A FEW YEARS ago homeowners were replacing their window air conditioners with central systems. Today, it seems the situation is reversed. Rising energy costs have made it more economical to cool rooms only when you're in them.

A spokesman for General Electric says many customers are buying 5,000-BUT units to use in place of or in addition to their central air-conditioning systems -- particularly in the kitchen. The hard-to-cool room can be turned down lower while the central system is set at an economical 80. This way, the central system isn't overloaded. Or people can turn off the central system and just use the window unit, if the plan to be in one room for most of a day or evening, says the GE spokesman.

General Electric's Walter Bennett recommends buying "the smallest unit air conditioner that will do the job." He explain s that an oversized air conditioner doesn't control humidity well, because it doesn't run constantly but cycles on and off.

This year, homeowners have their pick of air-conditioning systems, many of them heat pumps that both cool and heat.

Sears Roebuck and Co.'s computerized air conditioner went on the market last week in Washington. The window-unit air conditioner will sell for $549.95. It contains a "night-setback" device whose micro-processors turn the air conditioner up at night (or during the day, depending on when you want the extra cool). The micro processors are similar to the chips used in digital wristwatches and computer games.

"If you're not home until 7 p.m., you can set your conditioner to turn on at 6:30 instead of running the machine all day. It's more efficient," says Sears' H.T. Erfer.Erfer claims the 9,000-BTU conditioner is 9.6 efficient, a good rating.

The energy efficient ratio or EER, according to Walter Bennett of General Electric's major appliance and electric divisions, "tells you how many BUTs [British Thermal Units] you get for the watts [the measurement of electricity that the unit uses]. Usually the EER is 6.5 to 6.8. Anything over 7.5 is considered highly efficient."

The computerized Sears air conditioner has four fan settings. The fans slow down as the desired temperature is reached, and the conditioner then shuts off. During periods of high humidity, a control keeps the air circulating to dehumidify with minimal cooling.

The Amana company is not using micro processors on its air conditioners yet, says Chuck Mueller, manager of planning. "We feel touch controls on air conditioners and heat pumps are a bit gimmicky and result in no tangible energy savings."

Mueller agrees that the night-setback device is a good feature, but he says "you can get a timer at a hardware store -- like the ones people use with their lights -- and hook it up to your air conditioner. The homeowner will get the same energy benefits. At this point we [at Amana] don't see any great consumer values with computerization."

At General Electric, Zoneline Three, a through-the-wall heat pump is being marketed this year. The heat pump is a reverse-cycle air conditioner. Like a refrigerator, it exhausts warm air. The heat pump itself is not an electrical resistance heater, though often electrical heating coils (like the ones in your toaster or electric heater) are added for back up heat. Instead it transports heat. The Zoneline Three comes in 9,000, 11,500 and 14,100 BTUs; has an EER of up to 9.1; prices start at $815.

A closed-loop coil contains a refrigerant that expands and give off cold air. The refrigerant is then pumped back into the house and condensed to give off heat. Simply put, a heat pump removes the heat from the air in summer and the cool air from the room in winter. "With electric resistant heat, says Amana's Chuck Mueller, "the homeowner gets one watt for every watt put in. But with a heat pump, the homeowner gets 2.6 watts for every watt put in."

"Zoneline Three is bigger than your usual window unit air conditioner," explains Bennett. "It looks like the conditioners seen in motel rooms. We recommend it for cooling one or two rooms. People who add on to their home or who enclose a porch seem to love it."

Architects and designers have used the GE heat pump for the last two years, says Bennett. "With the price of electric heat going sky high, the heat pump is about two to three times more efficient than the electric heater." h

Amana's Mueller says, "We have to anticipate the consumer's needs two and three years down the road. So in designing the units we must get more energy for the least operating costs. One byproduct of worrying about the efficiency of the unit," he says, "is that the machine becomes more relaible -- it's no longer a boom market as it was in the '60s. What we have now is a replacement market in which consumers want an improvement over what they have now."

Amana's 1981 line of room airconditioners, called the "Energy Savers," has an EER of 7.7 or better. Each "ES" unit has a special control that cycles the fan motor with the compressor, saving energy.

The "ES" Amanas include: a 12,000-BTU unit that costs about $550; a 18,000 BTU unit for $580; a 24,000-BTU unit for $880.

Noise is probably the biggest problem users have with window air conditioners. This year, Amana has done something to quiet things down.

All Amana room air conditioners have an internal compressor whose parts are steel-spring mounted. Resilent rubber grommets cushion the compressor against vibration and in some models the compressor is further isolated from noise with a rust-resistant coil spring. The fan motor is mounted on rubber grommets to reduce mechanical vibration and noise. Amana has added a muffler to the compressor discharge line on most models. This decreases refrigerant pulsations and noise as well. l

Chris Greenbacker, sales engineer with Carrier and Koldwave air conditioners, says Koldwave has computerized air conditioners for the office building, but not yet for the home. Carrier makes a through-the-wall unit heat pump ($1,200 for a floor-mounted 14,000-BTU unit) and a high-efficiency window air conditioning unit ($700 to $800 for a 14,000 BTU-unit).

In keeping with the trend toward the use of small air-conditioning units, GE sells "Carry-Cool." It comes with a built-in handle and can be moved from kitchen in the day to the bedroom at night. It comes in 4,000, 5,000 and 6,000 BTUs and sells for $199 to $269. The line was introduced a decade ago and originally sold for $100 each. According to GE, it's now GE's largest selling room air conditioner sold in the United States.

Sears also carries small Kenmore air conditioners: 4,000 BTUs for $175.95 and 5,000 BTUs for $295.95.

Fedders makes a 5,000-BTU conditioner for $239.95 (EER is 5.7); another 5,000-BTU unit for $259.95 (EER is 7.0); and a 5,900-BTU unit for $279.95 (EER is 7).

Carrier's Siesta Portable is 5,000 BTUs, priced from $330 to $355.

Amana also makes a portable room air conditioner. Prices: $280 for a 6,000-BTU unit' $380-$500 for a 9,000-BTU unit.

If you're not in the market for a new conditioner this year, one way to insure that you'll get the most out the air conditioner you have, is to give it a good cleaning throughout the season, particularly the first time you turn it on. General Electric recommends routine filter cleaning monthly. The less lint and dirt on the coils and filter, the more efficient your conditioner will be. The outside part of the conditioner should also be cleaned -- the unit needs free air movement both indoors and out.

Cleaning an electrical machine requires extreme caution (make sure the conditioner is unplugged) and with an air conditioner, some dismantling and reassembling. In most cases, a professional air conditioner maintenance person can take care of the work -- faster and safer -- than the homeowner.