How to Install a Window AC Unit: A Step-by-Step Guide

A window air conditioner installation can be an affordable way to cool a room, but if you’ve never installed one before, you might wonder where to start. This guide walks you through window AC installation step by step, including how to pick the right window, size the unit correctly for the room, and seal everything up so you stay cool and efficient.

Before You Start

Installing a window AC unit can be a straightforward process if you follow the right steps. Here’s what to do first to help make this project go smoothly.

Choose the Right AC Unit

Before buying a new window AC for your home, follow these four steps:

1. Room Size

Measure the space and check the unit’s BTU rating to make sure it’s powerful enough. The table below has helpful square footage details to ensure the unit you purchase will work well in the room where you want to install a window AC unit:

Room Size (sq ft)	Recommended BTU
Up to 150	5,000 BTU
Up to 250	6,000 BTU
Up to 350	8,000 BTU
Up to 450	10,000 BTU
Up to 550	12,000 BTU

Pro tip: Increase BTU capacity by about 10 percent for very sunny rooms and decrease by about 10 percent for shaded rooms. Oversizing a unit is not more effective; a too-large unit short-cycles, which means it turns on and off too frequently, doesn't remove humidity properly, and wears out faster.

2. Power Source

Make sure the outlet you plan to use can handle the AC unit’s power requirements. Most units under 12,000 BTUs can be plugged into a standard home outlet, but units over that rating require a dedicated 120-volt outlet. Check the manufacturer's manual for the unit you plan to purchase for the details on the voltage you'll need.

3. Energy Efficiency

Look for an ENERGY STAR^®-rated model to save on electricity costs.

4. Window Type

Some units work best with double-hung windows, while others fit sliding or casement windows.

Next, we'll provide details on different window types so you can choose a compatible unit for your window AC installation.

Pick the Right Window

Not every window works for a window AC unit. Before you buy or install, confirm your window type is compatible:

• Double-hung windows: The standard. Almost every window AC unit is designed for this type, in which the bottom sash lifts up, and the unit rests on the sill.
• Single-hung windows: Also compatible with most standard units, since only the bottom sash needs to open.
• Casement or sliding windows: Not compatible with standard units. You'll need a special adapter kit designed for casement or sliding installations, or a different cooling solution.

Also, make sure that the window you choose is close to a grounded electrical outlet. Window AC units should never be used with an extension cord.

Installing a Window AC Unit in 6 Steps

Now that you've purchased the right AC unit and window to use, you're ready to start installation. Here's what to do next:

1. Gather Your Tools and Materials

Use this list of tools for your project. Having the right tools on hand will make your specific window AC installation go more smoothly.

• Window AC unit
• Measuring tape
• Screwdriver (Phillips and flathead)
• Level
• Foam insulation strips (usually included with the unit)
• Weatherproof sealant (for extra insulation)
• Support brackets (if required by your AC’s model)
• Window locking bracket (for safety)
• Special adapter kit for units installed in casement or sliding windows

2. Prepare the Window

Since you’ve selected the right window in your home for installing a new AC unit, simply follow these steps next:

• Open the window and clean the sill to remove dust and debris.
• Measure the window opening to make sure the AC unit will fit.
• If your unit has accordion side panels, attach them before lifting the unit into place. This is easier to do on the ground than at the window.
• Apply weatherstripping to the windowsill where the unit will rest, if your unit didn't come with a pre-attached seal.
• Check the filter and make sure it’s in place.

3. Start the Window AC Unit Installation

Now it’s time to lift the unit into place. Window ACs are heavy, so ask for help if needed.

• Lift the AC unit and place it on the windowsill, tilting the back end slightly downward for condensation drainage.
• Lower the window sash slowly until it rests against the top of the unit.
• Center the AC unit so it sits evenly in the opening.

Do You Need a Support Bracket?

Support brackets take the weight of the AC unit off the window sash, protect the window frame, and prevent the unit from falling out. A bracket is strongly recommended if:

• Your unit weighs more than 80 pounds
• You're installing on a second-story or higher window
• Your window frame is older or shows any signs of wear and tear
• Local ordinances require it (some cities do, especially for upper-story installations)

To install a bracket, you’ll want to:

• Position the bracket on the windowsill according to the manufacturer’s instructions.
• Use a pencil to mark where the screws will go.
• Drill pilot holes if needed, then secure the brackets with screws.
• Once installed, rest the AC unit on the brackets for some added support.

4. Secure the AC Unit

Once the unit is centered, secure it to the window frame:

• Use screws to fasten the unit in place, following the manufacturer’s instructions.
• Make sure the unit sits firmly against the window sash and doesn't shift.
• Install a window locking bracket on the upper sash to prevent someone from lifting the window above the AC unit. This bracket is a safety essential most homeowners overlook, and it's especially important if you have children or pets.

5. Seal and Insulate the Sides

Once the AC is secured, fill in the gaps around it. A tight seal is what separates efficient cooling from a unit that runs constantly and wastes energy.

• Extend the side panels until they touch the window frame, then screw or clip them into place.
• Use foam insulation strips to seal any openings between the unit and the frame.
• Cut foam strips to fit vertically where the accordion panels meet the window frame, a spot that often gets missed.
• If needed, apply weatherproof sealant around the edges for extra insulation.

6. Plug in and Test Your AC Unit

Now that your AC is installed, it’s time to power it up:

• Plug the unit into the nearby dedicated outlet. Do not use extension cords, as they can overheat and become a fire hazard.
• Turn it on and check for any vibrations or unusual noises.
• Adjust the settings to your desired temperature and let the unit run for at least 10 minutes to make sure it’s cooling properly.

Alternative Options for Cooling Your Home

Not every home or window is suited for a window AC unit. If you need an alternative way to stay cool, here are a few options to consider:

• Portable AC: A mobile cooling unit that can be placed in different rooms and vents through a window or sliding door. It’s simple to set up but generally less efficient than a window AC.
• Wall AC unit: Similar to a window unit but installed directly into an exterior wall. It requires cutting a hole in the wall, but it frees up window space and provides a permanent cooling solution.
• Central air conditioning: The most effective way to cool an entire home. It requires ductwork and professional installation but offers consistent temperature control throughout the house. See how air conditioners work to learn more.
• Ductless mini-split system: A high-efficiency cooling system designed for targeted cooling in specific rooms. It needs professional installation but provides long-term energy savings and improved comfort.
• Evaporative cooler: Also known as a swamp cooler, this unit adds humidity to the air and functions best in dry regions. It's an energy-efficient alternative, but less effective in humid areas.

Protect Your AC System with a Home Warranty

Installing a window AC unit doesn’t have to be complicated. With the right tools and steps, you can do it yourself in no time. You can extend a unit’s life with regular AC tune-ups. And once it’s in, you’ll want to know how a home warranty fits into the picture.

For homeowners with built-in cooling systems, First American's HVAC coverage helps take care of covered repairs and replacements when your central AC or HVAC system fails from normal wear and use.

It's easy to learn more about our Home Warranty Heating and Air Conditioning coverage and how HVAC plan limits work.

If you’re ready to cover your systems and appliances, get a free home warranty quote today.

FAQs About Window AC Installation

Can you put a window AC in any window?

Most window AC units are designed to work with double- or single-hung windows. You may be able to install them in windows with casements or sliding windows, but additional tools and hardware may be required.

Do I need a support bracket for my window AC unit?

A support bracket is recommended for any window AC unit heavier than about 80 pounds and for installations on second-story or higher windows. Brackets take the weight off the window sash, prevent damage to the frame, and reduce the risk of the unit falling. Many manufacturers require brackets for larger units.

Does a window AC unit need to be on its own outlet?

Typically, yes. Larger window ACs units, generally those rated 12,000 BTU or higher, require a dedicated 120-volt outlet due to the amount of power they draw. However, most smaller units can use a standard household outlet.

What size window AC unit do I need for my room?

Match the BTU rating to your room size:

• 5,000 BTU cools up to 150 square feet
• 6,000 BTU up to 250 square feet
• 8,000 BTU up to 350 square feet
• 10,000 BTU up to 450 square feet
• 12,000 BTU up to 550 square feet

Increase BTU capacity by about 10 percent for sunny rooms and reduce by about 10 percent for shaded rooms.

Does a home warranty cover window AC units?

Window air-conditioning units may be covered with optional or upgraded coverage, depending on your plan. Standard central air conditioning coverage typically excludes window units. Review your contract for specific coverage, terms, and limitations. Plan options and upgrades vary, so your actual contract governs what is covered.

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The contents of this article are provided for general guidance only. First American Home Warranty does not assume any responsibility for losses or damages as a result of using this information.

Please be aware that if you disassemble a covered system or item, it can invalidate your home warranty or manufacturer's warranty.