Buying a room air conditioner? Make sure you size up the room

Hotandhumid.

Sometimes, even the words seem to stick together. And as summer builds, people who aren't thinking about beaches, ice cream trucks, and swimming pools may be thinking about air conditioners.

The decision to buy a room, or window, air conditioner, says Robert Mason, owner of Window Air Conditioning Specialists, a Boston dealer, has nothing to do with the economy or energy prices.

''The window air conditioner business depends on one thing, the weather,'' Mr. Mason says, pointing to his phone with its row of buttons indicating several incoming lines. ''You can tell how hot it is just by listening to our phones. We've got three lines coming in here and on a hot day, they're all busy.''

This is why some people who remember how they felt last summer are thinking about buying an air conditioner now. They are trying to figure out energy efficiency rating (EER), British thermal units (Btu), and other factors that go into picking the right machine.

An air conditioner's EER is obtained by dividing the number of Btu it uses by the number of watts of power it consumes. The higher this EER number is, the more efficient the air conditioner. Generally, an EER of 7.5 or 8 is considered the minimum acceptable rating. In some states, only units with a minimum EER of 7.5 can be sold at all.

This interest in higher EERs is evident in shipments. According to figures kept by the Association of Home Appliance Manufacturers, shipments of units with an EER of 7.5 to 8.4 increased 223.9 percent from 1974 to 1981. Units with an EER of 6.5 to 7.4, by contrast, were up just 29.9 percent.

With all the attention that has been paid to the EER, Mr. Mason says, people often forget to consider where they are going to put the air conditioner and to buy the right amount of cooling power for that room. When he goes into a home before making an installation, he looks at a number of factors, including the direction the house is facing; sources of outside shade such as neighboring buildings and trees; the size and location of windows; the size of the room being cooled; what floor that room is on; the effectiveness of insulation; the number of people in the house and the amount of time they are home; and their ages.

If it is a small room, for instance, you can get by with less cooling power (represented by a lower Btu level) than you will need for a large room. However, if that room is on the top floor, you'll need more Btu to clear out the heat that has risen up there.

A medium-size living room or a large bedroom should have a unit with 8,000 to 12,000 Btu. The wide range leaves room for all those other factors like shade and window size.

The air conditioner should be large enough to cool the room it is in, no larger. If its Btu level is too low, you will be running it almost constantly to keep the room cool. If it is too high, you will be paying for power -- in both a higher price tag and higher electricity bills -- that you do not need.

For an 8,200-Btu unit, Mr. Mason says, users should expect to pay about $40 extra on their electricity bills for a season of cooling. That is assuming a 400 -hour New England cooling season, which may be a good deal longer -- and more expensive -- eleswhere in the country.

An air conditioner of that size will cost from $300 to $400, though you may be able to find some models at discount; and units with a lower EER may also cost less. Remember, though, that the extra $100 or $150 you spend on a higher EER-rated air conditioner may be saved in five to seven years of use. And a well-maintained unit can be expected to last 10 to 13 years.