No matter how you feel about the appropriateness of air conditioning in old houses, when the temperature hits the high 90s and the humidity's not far behind, you'll probably want some relief. Even if it's just one room, maybe a bedroom, having a place to cool off will make the rest of the house seem more bearable.

There's more to it than just sticking that yard sale unit in the window, however. Before you even look for an air conditioner, there are three issues you need to consider:

* What size unit you should buy.

* Where you will plug it in and what impact it will have on the wiring.

* How it will be mounted on the window.

Air conditioner sizes are meant to be read by engineers, not by ordinary humans. Instead of being in cubic feet, or something logical, they are sized in Btu, or British thermal units. One Btu is the amount of heat required to raise one pound of water one degree Fahrenheit. In the case of air conditioners, the Btu rating indicates how much heat the unit will remove.

Fortunately, there's a rule of thumb, based on square feet of the space to be cooled, to help you decide how big a unit you need (see accompanying chart). It's only a rough guide, to give you an idea what to look for. If you have ceilings higher than 8 feet, or a lot of windows on a south-facing wall, or some other special circumstance, you'll need the advice of the salesman. The point is tobuy only as much capacity as you need; the price, size, cost of operation and wiring needs rise dramatically with the larger units.

Wiring is an important element in choosing and installing an air conditioner. Air conditioning and old wiring don't mix. In electrician's terms, air conditioners pull a lot of juice. If you simply plug it into a wall outlet, it is probably going to require most of the amperage available on that particular circuit. If the wiring is old, it may not be heavy enough; it could heat up enough to be a fire hazard. (This is why you should never plug air conditioners into ordinary extension cords; they simply aren't heavy enough. If, for some reason, you have to use an extension cord, make sure you get one that is the same size as the unit's cord. The amperage will be listed on the unit; extension cords are also rated for amps -- check the fine print.)

Ideally, each air conditioner unit should be on its own circuit. Circuits are measured in two ways: amperage, which is the rate at which electricity is delivered through the wire; and voltage, which measures the pressure of the electricity. (Electricity, like water, has to be under pressure to get to the outlet points.) Basic circuits are 15 amps, 115-120 volts. A small unit runs on 120 volts, and may pull 7 amps -- or half the current available on a 15-amp circuit. Any unit that pulls more amps than that should have a 20-amp circuit.

If you're redoing the wiring anyway, install 20-amp circuits where you plan to plug in air conditioners. (The technical term is 20-amp duplex receptacle circuits.) It's not a special outlet; in the winter you can use it for a lamp or an iron or a radio.

If there's no reason to replace all the wiring, it still might be a good idea to install a 20-amp receptacle circuit in each room. That way there's one heavy-duty circuit in every area that you can depend on when you have heavy electrical demands -- using power tools, for instance, if you're doing a rehab.

Larger air-conditioning units may require more voltage -- 230-240 volt circuits -- but operate more efficiently by using less amperage. An electrician can install a 230-240 circuit, if you have "three-wire" service into the house. Old electrical service has two wires running from the utility pole to the house; newer service has three. (You can usually tell which you have by looking at the spot where the wires enter the house. If you can't tell, the electrician can, and installing 230-240 service is a job best left to a professional.) If you have two-wire service, you can change it, but it will require the combined efforts of an electrician and the local utility company.

One you get the wiring straightened out, you have to figure out how you'll support the air-conditioner unit in the window. The unit has to slant down and out, so condensation can run out. Smaller units generally are designed to be supported by the window. That works fine if you can position the unit so that it slants in the right direction. Otherwise, you may have to build up the surface on the inside.

If your windows aren't wood, you may not want the frames to carry the weight of the air conditioner. Instead you may have to build a platform for the unit, making sure it has the proper slant.

Larger units usually come with some kind of support brackets. Again, they may work fine if you have simple windows; vinyl windows or storm windows may complicate things.

Before you go out to look at air conditioners, measure the window. Then take a tape measure to the store so you can measure the units you're interested in.

By all means shop around. Air conditioners have gotten more sophisticated in recent years; among other things, they're rated for energy efficiency. There are also more casement units available and there are even movable units that require only a plug and small window vent. Be sure you've checked out all the options.

And besides, the stores are air-conditioned.

Next: Retrofitting central air.

Mr. Johnson is construction manager for Neighborhood Housing Services of Baltimore. Ms. Menzie is Home Editor of The Sun.

If you have questions, comments, tips or experiences to share about working on houses, write to us c/o HOME WORK, The Sun, 501 N. Calvert St., Baltimore, Md. 21278. Questions of general interest will be answered in the column; comments, tips and experiences will be reported in occasional columns.

Choosing the right size

This is a rough sizing chart for air conditioners. Multiply length times width of space to be cooled. Units above 12,000 Btu will probably require 230-240 volts.

Btu .. .. .. Sq. ft. .. .. .. Btu .. .. Sq. ft. .. .. Btu .. .. Sq. ft.

4,800 .. .. .. 200 .. .. ... 12,000* .. .. 500 .. ... 19,200 .. ... 800

6,000 .. .. .. 250 .. .. ... 13,200 .. ... 550 .. ... 20,400 .. ... 850

7,200 .. .. .. 300 .. .. ... 14,400 .. ... 600 .. ... 21,600 .. ... 900

8,400 .. .. .. 350 .. .. ... 15,600 .. ... 650 .. ... 22,800 .. ... 950

9,600 .. .. .. 400 .. .. ... 16,800 .. ... 700 .. ... 24,000 .. . 1,000

10,800 . .. .. 450 .. .. ... 18,000 .. ... 750

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