Window Air Conditioners: Types and When to Use One

What counts as a window air conditioner

Federal rules define a room air conditioner as a window-mounted or through-the-wall-mounted encased assembly that delivers cooled, conditioned air to an enclosed space. The same definition says the assembly includes a source of refrigeration and may include additional means for ventilating and heating. Our reading: encased means the compressor, coils and fans share one cabinet, so a window unit needs no separate outdoor part the way a split system does.

The Department of Energy's Energy Saver 101 guide says a room air conditioner provides spot cooling and can be either a window unit or a portable air conditioner. This page covers the window kind, which hangs in an opening in your wall, so the opening and the building matter as much as the machine.

The window unit types and what each one fits

The standard unit sits in a double-hung window, where the lower sash slides up and the unit rests on the sill. One maker's heat-pump window model lists standard double-hung windows with opening widths of 23.6 to 38.8 inches. The lower sash must also open far enough to leave a clear vertical opening of 13.8 inches, according to the same manual. That manual adds that the rear of the unit must be outdoors, with at least 11.8 inches of clearance around the unit. Our reading: those numbers belong to one model, so match your own window to the manual of the exact unit you are considering.

A casement window hinges at the side and opens with a crank, so it has no sliding sash to hold a standard unit. Federal rules define a casement-only unit as one designed for a casement window, with an encased assembly 14.8 inches wide or less and 11.2 inches high or less. A casement-slider is the second class, designed for a sliding or casement window, with an encased assembly 15.5 inches wide or less. ENERGY STAR gives casement units their own efficiency table, separate from the tables for other window units. Our reading: these are small, narrow units, so a casement window limits the cooling size you can choose, and the exact model's manual shows how it mounts.

A sliding window moves sideways, and a vertical unit is built taller than it is wide to fit that kind of opening. One maker's inverter vertical skinny window model says the window frame must have U channels. Its manual says that if there are no U channels, or the window is difficult, you should find a professional tradesman to install the unit. It also says both left and right sliding windows can be accommodated.

ENERGY STAR describes a U-shaped or saddle design that brackets the window or windowsill rather than sitting on top of it. It says this design lets you fully open and close the window where the unit sits. In most cases, ENERGY STAR says, the control panel and vent are inside the room while the noise-producing compressor is outside. ENERGY STAR lists three benefits: greater noise reduction, an unobstructed window view, and the ability to freely open and close the window. Our reading: a U-shaped unit still hangs on the sill, so the support and weight questions further down apply to it too.

Window units with heat

ENERGY STAR's key criteria list units with reverse cycle in one table and units without it in another. Federal efficiency classes draw the same line, with separate classes for room units with and without reverse cycle. Our reading: reverse cycle means the cooling cycle runs backward, so the unit moves heat into the room instead of out, which is how a heat pump heats. The federal definition also allows other heating means, so ask the seller whether a model's heat is reverse cycle or something else.

One maker sells an inverter window heat pump, and its manual lists a heating range of 41°F to 75°F outdoors. The manual says that below 41°F outdoors the machine shuts off and shows LO. Its troubleshooting table tells the owner to wait for the outdoor temperature to rise above the minimum heat pump operating temperature. Our reading: a window heat pump is a mild-weather heater with a stated lower limit, so compare that limit with a cold snap in your area. A window unit with heat warms one room, not the house; for heat pumps as a whole-home Florida system, see heat pumps in Florida.

When a window unit is the right tool

Our reading: a window unit fits when the job is one room, such as a bedroom, an addition or a space with no duct, and the window can hold the unit. It is a poor fit when the goal is cooling a whole house, when the window cannot carry the unit, or when a lease or association rule bars it. Read the lease or association rules first, because that document, not a manual, decides whether a window unit is allowed. A window unit also takes over the window, which matters if you need that opening for light or fresh air.

On size, ENERGY STAR says bigger is not always better. It warns that an oversized unit cools the room before it has removed the moisture. That can leave cool, moist air that makes the room feel damp and clammy, in ENERGY STAR's words. In humid Tampa Bay summers that clammy result matters; the moisture side is covered in why a house stays humid despite the AC. The sizing math lives in two other pages, what BTU means for an AC and the Florida sizing guide.

If you want one room cooled without giving up the window, a ductless mini-split is the usual alternative, covered on the mini-splits for one room page.

Mounting and safety: the window must carry the unit

A window unit hangs on a building opening, so the window frame and wall carry its weight. One maker's vertical-window manual says to choose a place that can withstand the weight of the air conditioner without causing excessive noise and vibration. A support-bracket maker's guide says to make sure the supporting surface will safely bear the combined weight of the equipment and all the additional hardware. That guide covers an accessory bracket, not a particular air conditioner, and it says the bracket should only be installed by professionals. It also says to examine the support at least every three months for any sign of instability.

A heat-pump window manual says the top rail must be securely fastened with all four screws, for safety reasons. It also says you must use all supplied parts and the installation procedures it describes. A cooling-only manual from the same maker warns that if the installation bracket is damaged, there is concern of damage due to falling of unit. Our reading: the manual for your exact model is the mounting instruction, and a bracket or sill that looks weak is a reason to call a professional.

On power, the same heat-pump manual says not to modify the cord length or use an extension cord to power the unit. It warns that aluminum house wiring may present special problems and tells you to consult a qualified electrician. The cooling-only manual says that if the outlet is not adequately grounded, a qualified electrician should install the proper receptacle. This page gives no wiring steps, because outlet and circuit work is for a licensed electrician.

A tight fit also matters for comfort, since ENERGY STAR says an improperly installed room air conditioner leaks as much air as a 6-square-inch hole. It advises following the instructions and using the insulation materials included with the unit. It also says to make sure the unit is level so the drainage system works. The Department of Energy adds that for window air conditioners you should inspect the window seals to keep cool air from escaping.

Tampa Bay: storms, salt air and taking the unit out

The National Hurricane Center says the Atlantic hurricane season runs from June 1 to November 30. Our reading: a window unit is a movable hole in the wall, so its owner needs a storm plan that covers the unit, the cord and the gap it leaves. ENERGY STAR advises that at the end of the cooling season you remove the room unit if possible, or use an appropriately sized cover. A maker's removal instructions begin by turning the air conditioner off and disconnecting the power cord. The same manual says that in a thunderstorm the power must be cut off to avoid damage to the machine due to lightning. For the central-system side of storm work, see storm preparation for a central system.

The vertical-window manual says not to install the unit in an explosion-hazardous or corrosive environment. Our reading: salt air near the Gulf corrodes metal, and the Midea manuals read for this page do not mention salt air, so ask the seller what the maker says about coastal use. Salt damage to the outdoor coil of a central system is covered in a guide to salt and the outdoor coil.

What to ask before you buy or mount one

1. What kind of window do I have, and which unit type does that maker's manual list for it?
2. Do my window's width and height fall inside the listed range for this exact model?
3. Can the frame, sill and wall carry the unit, and who is mounting it?
4. If it has heat, is that reverse cycle or another source, and what outdoor temperature does the maker list for heat?
5. Does the manual call for a grounded outlet, and who checks the outlet before the unit is plugged in?
6. What is the plan for the unit and the opening when a storm is forecast?