How to Install a Window Air Conditioner Safely: Step-by-Step Guide

Before you start: size, window and power

• Size the unit. A typical guide is about 20 BTU/h per square foot for a normal room, more for sunny or kitchen spaces. Use our BTU calculator before buying.
• Measure the window. Note the minimum and maximum width the unit's accordion panels can cover and the minimum opening height. Check the sill depth and whether a storm window, screen frame or exterior trim is in the way.
• Check the electrical supply. Read the nameplate for voltage and amps and compare it with the outlet and circuit (see the electrical section below).
• Know the rules. Some buildings and cities restrict window units, require brackets, or ban them above certain floors. Check before installing over a walkway.

Tools and materials

• Tape measure and short spirit level
• Screwdriver set and a drill/driver
• Support bracket rated for the unit's weight (many brands sell them; required for heavy units and recommended for most)
• Foam weatherstrip or the foam strip supplied with the unit
• Wood or metal sash lock or an L-bracket to stop the lower sash being raised
• Exterior-grade screws suited to your window frame
• A helper. Window units weigh roughly 40–100 lb (18–45 kg), and lifting one out over a sill is awkward.

Step-by-step installation

1. Unbox and inspect. Remove packing, check the fins and cord for damage, and fit the side accordion panels and top rail according to the manual.
2. Install the support bracket on the outside sill or wall following its instructions, so its arm sits under the outer half of the unit. Adjust it to give the required tilt.
3. Open the window and clean the sill.
4. Lift the unit into place with your helper, setting the bottom channel over the sill and against the inside edge as the manual shows. Keep a hand on the unit at all times until the sash is down.
5. Lower the sash firmly onto the unit's top rail, behind the rail flange.
6. Set the tilt. Most units need the outdoor side slightly lower, often about 1/4–1/2 in (6–13 mm) across the depth, so condensate drains outside. Check with a level.
7. Extend the side panels to the window frame and screw them to the sash or frame where the manual allows.
8. Secure the sash. Screw the supplied L-bracket or a sash lock so the lower sash cannot be raised. This keeps the unit from falling inward and stops it being pushed in from outside.
9. Seal the gaps. Put foam between the upper and lower sash where they overlap, and around the panels. Gaps waste cooling and let in rain, insects and humid air.
10. Plug in and test. Plug directly into the outlet, press the test button on the plug if it has one, and run on high cooling for 15 minutes. Check that air is cold, there is no rattle, and water drips outside rather than in.

Electrical: circuits, plugs and extension cords

Never use an extension cord or power strip with a window air conditioner. AC units draw a heavy, continuous load, and ordinary cords overheat at the plug and along the run. Most manufacturers prohibit them outright. If the cord will not reach, move the unit or have an electrician add an outlet.

• Small 115 V units (roughly 5,000–8,000 BTU/h) draw about 4–7 A and can usually share a standard 15 A or 20 A circuit, provided not much else runs on it. The National Electrical Code limits a cord-connected room AC to 50% of the circuit rating when other loads share the circuit.
• Larger 115 V units (roughly 10,000–15,000 BTU/h) draw about 9–13 A and are best on a dedicated circuit. On a dedicated circuit the code allows up to 80% of the rating, so a 12 A unit fits a dedicated 15 A circuit.
• 230 V units (often 18,000 BTU/h and up) have a different plug shape and always need a dedicated 230 V circuit with a matching receptacle. Installing that is electrician work.
• Match the plug to the outlet. Never remove a ground pin or use an adapter that defeats it. The bulky plug head on many units is a leakage-current detection device (LCDI) that shuts power off if the cord is damaged; do not cut it off.

If lights dim when the compressor starts, or the breaker trips, read AC breaker keeps tripping. You can estimate running cost with our AC running cost guide.

Common mistakes

• Leveling the unit or tilting it inward, which sends condensate onto the floor (see window AC leaking water).
• Skipping the bracket on a heavy unit or a narrow sill.
• Leaving the sash unsecured, so the unit can slide or be pushed in.
• Blocking airflow with curtains, furniture, or exterior shrubs and screens.
• Drilling into the base pan to make extra drain holes, which can hit refrigerant tubing.
• Using an extension cord or an undersized outlet.
• Buying too big a unit. An oversized unit short-cycles and leaves the room clammy.

Removal and winter storage

In cold climates, remove the unit for winter or fit an insulated cover on the outside and seal the indoor side. To store it, drain the base pan, clean the filter, let it dry, and keep it upright so oil stays in the compressor. After moving or storing a unit on its side, let it stand upright for several hours (the manual will give a time) before running it.

Frequently asked questions