What is the most cost-efficient way to air condition a home - using a central unit or a window model?

If you don't have an air conditioner and are considering a purchase, use this rule of thumb: If you intend to cool three or more rooms at once, a central unit might prove to be most cost-effective. On the other hand, if you plan to cool only one or two rooms, consider a window air conditioner. And, under certain conditions, you might be better off with both systems.

If you have a home with many rooms, but occupy only one on a frequent basis, a central unit combined with an independent unit in the room you use most can save you dollars in the long run. Not everyone can afford the luxury of two systems, but if you can swing the initial investment, the long-term savings will pay for the second system in no time.

If you are put off by lugging a window unit up from the basement each year, don't use the unit in the window; build it into the wall instead. When in the wall, the unit can be left in place year round without obstructing the view through a window. Also, if you have an air-conditioning unit mounted in a bedroom window, you might be endangering the occupant. Normally, in the event of a fire there are two ways to escape from a bedroom - the door and the window. A window unit could cut down on the odds of escape.

The energy efficiency of a newer unit can reduce costs by up to 25 percent. Regardless of the type or age of the unit, be sure to change filters after every 90 days of use.

Before going to the store to purchase a new unit, measure the size of the room you want to cool. The length, width and height are used to calculate the volume of air in the room that will have to be cooled. Thus, the appropriate size of the air-conditioner can be determined.

An energy saver is another important feature to look for. As thermostats do for the entire unit, the energy-saver feature will cycle the compressor on and off so that it doesn't operate continuously. You don't have to know whether the compressor is on or off as long as you are aware that the energy-saver feature will reduce your energy expenditure.

Your personal comfort also is important. A three-speed fan control (low, medium and high speeds) is a must. Using a thermostat in conjunction with the fan speed allows the user to bring the room temperature down to the desired comfort level very quickly. The fan can be adjusted to a lower speed to maintain the temperature.

Choose a model with a timer. Setting the unit to come on a half hour or so before you get home will assure comfort on arrival. Conversely, the timer can be used to turn the unit off once you have fallen asleep. Portable electric timers - found in a local home center - are geared for small appliances, not built-in air-conditioners. A portable timer that would operate a wall unit would not be a cost-effective purchase.

A central air-conditioning unit is a bit more complex to purchase because the equipment occupies more space, and ducting is involved. As with the wall unit, the energy saver is available with the central unit. If you want maximum comfort at minimal operating expense, be sure to include several cold-air return locations. Air conditioners also act as a sort of dehumidifier. An ounce of prevention can be worth a pound of cure when it comes to high humidity, so don't rule out a built-in dehumidifier.

Another excellent accessory for any central system (heating or cooling) is an electrostatic filtering system. This is an option you'll want if a member of your family has allergies. The filter unit never has to be replaced; all you have to do is give it a thorough cleaning every once in a while.

Finally, be sure to keep energy efficiency in mind when selecting any large electric appliance.