Although most room air conditioners are installed in windows, many homeowners now prefer through-the-wall installations.

Putting the air conditioner through the wall makes it possible to place it where it is less likely to detract from the room's decor. The change in location may also lead to more efficient air circulation with less chance of annoying drafts, and relocating the air conditioner returns the window to its original purpose - furnishing light and air to a room.

Freeing the window has other advantages. The high cost of energy makes people reluctant to use more of it than is necessary, and opening a window for both light and air can cut the cost of the electric bill.

It is best to install a through-the-wall air conditioner that has been specifically designed for that purpose. They come with a special metal sleeve that is placed permanently in the wall, after which the air conditioner slides into it from the inside. Although the sleeve is permanent, the air conditioner can be easily removed for service or replacement.

A question that often arises is whether a window air conditioner can be installed through the wall. In most cases the answer is yes, but a number of factors must be considered. The first is the thickness of the wall through which it must fit.

All room air-conditioning units have two basic sections or air-moving components: the evaporator, the system that actually cools the air inside the room and recirculates it (warm air is drawn in, some heat is extracted from it, and it is blown back out into the same room area); the condenser and its coils. This is found in the outer end of the cabinet and faces outside. The condenser absorbs the heat pumped into it by the evaporator and dissipates it to the outside air by use of a second fan on the outer end.

Air for cooling the condenser coils is drawn in through louvers on or near the back end of the cabinet. Units designed for through-the-wall installation have these louvers on the back, and some window units have them on the back, but most window units have them on the side and these louvers must not be restricted by the thickness of the wall into which the air conditioner will fit. It is important to measure their distance from the front before trying to install a unit through the wall.

With a typical frame wall about six inches thick, there will be no problem, but if the outer walls are as much as 10 inches or 12 inches thick, there may be a problem because the air conditioner will not protrude far enough to permit a free flow of cooling air into and around the condenser coils.

In addition to making sure the unit will protrude enough to provide an adequate flow of air through the louvers, mounting a window unit through the wall presents one or two other problems. Window units that have cabinets that permit the chassis to be removed from the inside are most adaptable to TTW mounting because the cabinet can be secured inside its framed opening and the chassis will slide out from the inside when service is needed.

But so-called "button-down" type window units require taking the entire unit - cabinet and all - out of the wall when service is needed. This complicates matters and makes it hard to get a watertight fit when it is inside the opening.

In either case, an opening of adequate height and size must be cut to allow for framing, and proper bracing and fasteners must be provided to secure that unit solidly inside this opening so it fits level when in place.

The do-it-yourselfer who is familiar with basic carpentry techniques should have no trouble cutting through a wood frame wall to create a suitable opening for any air-conditioner. If a unit is being installed the directions will tell the size opening needed for the metal sleeve. If a window unit is being installed, the opening should be sized to provide clearance for the cabinet after the framing lumber (usually 1x6 or 1x8) is in place inside the cutout.

Actually, the width should be slightly larger than the width of the unit because each side can be filled with pieces of plywood or similar material, much the same way that panels are used on each side when the air conditioner fits in a window. The height should be as close as possible to the proper height to insure a snug fit after caulking or weatherstripping is applied, but not so tight that it will be difficult to insert or remove the cabinet.

When choosing a place for the opening, select a space in the wall where no wires or plumbing lines will interfere. Start cutting from the inside, removing plaster or gypsum board first to expose the studs. For this you can use a saber saw or a hammer and chisel, but stay as close as you can to the outlines drawn on the wall ahead of time. When the studs are exposed, cut away the one that is in the way, then frame out with short vertical pieces at each side and along the top and bottom.

Drill a hole through to the outside in each of the four corners of this framed opening, then use these holes as a guide to cut away the siding or shingles from the outside.

Framing with 1x6 or 1x8 boards is advisable to insure a snug fit and to provide some exterior trim against which caulking can be applied to insure a watertight fit. After the air conditioner is in place add weatherstripping or ropetight caulking around the outside of the cabinet to keep water from seeping in, and finish off the inside with wood trim, if desired. CAPTION: Illustration 1, Air conditioner opening outside needs no trim; Illustration 2, Trim can be nailed up on inside.