How to Install a Window Air Conditioner?

In this Pick HVAC How-to guide to installing a window air conditioner, you’ll learn the basic installation methods for various window types plus step by step instructions and tips. 

Hot days and warm, humid nights may have prompted you to purchase a window air conditioner. And installing a window AC can be a little more challenging than it might seem. You just open the window, set the AC in place and close the window? No, it’s more complex than that. 

We wrote this guide because readers often search how to install window AC and similar queries. 

Since window air conditioners are usually installed by the user, you need to be informed about some basics as well as the particulars. 

The installation of any window air conditioner will depend on two main factors: (1) the type of window you will be installing it in, and (2) the type of air conditioner you purchase. Let’s unpack what needs to be considered in your situation. 

Pro Tip: Read our Window AC Review and Buying Guide before you buy. It will assist you in choosing the right size for the space and a window air conditioner with the features and performance you desire.

Installing AC in a Sash Window 

A sash window is best described as a window that has one or more moveable panels that open and close vertically – up and down. Some are “single hung”, meaning that only the bottom sash opens by sliding it up. Others are “double hung”, meaning that both sashes will open; the bottom sash opens upward while the top sash opens downward. In either case, the AC will be installed in the lower sash opening.

There are three types of window air conditioners that work best in sash windows: “standard” AC units, “saddle” AC units and “U-shaped” AC units. Here is what is normally involved in installing each type. Several of the steps given for the standard AC unit also apply to the other two types.

1. Standard Window AC Installation

Here are a handful of steps for how to install a window air conditioner in a sash window. 

Measure Your Window Opening Before Purchasing:

Before buying a window AC, lift the bottom sash up and measure the width and height of the opening. Every AC unit will list the minimum height as well as the minimum and maximum width of the opening it will fit in, so be sure to write down both measurements before you go shopping, and take them with you.

Unbox the Unit:

Normally, everything you will need for installation of the window AC will be supplied inside its box. Occasionally, there may be a situation where you will need to make a trip to your local hardware store to get something like foam weatherstripping to make everything fit right for your installation. Be sure to set all pieces nearby so they are easily accessible and will not get misplaced.

Attach Top Mounting Rail:

If your AC unit already has a rail on top, you can skip this step, but many provide this part for you to install. It will probably be a “U”-shaped bar that is the same width as the unit. This is to be fastened to the top of the unit with screws after putting a strip of insulation on the bottom of the “U”. There should already be holes in the top of the unit for these screws to match with those in the bottom of the rail.

Pro Tip: You might want to set the AC on a countertop or workbench to attach the top rail and side panels without stooping over. If you’re concerned about scratching the surface, place an old towel on top of it, and the AC on the towel.

Attach Side Accordion Panels:

There are plastic or vinyl accordion panels that are designed to fill the space between your AC unit and the side of the window opening once the unit is in place. These are normally marked “R” and “L” for the right and left sides of the AC when looking at it from its front. These panels slide down the side of the AC in grooves made for them, then the top and bottom frame of the panels slide sideways into the top and bottom rails of the AC. Some units may not have the grooves on the sides, but will provide screws to attach these panels to the sides of the unit. 

Prepare the Window Opening:

As you look at your closed window, you will see that the bottom sash fits behind a lip on the bottom sill of the window frame. When you install your AC unit into the open window, its bottom rail should fit where the bottom sash was when it was closed. The top rail will be against the inside of the bottom of the sash when it is open part way. Normally, this will result in the AC unit having a slight tilt toward the outside of your house. This is important, because evaporated water from your room as well as rain needs to be able to run away – not toward your window.

Rare exceptionIf the design of your window is such that this will not be the case, you might have to cut a piece of wood to lay behind the window sill so the AC unit will slant slightly outward after it is installed. This will usually not be a problem with most modern windows, but some older styles were made before window AC’s were common, and so a small modification might be in order. If it is provided, you may be instructed to place a strip of insulation in the space where the bottom rail of the AC unit will sit on the sill.

Large units: Window ACs 15,000 BTUs and larger can weigh a lot. For very heavy AC units, your instructions may have you install some kind of brace or shelf under the outside part of the unit to support its weight. It may be provided, and it should be installed now, since the window opening will be blocked once the AC is in place. If not provided, there are several kinds and sizes available online or in your local big box store. As a final part of this step, check to see if you have another strip of insulation to put on top of the top rail where the sash will contact it. This insulation will create a better seal – keeping out air drafts and bugs too.

Insert the Unit in the Window Opening:

Two notes of caution on this step.

(1) Your window AC is heavy – anywhere from 50 to 140 pounds. It’s best to have a helper when placing it into the window opening. Not only can it be difficult to lift, but it must be carefully positioned on the window sill. It will also be overbalanced when it is placed on the sill and easily dropped out the open window. Be wise and enlist a helper.

(2) When handling the AC unit, avoid even touching the back side if possible. This is where the condenser fins are. They are very sharp and very fragile. Not only will they cut your fingers, but if they are bent, it will affect the efficiency of the unit’s ability to get rid of hot air to the outside.

Pro Tip: Wear a pair of good-fitting leather or mechanic gloves for hand protection in case you do get your fingers on those fins! You’ll be glad you had the gloves on.

OK – with your helper, position the AC unit into the center of the window opening so the bottom rail sits into the space where the sash was when it was closed. While holding the top of the unit so it can’t fall out, lower the sash until it is right down on the unit on the outside of the top rail. The unit should now be where it cannot fall out because it is “trapped” between the window sill on the bottom and the window sash on top. Check again to make sure the AC unit is tilted slightly outward for proper drainage. If not, make whatever adjustments are necessary.

Secure the Unit and Window:

As your AC unit now sits, all it wouldn’t take much for it to tumble out the window is for the sash to be raised slightly. Many units provide screws to be screwed through holes in the top rail into the bottom edge of the window sash. This will keep the window from being raised and keep the top of the AC unit secure. 

Many AC units also provide a small “L”-shaped bracket to screw the sash track tightly down against the top of the sash. This gives you double security that the sash cannot be raised.

However, if you live in an apartment or some other rental dwelling, you may not want to drill holes in the window sash and jeopardize your security deposit. Here are some alternatives to using screws in the sash.

  1. Cut a board or dowel that will fit tightly in the sash track between the top edge of the sash and the inside of the top of the frame.
  2. If your window is made of vinyl, put a small “C” clamp on the edge of the sash track right above the sash, preventing it from going up.
  3. Look online or in stores for the various kinds of sash window stops, clamps and bars that will keep your window closed and your AC unit secure.

Open Accordion Panels, and Seal/Secure Them:

The vinyl accordion panels that you inserted in the sides of the AC unit at the beginning should now slide open on each side to block the open areas beside the AC. Ideally, they should slide right alongside the AC where it contacts the sill below and the sash above. They may need to be secured with screws or tape to keep them in place. Check all around the AC unit to see if light is coming in and seal with tape or insulation if needed.

Plug In and Enjoy!

Your window air conditioner should be ready to use. Plug it into the appropriate power receptacle, set programs, timers and temperature, and enjoy the benefits of a cooler, less humid room.

2. “Saddle” Window AC

Saddle Window AC

The saddle window AC unit is made to fit over the window sill and hang mostly below the window opening, just as a saddle for a horse hangs over the horse’s back. Here are steps to install this type of unit.

Measure Your Window Opening Before Purchasing:

The only manufacturer of this type of window AC is Soleus Air, and the four models are all the same physical size. Your window opening can be 24” to 48” wide, no less than 16” high, and the total width of your sill can be no more than 11”.

Unbox the Unit:

All necessary parts are included, so be sure to unpack everything and set parts aside.

Insert Supports:

There are two adjustable supports that screw into the unit that will rest against the exterior of your house below the window. Locknuts will hold them in place after they are adjusted.

Prepare the Window Opening:

There is a high density foam block that is to be placed on the center of the window sill where the AC unit will rest. Remove the adhesive backing and stick it firmly on the sill.

Insert the AC and Adjust:

With a helper (this unit weighs 80 lbs.) carefully place the AC unit over the window sill, sliding it outward until it contacts the inside of the sill beneath the opening. The unit will be greatly overbalanced outward, so hold down on the inside portion while adjusting the supports against the exterior of your house. The unit should have about a ½” outward tilt. Tighten the locknuts against the unit when adjusted.

Install Window Sliders:

There are window slider brackets to be stuck on each side of the unit in line with where the sash will be when it closes down on top of the unit. Window sliders can now be slid into the brackets and then opened sideways into the sash track on each side of the window frame.

Close and Secure the Sash:

Place the provided adhesive strip of insulation across the top of the AC unit and sliders to seal the sash when it is closed. Close the sash. Secure it with clamps, adjustable bars or some other method of keeping the window sash from being opened.

Plug In and Enjoy!

Plug in the unit, set programs, timers, temperature and enjoy a cool room.

3. “U-Shaped” AC Units

U-Shaped AC Units

The U-shaped AC unit sits on the sill of a sash window, but rather than having the sash close down on top of it, it will close down inside the U-shape of the unit. The result is that the window closes most of the way down, leaving much smaller open spaces to seal on the sides of the unit. Here are the installation steps for this type of AC unit. These units are also very secure – less likely to fall out than standard window air conditioners.

Did you know? These U shaped window air conditioners are also called over the sill air conditioners due to their shape.

Measure Your Window Before Purchasing

Midea or Nada:

You’ve got one brand option. The only manufacturer of this type of window AC is Midea. It is offered in three models, all of which have the same physical dimensions. They will fit a sash window that has an opening between 22” and 36” wide and at least 13 ¾” high.

Unbox the Unit:

All parts needed for installation are inside. Set them aside within easy reach.

Assemble the Support Bracket:

This AC unit will rest on a provided support bracket placed on the window sill. It has side extension arms that must be adjusted outward according to the width of your window.

Install the Support Bracket:

The support bracket should be installed over the sill with the front of the bracket against the inside of the window frame. On the outside of the house, there are braces that will angle back to the side of the house. They should be adjusted so the support bracket has a slight outward tilt. Center the support bracket on the window and adjust the side extension arms out into window track. Secure the extension arms to the side of the frame with screws.

Insert the AC and Adjust:

With a helper (this unit weighs about 70 lbs.) place the AC into the window, making sure that the channels on the bottom of the unit align with the bracket. While holding the unit, partially close the window into the U-shape and square the unit with the window. Now install the “open window” brackets to each side to secure the unit from side to side movement.

Seal and Secure:

There are special side hinges which will hold foam pieces in place between the unit and the side of the window opening. You cut these to fit your particular window. Next, you slide “anti-tip” brackets out to contact the window frame on each side and secure them with screws.

Close the Window and Seal:

Close the window into the U and extend the integrated window locks to keep the window from being opened. There is also a small “L” bracket that can be installed at the top of the window in the track for additional security. Install a provided strip of foam insulation between the two sashes, check for other gaps and seal with foam or tape as needed.

Plug In and Enjoy!

Plug the unit into a nearby receptacle, set programs, timers, temperature and enjoy quiet cooling.

Learn More about Saddle ACs and U Shaped ACs

Because we’re on top of the window air conditioner industry like nobody else, we’ve completed a Reviews, Prices and Buying Guide for the Soleus Air and Midea window air conditioners discussed above. Here it is.

Installing AC in a Sliding or Casement Window – Vertical Window ACs

There are only a few AC models specially designed for sliding windows, and of course, Pick HVAC has prepared a complete Buying Guide to the 6 Best Window ACs for a Sliding Window.

These units are narrower and taller than the standard window AC unit. The “saddle” AC unit (above) will fit over the sill of an open sliding window, and the manufacturer also offers a separate kit to fill in the space above the unit. Casement windows are those that have a crank to pivot them open and closed.

What if you Have a Sliding or Casement Window and a Standard AC?

If they don’t buy an AC specifically for a sliding or casement window, most people with sliding or casement windows find a way to adapt their standard AC unit so it will meet their needs. The three primary concerns are: (1) providing a way to prevent the unit from tipping out the window, (2) closing off the opening above the unit, and (3) sealing any gaps or cracks around the unit to prevent insects and dust from entering into the room. Here are a few suggestions, all of them requiring some handiwork:

A sliding window will need to be opened until the unit is installed, but a casement window will need to be completely removed.

  • A simple wood frame can be constructed to fit inside the opening. It needs to be fastened to be secure, and it must provide a solid piece across the top for the top rail of the AC unit to rest against.
  • The AC can be supported by a board or metal bar that fits across the opening for the top rail to rest against. These must be secured so they cannot shift and allow the unit to tip out the window.
  • After a sliding window is closed against the unit, the opening above can be closed off with glass or plexiglass cut to size. In a casement window, all spaces above and beside the installed unit will need to be closed off. Plywood and plexiglass are commonly used materials.
  • All gaps and cracks can be sealed with foam strips and tape, both of which are readily available at local hardware stores.

There are many window AC’s available to fit your hot weather needs. Installing them can be quite simple or fairly difficult, depending on your windows and the units you want to use. Whatever your situation, you can have cool air in your home this year provided by a window air conditioner.

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