Installing a window air conditioner takes 20–45 minutes and requires no special tools — a Phillips screwdriver and a tape measure are all most installations need. The process involves measuring your window, preparing the opening, placing the unit, extending the side panels, lowering the sash, and sealing the gaps.
This guide walks you through every step, covering standard double-hung windows, sliding windows, and common problems like uneven sills, narrow openings, and upper-floor safety.
Before You Start: Pre-Installation Checklist
Before unboxing your new window AC, verify these four things:
1. Window Compatibility
Measure your window opening with the sash fully raised:
| Measurement | What to Measure | Minimum Required |
|---|---|---|
| Opening Width | Inside frame edge to inside frame edge | Check AC specs (typically 23"–36") |
| Opening Height | Sill to bottom of raised sash | Check AC specs (typically 13"–20") |
| Sill Depth | Inside edge of sill to where window closes | 4" minimum (most units need 4"–6") |
| Sill Condition | Level, sturdy, no rot or damage | Must support unit weight |
Measure twice. The #1 reason people return window ACs is buying a unit too wide or tall for their window. Every AC model lists minimum and maximum window dimensions in its specs — check these against your actual measurements before purchasing.
2. Electrical Requirements
| AC Size | Circuit | Outlet | Amp Draw |
|---|---|---|---|
| 5,000–8,000 BTU | 15A, 120V | Standard NEMA 5-15 | 4.5–7.5A |
| 8,001–14,000 BTU | 15A or 20A, 120V | NEMA 5-15 or 5-20 | 7.5–12.5A |
| 14,001–18,000 BTU | 20A, 120V | NEMA 5-20 (T-slot) | 12.5–15A |
| 18,001–25,000 BTU | 20A–30A, 230V | NEMA 6-20 or 6-30 | 8–13A |
The AC must plug directly into a wall outlet. Extension cords are not safe for air conditioners — they cause voltage drops that overheat the compressor motor and create fire hazards. If the outlet is too far away, have an electrician install a closer one ($150–$300).
3. Location Selection
Pick the right window for installation:
- Choose a window near the center of the room for even air distribution
- Avoid windows directly above electronic equipment — condensation can drip
- Select a shaded window if possible — direct sun on the condenser reduces efficiency by 5–10%
- Ensure outdoor clearance — the back of the unit needs at least 12–20" of unobstructed space for airflow
- Verify no bushes, walls, or structures block the outdoor side
- Upper floors (3rd+): Most building codes and HOAs require a support bracket for units above the 2nd floor
4. Tools and Materials
Most window ACs include everything needed for basic installation. Have these items ready:
| Item | Included with AC? | Cost If Not |
|---|---|---|
| Side panels / accordion extensions | Yes | — |
| Mounting hardware (screws, L-brackets) | Yes | — |
| Foam seal strip (top) | Yes | — |
| Phillips screwdriver | No | $5–$10 |
| Tape measure | No | $5–$10 |
| Level (optional but recommended) | No | $5–$15 |
| Weatherstrip foam tape (extra seal) | No | $5–$8 |
| Support bracket (for heavy units or upper floors) | No | $15–$40 |
| Rigid foam insulation board (winter seal) | No | $8–$15 |
Step-by-Step Installation: Standard Double-Hung Window
Step 1: Unbox and Inspect (5 minutes)
Remove the AC from the box and verify all parts are present. Check the packing list — you should find the main unit, two side panel extensions (accordion-style or rigid), a foam seal strip, mounting screws, L-brackets, and the installation guide.
Inspect the unit for shipping damage. Look for bent fins on the condenser coil (the aluminum grid on the back). A few bent fins are normal and won't affect performance, but large areas of damage indicate rough handling — consider returning.
Save the box and styrofoam inserts. If you remove the AC for winter, the original packaging is the safest way to store it. It also makes the unit easier to carry.
Step 2: Prepare the Window (5 minutes)
Open the window fully by raising the lower sash as high as it will go. Clean the windowsill of dirt, debris, and old weatherstrip residue. If the sill has peeling paint or is uneven, sand it lightly so the foam seal makes good contact.
If your window has a screen, remove it entirely. You can't install a window AC with the screen in place. Store the screen for reinstallation when you remove the AC.
Place the foam seal strip (included with most ACs) along the top of the lower sash rail. This creates an air-tight seal between the sash and the AC unit when you lower the window onto the unit.
Step 3: Attach Side Panels to the Unit (3 minutes)
If your AC has slide-out accordion panels, extend them to roughly match your window width. Some models have the panels pre-attached; others require you to slide them into tracks on the sides of the unit.
Don't fully extend the panels yet — you'll adjust them to fit once the unit is in the window.
Step 4: Place the Unit in the Window (5 minutes — two people recommended for units over 60 lbs)
This is the trickiest step and the one where injuries happen. Here's how to do it safely:
- Position yourself inside the room, facing the open window
- Lift the unit by its base, not by the front panel or controls
- Rest the bottom of the unit on the windowsill with about 1/3 of the unit extending outside
- Slide the unit outward until the bottom lip of the unit's frame rests securely on the sill
- The unit should tilt very slightly backward (toward the outside) — about 1/4" to 1/2" — so condensation drains outdoors rather than into your room
For units over 60 lbs, use two people. One person guides the unit into the window while the other supports it from below (outside) or holds it steady. Dropping a window AC can cause serious injury and damage the unit beyond repair. Better yet, install a support bracket first (Step 4B below).
Step 4B (Optional): Install Support Bracket First
If you're on an upper floor, have a heavy unit (60+ lbs), or want extra security, install a support bracket before placing the AC:
- Position the L-shaped bracket below the window on the exterior wall
- Level the bracket and mark screw holes
- Drill pilot holes into the wall (not into the window frame)
- Secure with appropriate fasteners (lag bolts for wood, masonry anchors for brick/concrete)
- Place the AC on the bracket, which bears the weight instead of the windowsill
Step 5: Lower the Window Sash (2 minutes)
Carefully lower the window sash onto the top of the AC unit. The sash should press firmly against the foam seal strip you placed in Step 2. This creates the primary air seal between indoor and outdoor air.
The sash should sit flat on the unit without gaps. If there's a gap, the foam strip may be too thin — add weatherstrip tape until the seal is snug.
Step 6: Extend and Secure Side Panels (5 minutes)
Extend the accordion side panels outward until they contact the window frame on each side. The panels fill the gap between the AC unit and the window frame edges.
Secure the panels to the window frame using the included screws. Most kits provide small L-brackets that screw into both the panel and the window frame. This serves two purposes: it seals the gap against air infiltration and prevents someone from pushing the panels in to open the window (security).
Step 7: Secure the Window Sash (3 minutes)
This is a critical safety and security step that many people skip. You need to prevent the window sash from being lifted, which would release the AC unit.
Method 1 (Included hardware): Most ACs include an L-bracket that screws into the upper sash track, creating a stop that prevents the sash from being raised. Install this on the inside of the upper sash channel.
Method 2 (DIY alternative): Cut a piece of wood (a 1x2 works well) to the exact length of the upper sash track and wedge it between the top of the lower sash and the top of the window frame. This physically prevents the sash from moving.
Step 8: Seal All Gaps (5 minutes)
Air leaks around the window AC waste energy and let in bugs, dust, and outdoor noise. Seal these common leak points:
| Gap Location | Sealing Method | Material |
|---|---|---|
| Top of unit to sash | Foam seal strip (included) | Closed-cell foam |
| Side panels to frame | Weatherstrip tape | Adhesive foam tape |
| Between side panels and unit | Included seal or caulk rope | Foam or rope caulk |
| Below sash and above AC | Additional foam strip if gap exists | Adhesive foam tape |
| Between upper and lower sash | Sash lock should close this | Lock the sash |
Rope caulk is your best friend. This putty-like sealant ($3–$5 per roll) presses into any gap and removes cleanly without damaging paint or wood. Use it around the edges of side panels and anywhere foam strips don't quite seal.
Step 9: Plug In and Test (2 minutes)
Plug the AC directly into a wall outlet. Turn it on to the highest cooling setting. Within 5 minutes, you should feel cold air blowing from the front vents.
Check for these indicators of correct installation:
- Cold air blows from front vents (indoor side)
- Warm air blows from the back (outdoor side)
- No water dripping indoors
- No unusual vibrations or rattling
- Side panels are secure and don't flex when pushed
Installation Variations
Sliding (Horizontal) Windows
Sliding windows move side to side, so the standard horizontal installation won't work. Options:
- Casement/vertical window AC: Purpose-built units that install vertically in the opening. See our casement window AC guide.
- DIY adapter panel: Create a plexiglass or plywood panel that fills the unused window space above or beside the AC. Cut a hole in the panel that matches the AC's profile.
- Through-the-wall conversion: If you own the home, a through-the-wall installation eliminates window compatibility issues entirely.
Casement (Crank-Out) Windows
Standard window ACs don't work with casement windows because the sash swings outward rather than sliding up. Your options are limited to casement-specific AC units (vertically oriented) or portable ACs with window exhaust kits. See our casement window AC guide for specific models.
Vinyl and Aluminum Windows
Modern vinyl and aluminum window frames are thinner than wood frames, which can cause stability issues. Tips:
- Use the L-bracket security hardware rather than relying on the sash weight alone
- Add extra foam weatherstrip to compensate for thinner frames
- For very thin frames, a support bracket is recommended even for lighter units
- Be careful with screws — vinyl frames crack more easily than wood
Common Installation Mistakes (And How to Avoid Them)
Mistake 1: No Backward Tilt
The problem: Unit is perfectly level or tilts slightly forward (toward the room). Condensation can't drain outside and instead pools inside the unit or drips onto your floor.
The fix: The unit should tilt 1/4" to 1/2" toward the outside. Many units have adjustable feet or built-in tilt. You can also place a thin shim (a popsicle stick works) under the front of the unit.
Mistake 2: Using an Extension Cord
The problem: Extension cords cause voltage drops, especially with the high amp draw of a compressor starting up. This can overheat the cord (fire risk), damage the compressor, and void your warranty.
The fix: Plug directly into a wall outlet. If the outlet is too far away, have an electrician install a new one. The $150–$300 cost is far less than a house fire or replacing a burned-out unit.
Mistake 3: Not Sealing the Side Panels
The problem: Accordion side panels left unsealed allow hot air to infiltrate around the unit, reducing efficiency by 10–20%. They also let in bugs, dust, and allergens.
The fix: Screw the panels to the window frame, then seal all edges with foam weatherstrip tape or rope caulk.
Mistake 4: Over-Tilting the Unit
The problem: Excessive backward tilt (more than 1/2") can cause the unit to slide out of the window and can prevent the internal pan from distributing condensation properly across the condenser coil.
The fix: A 1/4" to 1/2" tilt is ideal. Use a level to check — the bubble should be just slightly off-center toward the outdoor side.
Example: First-Time Installation in an Older Home Tom bought a Frigidaire 8,000 BTU unit for his 1920s home in Baltimore. The old wood window frames had slight warping, so the sash didn't create a perfect seal. His solution: extra-thick closed-cell foam tape along the sash contact point, rope caulk around the side panels, and a window lock bracket to secure the sash. Total extra material cost: $12. The installation took 40 minutes and eliminated all air leaks.
Example: Upper-Floor Apartment Installation Jessica lives on the 4th floor of a Chicago apartment building. Her building requires support brackets for all window ACs. She installed a universal AC bracket ($28 on Amazon) first, then placed her LG 10,000 BTU unit on it. The bracket bears the weight, the windowsill just acts as a guide. Her building manager approved the installation after verifying the bracket was lag-bolted into the brick facade.
Example: Installing a U-Shaped AC David chose a Midea U-Shaped 8,000 BTU unit for his bedroom. The installation is slightly different: the unit's bracket mounts to the sill first, then the U-shaped chassis drops into the bracket with the middle channel straddling the closed lower sash. The window sash actually closes through the channel, so the window can open and close freely. Installation took 35 minutes and David describes the noise difference as "dramatic" compared to his previous standard window unit.
Removing Your Window AC for Winter
If you live in a cold climate, removing the window AC for winter saves 5–10% on heating bills by eliminating a major air leak. Here's the process:
- Turn off and unplug the unit 24 hours before removal (lets refrigerant equalize)
- Remove the window sash lock bracket and security hardware
- Raise the window sash fully
- Slide the unit inward carefully (same two-person lift for heavy units)
- Clean the unit: Vacuum the coils, wash the filter, wipe down surfaces
- Drain any residual water by tilting the unit slightly
- Store in a climate-controlled area if possible, upright (never on its side)
- Reinstall the window screen and close the window fully
If you leave the unit installed year-round, use an insulated outdoor cover ($10–$20) to block cold air from entering through the unit. Also cover the indoor side with rigid foam insulation board cut to fit, sealed with weatherstrip tape. This reduces heat loss but doesn't eliminate it — removal is always better.
Electrical Safety Essentials
| Safety Rule | Reason |
|---|---|
| Plug directly into wall outlet | Extension cords cause voltage drops and fire risk |
| Dedicated circuit preferred | Sharing a circuit with other appliances may trip the breaker |
| GFCI outlet NOT required | Window ACs don't need GFCI (unlike bathroom appliances), but it won't hurt |
| Check outlet condition | Loose outlets cause arcing — replace if the plug feels wobbly |
| No power strips or multi-taps | Same issue as extension cords — insufficient wire gauge |
| Wait 3 minutes after turning off before restarting | Allows compressor pressure to equalize, preventing startup strain |
Key Takeaways
- Measure your window first — width, height, and sill depth must match the AC's specified range.
- Never use an extension cord — direct wall outlet only, every time.
- Slight backward tilt (1/4"–1/2") ensures condensation drains outside.
- Seal every gap with foam tape, rope caulk, or included weatherstrip. Air leaks cost 10–20% efficiency.
- Support brackets are essential for upper floors and units over 60 lbs.
- Secure the sash with the included L-bracket or a wood shim to prevent the window from being raised.
- Remove for winter if possible — it saves 5–10% on heating costs.
- The whole job takes 20–45 minutes with basic tools. No HVAC skills needed.