How To Install a Window AC Unit In a Horizontal Sliding Window – Step-by-Step Guide

Many people are purchasing air conditioners that fit into a window because they’re cheaper than portable units or central air.

However, most of these units are designed to fit vertically opening windows. 

If you have horizontal sliding windows and want a window AC, you may have noticed that ACs made for this type of window are significantly more expensive. 

Quick Questions Before Starting

It’s possible to adapt ACs made for vertical windows to fit in a sliding window. If you live in an apartment or if you only need an air conditioner a few months of the year, this is a perfect solution for you.

How Difficult is This to Do?

• Installing the AC unit is about a medium difficulty level; however, it’s best to have a friend help you lift your air conditioner and hold it in place.

How Long Does it Take?

• This project will take you approx 1:30.

How Much Do Materials Cost?

• Depending on the supplies you already have, this project will cost you $250-$700.

Items Needed

Here’s a list of all the items you'll need to complete this project and their estimated price. Many people already have some of the tools necessary to complete this project. If not, you can pick them up at any local hardware store.  

How To Install a Window AC Unit In a Horizontal Sliding Window

Before starting on the project, make sure to measure your air conditioner's width and height. Include any vents that might stick out on the sides, as well as the height of the window’s opening.  

Choosing a Window 

The window should be near a three-pronged outlet—using an extension cord may not provide enough power for the unit to function properly. Inspect the windowsill; if it’s old, weak, or shows signs of damage, it may not support the unit's weight. You’ll need to replace the windowsill or select a different window.

Weatherstripping 

Cut a piece the same width as the window’s opening. Follow the instructions to place the weatherstripping correctly and adhere it to the windowsill so that the unit will be on top of it when in place. Firmly press the weatherstripping to help it adhere to the windowsill.

Support Brackets 

With an electric drill or screwdriver, secure the bracket to the ledge of your window with screws. Check to see if your AC needs a tilt or any leveling. Depending on your unit, either use a level to make sure the brackets are straight or angle them between 1/4 and 1/2 inch. 

The instructions included with your air conditioner will show you how to assemble the bracket, if it’s pre-assembled, and where to install the bracket along the windowsill. To prevent the AC from falling out the window on either side, you should place a bracket in the center of the windowsill. 

Depending on your unit's size and weight, the exterior bracket may attach just below your windowsill or further down the side.

Assemble the AC Unit 

You might need to attach air conditioner panels along the sides and top of the unit. The panels will attach with screws; any special tools you need should come with the AC unit. These panels frame the AC and prevent warm air, allergens, and pollution from entering your home. You can add weatherstripping on the sides of the panels to create an air-tight seal.

Closing the Gap

Subtract the height of your unit from the height of your selected window. You can cover this gap with the plastic curtain that comes with your AC unit. Cut the plastic curtain with scissors to cover the gap between the unit and the top of the frame. 

For example, if the unit is 30 inches high and the window opening is 50 inches tall, cut the curtain to 20 inches in length. Slide the curtain into the frame that came with the unit, and attach it to the unit, sliding the frame neatly into the side panels. 

Attach the frame to your air conditioner using a screwdriver; if necessary, some units slide into place.

Enjoy Your AC

Plug your unit in and wait for your room to cool off.

Tips and Safety Considerations When Installing a Window AC Unit In a Horizontal Sliding Window

• When purchasing an AC unit, look for an energy efficiency rating (EER) of 10 or more for better efficiency. It's also important to know the British thermal units (BTUs) of the AC. If your unit doesn't have the correct amount of BTUs, it won't reach desired temperatures, or it'll turn on and off too quickly, leading to an uneven temperature in the space. Check the box to see the manufacturer's recommended room size; you can also find online tools to find how many BTUs your room requires.
  
  
• Some AC units require a tilt to prevent condensation from dripping down the wall inside your home. The tilt should be no more than half an inch, but check the installation guide for the manufacturer's recommendation. Other units don't require a slant; they have a drip pan inside them at an angle to collect the condensation.
  
  
• Window air conditioners weigh between 50 and 100 pounds, so lifting the appliance and holding it in place while securing it with screws definitely requires assistance. You may injure yourself or accidentally drop the unit out the window if you try and install an AC by yourself.

Common Issues 

Be on the lookout for these common problems with installing a window AC in a horizontal sliding window.

Size

The most common problem is installing a unit of the wrong size. The most efficient and effective window air conditioner has a cooling capacity of 20 BTUs per square foot of space in the room. 

If your desired room has high ceilings or is in direct sunlight, you may need a slightly larger capacity—bigger is not always better. An oversized unit in a small room won’t cool better than a smaller unit. A smaller AC running for a longer time is more efficient than a larger unit turning itself on and off repeatedly.

Weight Distribution

It may be tempting to open the window, slide the air conditioner in, and close the window firmly onto the unit. Doing this forces the window and the window sash to bear the weight of the air conditioning unit and can cause damage to the window. When installing your AC unit, take the time to attach brackets to help support and distribute the air conditioner's weight.

Tilting

Air conditioners cool air by removing moisture from it, causing water to pool inside the AC and drain from the back. Some air conditioners have gutters and channels built into them, allowing them to drain when the unit is level. Others require a slight tilt for the AC to drain correctly.

Conclusion 

Window-mounted air conditioning units are a cost-effective alternative to central air that can provide immediate relief on a hot day. Make sure that your window unit is the correct size for your window and has adequate support. Ask a friend to help you to prevent personal injury and damage to the unit.