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Installation Deep-Dive: What It Takes to Install a 15,000 BTU Through-The-Wall AC

Savvy tone: “Yes, you could DIY—just know the traps before you grab a drill.”

Installing a through-the-wall AC can feel like you’re one bad measurement away from turning your home into Swiss cheese. But with the right prep, the right tools, and the right expectations, installing a 15,000 BTU through-the-wall unit is absolutely achievable—whether you’re doing a near-DIY job or supervising a professional.

And if you’ve ever compared through-the-wall vs. window AC vs. split system setups, you already know the through-the-wall option gives you that sweet spot of power, permanence, and a cleaner aesthetic—all without hogging your window or flooding your room with noise.

This guide breaks down everything: structural considerations, the wall sleeve, wiring, drainage, insulation, weatherproofing, and how to avoid the mistakes that turn clean installs into “why is there water on my floor?” nightmares.

We’ll also tie everything back to The Furnace Outlet’s pillar page, emphasizing space-saving design, because that’s one of the biggest perks of installing a built-in unit over a bulky window AC.

Let’s dive in—tools first, regrets never.

1. What Makes Through-the-Wall AC Installation Different?

Before we get into studs, sleeves, and electrical panels, it helps to understand what makes this type of installation unique.

Unlike window ACs, a through-the-wall unit:

Requires cutting into an exterior wall
Needs permanent framing for structural support
Uses a wall sleeve to contain and support the unit
Must be sealed for weatherproofing
Requires a proper drainage slope
Needs safe electrical compatibility
Is designed for multi-season use

The payoff?
A clean, space-saving design that doesn’t hog your window, block light, reduce security, or look like a giant metal rectangle hanging out of your house.

The Furnace Outlet highlights this space-saving benefit in its pillar page—and this installation guide will show you exactly what goes into achieving that sleek built-in look.

2. Tools & Supplies You’ll Need

Make sure you have these before you begin:

Major Tools

Stud finder
Level
Reciprocating saw or oscillating multi-tool
Drill + bits
Tape measure
Hammer
Pry bar
Utility knife
Safety gear (gloves, goggles, dust mask)

Materials

Wall sleeve (manufacturer-specific)
2×4 lumber (for framing the opening)
Exterior-grade screws
Shims
Caulk (exterior-grade)
Expanding foam insulation
Flashing / drip cap
Weatherproof gasket
Electrical outlet upgrades (if needed)
Support brackets (depending on local code)

Electrical Requirements

A 15,000 BTU unit typically requires:

A 230V dedicated circuit
Proper amperage (often 15A or 20A depending on the model)
A compatible outlet

Always verify with the unit’s specs.
→ Electrical Code Guide

3. Step 1: Choose the Right Location

This isn’t like hanging a picture—you can’t patch a 16" × 26" hole with toothpaste and optimism.

Choose a location that allows:

✔ Access to dedicated electrical power

You don’t want an extension cord, ever.
→ NFPA Electrical Safety

✔ Proximity to an exterior wall

Interior walls won’t work.

✔ No hidden obstructions

Use your stud finder to locate:

Studs
Pipes
Electrical wiring
Ductwork
Horizontal blocking

✔ Correct height

Generally:

Bottom of opening should be 10–14 inches above the floor
Avoid placing directly behind furniture
Keep enough clearance for airflow on both sides

✔ Outdoor clearance

Make sure there’s at least 12–24 inches of space outside for exhaust air to disperse.
→ Energy Efficiency Installation Notes

4. Step 2: Mark and Cut the Wall Opening

This is the moment where your wall goes from “perfectly intact” to “let’s hope this is straight.”

A. Mark the Opening

Use the wall sleeve’s template or measure the exact interior dimensions provided by the manufacturer. Add 1/8" for fitting and adjustments.

B. Remove Drywall First

Cut from the inside:

Score drywall with a utility knife
Remove cleanly in sections
Use caution near stud edges

This gives you a good view of what you’re working with—before you start cutting the exterior wall.

C. Cut the Exterior Wall

Use a reciprocating saw to cut the siding or brick if applicable.
Go slow.
Make sure your cut slopes slightly downward on the exterior side (this helps drainage).

5. Step 3: Frame the Opening with 2×4 Lumber

A through-the-wall AC must be supported properly.
Do NOT rely on drywall or siding to hold the weight of a 70–95 lb unit.

A. Install Header & Footer (Top & Bottom Supports)

Cut 2×4 lumber to fit horizontally between studs.

B. Add Side Supports

Vertical 2×4 pieces help transfer weight to the floor plate.

C. Confirm Squareness

Use a level.
Use shims.
Use your eyes.
Correct framing is everything.

D. Test Fit the Wall Sleeve

You should have:

A snug fit
No crushing pressure
No loose gaps

Proper framing ensures the unit sits perfectly and avoids vibration noise later.

6. Step 4: Install the Wall Sleeve

Think of the sleeve as the cradle that supports the unit for life. If the sleeve is crooked—even by a hair—your AC will drain poorly, vibrate more, run louder, and potentially leak.

A. Slide the Sleeve into the Opening

Make sure the sleeve’s rear edge slopes slightly downward (¼ inch is typical).
This ensures condensation flows outside—not inside your home.
→ HVAC Drainage Guidelines

B. Level It Left-to-Right

Use your level. The downward slope should exist ONLY toward the outside.

C. Fasten the Sleeve

Secure with recommended exterior-grade screws through pre-drilled holes.

D. Seal Exterior Gaps

Use:

Caulk between sleeve and siding
Flashing along the top edge
Foam insulation for small gaps

E. Interior Gasket Setup

The interior side of the sleeve typically has a gasket to prevent air leakage.

A tight seal = better efficiency.

7. Step 5: Electrical Requirements & Safety

This part is where you slow down and pay attention.

A 15,000 BTU unit usually requires:

✔ 230V Circuit

Most outlets in a home are 115V—not enough.

✔ Dedicated Circuit

No sharing with microwaves, space heaters, or hair dryers.

✔ Proper Breaker Size

Typically 15–20 amps, depending on the manufacturer.

✔ Correct Receptacle Type

These ACs use special plug shapes (NEMA-rated).

✔ GFCI Requirements (Region Dependent)

Some areas require GFCI-protected outlets for exterior-wall appliances.

Electrical mistakes can cause fires, shocks, or damage to the unit.
Always follow NEC and local codes.

If in doubt:
Hire an electrician.
This is the one step even seasoned DIYers skip—and regret.

8. Step 6: Install the Actual AC Unit

This is the part where your back says, “Are you sure you don’t want help?”

A. Slide the Unit into the Sleeve

Go slowly.
It should glide—not scrape.
If it scrapes, stop and recheck squareness.

B. Check Airflow & Clearances

Through-the-wall ACs need space behind them for exhaust airflow.
Do not install near:

Fences
Shrubs
Deck railings
Narrow alleyways

C. Secure the Unit

Once fully inserted, fasten the unit according to the manufacturer’s instructions.
Usually, this includes interior mounting screws.

D. Add Trim Kits

Most units include interior trim to cover framing edges and gaps.

E. Test for Proper Drainage

You should see condensation dripping outside, not inside.

If it drips inside?
Your slope is wrong.
Fix it NOW—not in July.

9. Step 7: Seal, Insulate & Weatherproof Everything

This is the most underestimated step—but one of the most important.

A. Seal Interior Gaps

Use foam insulation or backer rod + caulk.

B. Seal Exterior Gaps

Exterior-grade silicone caulk only.

C. Install Drip Cap

A drip cap prevents rainwater from sneaking behind the siding.

D. Check for Air Leaks

A small air leak can decrease efficiency by 15–25%.

10. Structural Concerns You Should Always Respect

A few critical warnings:

Never cut load-bearing studs

If your chosen location hits a load-bearing section, relocate the install.
Do NOT cut studs without a structural engineer’s approval.

Mind brick or stone exteriors

Cutting masonry requires special blades and often professional help.

Beware of wall cavities with wiring or plumbing

Use your stud finder and small exploratory cuts.

Ventilation matters

Do not install in:

Closets
Enclosed porches without enough airflow
Small alcoves
Behind furniture

11. Maintenance Setup: Install for the Future

Install it right today, maintain it easily tomorrow.

When framing:

Leave enough clearance for coil cleaning
Ensure easy access to air filters
Use sleeves with removable panels
Keep wiring accessible

Proper installation = fewer headaches during seasonal cleaning.

12. Safety “Traps” to Avoid (Savvy Style)

You could DIY this—but here’s what causes problems for homeowners:

❌ Trap 1: Crooked Wall Sleeve

If it’s not sloped outward, you’ll get water inside.

❌ Trap 2: Using an Existing Sleeve That Doesn’t Fit

Sleeves are manufacturer-specific.

❌ Trap 3: Not Upgrading the Outlet

A 230V unit in a 115V outlet is a no-go.

❌ Trap 4: Not Checking Outdoor Clearance

Airflow blockage = overheating = early failure.

❌ Trap 5: Poor Sealing

Drafts, pests, and lost efficiency.

❌ Trap 6: Cutting a Stud Without Knowing

This one doesn’t need explanation.

13. How This Ties Back to The Furnace Outlet Pillar Page

The main pillar page emphasizes:

Proper installation
Space-saving design
Efficient operation
Long-term durability
Correct sizing

A through-the-wall AC installation delivers all of these—if it’s done correctly.

The space-saving design is one of its biggest advantages over window ACs, and this guide shows exactly how the installation achieves that clean, integrated look.