How to install a window air conditioner

A window air conditioner install has exactly three ways to go wrong: the unit falls, the water drains inward, or the gaps leak. Everything below is about those three.

Two measurements decide whether the unit you are about to buy will actually go in.

• Sash opening width — measure the clear width between the window channels, not the frame outside. The unit body must be narrower; the side curtains fill the remainder.
• Sill depth and what is behind it — a storm window, a screen that does not remove, or a security grille will stop an install that the width measurement said was fine. Certified units here run from 15.3 to 22.0 inches deep.

Then read the weight, because it determines how many people you need:

Anything over about 50 lb is a two-person install, and the second person is not there to help lift — they are there to bring the sash down while you hold the unit steady with both hands.

1. Clear and clean the sill. Remove the screen if it comes out. Wipe the sill — the foam gaskets you are about to apply will not stick to grit.
2. Fit the top flange and side curtains to the unit while it is still on the floor. Every fixing you can do at floor level is one you are not doing while holding 50 lb at a window.
3. Apply the foam sill gasket along the sill where the unit will sit, and the adhesive foam strip to the top of the lower sash.
4. Lift together and seat the unit on the sill, pushing it out until the mounting rail contacts the outside of the sill. Keep both hands on it. Do not let go to adjust anything.
5. Bring the sash down onto the top flange and hold it there. This is what the second person is for. The unit is not secure until this happens.
6. Screw the top flange to the sash using the screws supplied. This is the fixing that stops the sash being raised and the unit going out of the window. Do not skip it.
7. Extend the side curtains to the window channels and screw them to the sash frame.
8. Check the tilt — see below.
9. Fit the sash lock bracket in the upper channel so the sash cannot be lifted.
10. Seal every remaining gap — see below.

A window air conditioner produces condensate continuously. That water has to end up outside, and on most units gravity is the only mechanism that puts it there.

So the unit sits with the outdoor end marginally lower than the indoor end. Set it dead level and water pools in the base pan, then eventually finds its way onto your floor.

Two important qualifications, and both come from the manual rather than from us. First, your unit specifies its own drop — and some models, including several U-shaped ones, are engineered to install level. Second, more tilt is not better. Excessive tilt loads the mounting hardware in the wrong direction and can push the unit toward sliding.

A phone level app against the top casing is entirely adequate for checking this. You are looking for a fraction of a degree, not a slope.

Every step above is about the unit staying put. This one is about what it costs you.

The most efficient certified 8,000 BTU unit uses 375 kWh a year — about $69. That figure is measured under laboratory test conditions, with no air leaking around the machine. A unit installed with open gaps around the side curtains is pulling hot outdoor air into the room continuously while it runs, and no certification number accounts for that.

The gaps to close, in order of how much they leak:

• The gap between the two sashes. When the lower sash is raised, its top rail no longer meets the upper sash’s bottom rail — you have opened a full-width slot. Foam strip, cut to length, wedged in. This is the biggest single leak and the one most often ignored.
• Behind and around the side curtains. The accordion curtains block the opening but do not seal it. Foam or weatherstrip the perimeter.
• Under the unit at the sill. The foam sill gasket goes here, and it also stops the unit sliding.
• The outdoor side of the sill. Worth a look once everything is in — a gap here lets exhaust heat drift back toward the condenser intake, which hurts efficiency directly rather than through infiltration.

Do not use expanding foam anywhere in a window air conditioner install. You will not get the unit out at the end of the season without damaging the window.

Run it and watch for water on the inside sill for the first hour. If any appears, the tilt is wrong, and it is far easier to correct now than in August.

Then leave it alone except for the filter. A blocked filter starves the evaporator of airflow, which drops capacity and raises run time — the same failure mode that dust causes in a fan, with a larger bill attached. Check it monthly through the cooling season; your manual will give the interval it wants.

And if you are still choosing, size the unit from the room before anything else. A correctly sized unit installed adequately beats an oversized one installed perfectly.