To most people the arrival of warm weather means using air conditioners. May undoubtedly will be hesitant about turning them on due to the high cost of energy these days; in fact, most will think twice about doing so on marginal days or nights, because in many communities the cost of running an air conditioner this summer will be higher than ever before.

In spite of this, most people will use their air conditioners when really hot weather arrives. Fortunately, there are steps one can take to improve efficiency and thus help to substantially lower operating costs.

When shopping for a new air conditioner, look for one with a high energy-efficiency ratio (an EER of at least 7.5; and buy one that is no larger than you absolutely need for that particular room; (dealers use charts to help compute the required size unit).

In fact, most experts recommend that you buy one at least 10 to 15 percent smaller than the room needs because then it will run without cycling on and off as frequently. This not only means it operates more effeciently and costs less to run (smaller units draw less power), it also means it will do a better job of removing humidity (the longer it runs, the more humidity it removes) and occupants will be comfortable at higher temperature settings.

Oversize units often run for such short periods that they don't stay on long enough to lower humidity, leaving people warm and clammy even when temperatures have been brought down low enough for comfort.

Before using a room air conditioner at the start of the season, regardless of whether it is intalled in a window or in a permanent opening cut into the wall, you should thoroughly clean the condenser coils and the various air-intake and discharge grills on the outer half of the unit (the part that protrudes from the window or wall opening). In many cases this can be done from the outside, but in others it may be easier to slide the unit inward to work from the inside.

Cleaning coils and grilles is important because this part of the machine serves to get rid of the heat that is absorbed from the inside. It does this by dispersing it into the outside air -- it is essential that there be a free flow of air to and around these coils. Accumulations of dirt, leaves or other windblown debris an prevent this free flow and interfere with efficient operation, as will any branches, vines or thick shrubs that are close enough to the unit to block partially any of the openings on the outside.

If the unit is designed to slip easily out of its cabinet or case, it is a good idea to use a vacuum and a brush to remove dust and dirt from the inside: fan blades, cooling fins and coils on the inside as well as on the outside half of the machine. Here again, unobstructed airflow is the key to efficiency; removing anything that gets in the way will help lower the load on the unit when it is running.

Here are some other steps you can take to help lower the cost of running your air conditioner this summer:

1. Be sure the filter is cleaned or replaced before the unit is put on for the first time. A dirty filter is probably the most common cause of inefficient operation. It obstructs the airflow and makes the machine work much harder and longer than it should to keep the room cool. Check the filter at least once a month during the season and wash or replace it if it gets so dirty you have trouble seeing through it when you hold it up to a bright light.

2. Set the temperature control or thermostat to a warmer temperature than you have in the past. Raising the temperature setting by only 2 degrees (from 70 to 72, for example) will reduce operating costs by as much as 10 percent in most cases. Remember that every time the unit runs for even a short while it also dehumidifies the inside air, and lowering the indoor humidity contributes almost as much toward keeping occupants comfortable as lowering the temperature.

3. Do everything possible to seal openings or cracks around the air conditioner where it fits into the window, or through its opening in the wall. Even small openings will allow a surprisingly large amount of hot air to enter, adding considerably to the amount of heat that must be removed from the inside. Use caulking around the outside, and foam-type weather stripping or rope-type caulking around the inside.

4. Try to "button up" or insulate the other windows in that room to keep out the sun's heat as much as possible. Draw shades or blinds on windows where the sun shines during the day, or add a transparent, heat-reflecting "solar" film to the glass (those reflect away the sun's heat before it can enter the room). Awnings on the outside will also accomplish the same thing. Leave storm sashes in place on windows that are seldom opened.

5. Many air conditioners have air vents that can be opened or closed -- when open they exhaust stale air from inside and fresh air can be drawn in from other rooms. These should be left closed unless really needed to eliminate odors, cigarette smoke, etc. Even then, close the vent as soon as possible. Better yet, if you must open the vent, turn off the compressor and leave only the fan running until the need for fresh air is over. cThen close the vent before turning the air conditioner back on.

6. If the air conditioner is on the side of the house where it gets a lot of late afternoon and evening sun while it is running, erect some type of awning or shade over it. Allowing the sun to shine directly on the outer half of an air conditioner while it is running only makes it work harder to get rid of the heat absorbed from the inside, and thus adds to the cost of operation.

7. On units that have a fan-cycle switch -- a switch that lets you set the fan to run either continuously or cycle on and off with the compressor -- set the switch so the fan runs only when the compressor cycles on, then shuts off when the compressor shuts off.

8. Install a timer to control the air conditioner and set it to turn the unit off in the eary morning hours before you get up when forecasts indicate it will get cool enough during the night to permit sleeping comfortably without air conditioning. The timer also can be used to turn the machine on in the evening before you get home. Most people, when they walk into an overheated room, set the thermostat as cold as possible in the misstaken notion that it will cool things off quickly.

9. If a unit is being used to cool more than one room (or a large L-shaped room), one part of the room may get too cold while another part is too warm. This is because the built-in fan cannot circulate the cooled air far enough to make full use of the machine's cooling capacity. To solve this problem, use one or two portable fans to circulate the cooled air when the machine is running. You will be able to set the thermostat to a higher temperature because the fans will help cool off the more distant areas.