to pick an air conditioner that is suitable for your house, and for your climate. For more on choosing the best unit for your needs, and on dealing with contractors, see above.

Trethewey, Richard with Don Best. This Old House: Heating, Ventilation, and Air Conditioning. Boston: Little, Brown and Company, 1994.

Wilson, Alex and John Morrill. Consumer Guide to Home Energy Savings. Washington, DC: American Council for an Energy-Efficient Economy, 1996.