How to Install a Window AC Unit? | 8 Steps, Done Right

A window AC unit installs in eight steps: check the window and outlet, fit the mounting rail, set the unit tilted slightly outside, and seal every gap.

Set a window air conditioner too flat and condensed water drips into your room instead of running outside. Tilt it too far and the compressor struggles.

This guide covers the full install in the order a first-timer should work through it, including the support hardware most people skip and the mistakes that let a unit fall out of a second-story window. Every step below comes from GE Appliances and LG installation documentation, plus the safety practices both brands repeat.

Check The Window And Outlet Before Anything Else

The install only works if the window sits within reach of a grounded three-prong wall outlet and the window itself opens to fit the unit’s mounting kit.

Window AC instructions assume a double-hung window — one that slides up and down in two sashes. Sliding, casement, and awning windows need a special bracket kit, and some simply won’t hold the unit safely. Measure the opening against the width range printed in your unit’s manual before you unbox anything.

Power matters just as much. GE’s installation guide requires a standard three-prong grounding outlet, and the grounding prong must never be removed to fit an old two-slot receptacle. Plug the unit directly into the wall. Extension cords and power strips are a fire risk on a load this heavy, so if the cord won’t reach, that window is the wrong window.

Attach The Mounting Rail And Side Panels

The mounting rail goes on first, then the accordion side panels bolt to the case with screws before the unit ever leaves the floor.

Both GE and LG follow the same sequence. Cut the window sash seal to the width of the opening and set it aside. Attach the accordion panels to the case with the screws in the kit — — and install the top mounting rail so it faces the back of the unit.

These panels do more than look tidy. They hold the unit centered and square, which is what keeps the tilt accurate once the window closes on top of it.

Set The Unit In And Get The Tilt Right

Center the unit in the opening with the bottom sash raised, then lower the sash behind the top mounting rail so the window itself holds the unit in place.

Now the slant. GE’s instructions call for the same: level or slightly tilted to the outside, checked with a level, never tilted inward and never steeply angled.

Secure, Seal, And Brace The Unit

Extend the side curtains to the window frame, drive the fasteners, and close off every gap with the foam and weatherstripping in the kit.

Drill pilot holes with a 1/8 inch bit, then fasten the side bracket and the window lock bracket — that lock bracket stops the lower sash from opening on its own, which matters more than it sounds. Slide the window lock bracket into place and check that the sash cannot be lifted from outside.

Support comes next. Window AC units need bracing underneath from metal brackets, mounting rails, or a support bracket, and shims must be secured so they can’t work loose. GE’s steps finish with foam gaskets stuffed at the bottom and top of the opening to block air and insects.

Two banned shortcuts: never wedge bricks, wooden blocks, or phone books between the unit and the sill, and never rest anything on top of the AC. Both get named in manufacturer warnings because both send units through windows.

If you’re still comparing models before you install, this tested breakdown of top window AC units for US homes covers capacity, noise, and fit.

Once everything is fastened, LG’s support page lays out the same window air conditioner installation steps in full, including the bracket order for their units.

What Installation Costs, And When To Pay Someone

Manufacturer documentation doesn’t publish a universal install price, and prices vary by unit size, window type, and region.

DIY installs cost nothing beyond the kit in the box. Professional installation is worth considering when the window is above the first floor, the opening isn’t double-hung, the unit weighs more than you can safely lift and hold while fastening, or the window frame can’t take bracket screws. Any of those conditions makes paid help the cheaper option.

One more install-day habit: keep the grounding prong, keep the unit off extension cords, and check the tilt with a level before you tighten the final screws.