If your house doesn't have central air conditioning and you find yourself wilting in the heat and humidity, you may be thinking this is the summer to buy a room air conditioner.

Before you do, however, here is advice from the Better Business Bureau:

The most uncertain things to consider before you purchase an air conditioner are the unit's cooling coacity compared to the area it will serve, its energy-efficiency rating (EER) and the cost and complexity of installation.

Buying a unit that is too or too small for a room can be a costly mistake, BBB officials said. An oversized unit may be more expensive than necessary, could cause unconfortable variations in temperature and is likely to cool the area so quickly that moisture remains in the air. And undersized unit cannot be expected to perform well.

In calculating a machine's cooling capacity, remember that is its measured by the number of British thermal units (Btus) of heat it can remove from the air in one hour. This capacity should be clearly marked on the nameplate of the unit.

Although some advertisments will mention a unit's horsepower or amperage, these have no relation to cooling capacity, the BBB reported.

Before buying an air conditioner dealer, who should be able to determine apporopriate capacity. Also, note the number of openings, window exposures, type of lighting and insulation, which are all factors to consider.

The energy-efficiency rating is a measure to gague consumption and a way to gauge operating costs. EERs range up to 11.6; the higher the number, the more efficient the unit. An air conditioner with higher EER may be worth, in the long run, a higher initial cost. EERs are a part of the labeling of all new units. If you are considering buying an old unit, the EER can be calculated by dividing the cooling capacity (Btus per hour) by the wattage or power.

Ease of installation also is important. Window units are usually designed for a minimum of installation effort. Be sure to read the follow the manufacturer's instructions. Before installing any unit, verify that the electrical wiring is adeguate for operation. Inadequate wiring wastes power, cuts efficiectncy and is very dangerous.

Other BBB installation tips:

Use a line not already loaded with appliances.

Make sure the outlet is properly grounded. Never remove the grounding prong from a three-pronged outlet.

If possible, install the unit in a shaded window, but avoid areas where outside dust, odors or excessive pollen will be drawn into it.

Never block the back of the unit.

Don't forget the importance of a warranty when shopping. Does the warranty apply to parts, labor or both and is it backed by the dealer as well as the manufacturer? Are there local facilities that will take care of problems or would it have to be shipped to the manufacturer for repairs?