What Size Through-the-Wall AC Do You Need? Tony’s BTU, Room Layout & Climate Rules

Most homeowners think picking a through-the-wall AC is simple: measure the room, check a BTU chart, and buy whatever “seems right.”
Tony shakes his head every time.

Through-the-wall ACs do not size like window units…
They do not behave like portable ACs…
And they definitely don’t perform like central air systems.

A wall unit needs the correct BTUs, correct airflow, correct sleeve fit, correct room layout, and correct climate adjustment — or it will run loud, run constantly, and never cool the way you expect.

This is Tony’s real-world sizing guide. Not theory. Not a generic BTU chart.
This is how an HVAC tech sizes wall units on real job sites — day in, day out.

Let’s size this thing right.

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1. Why Sizing a Through-the-Wall AC Is Different Than Sizing a Window Unit

Wall ACs are fixed in place.
They don’t leak air around the frame…
They don’t rattle against the window…
They push air more efficiently…

Which means they cool more aggressively — and punish you harder when undersized.

Undersized wall AC =

• long runtime

• hot corners

• sticky air

• warm afternoons

• compressor strain

Oversized wall AC =

• short cycling

• terrible humidity control

• noise

• uncomfortable temperature swings

Tony’s rule:

“BTUs matter — but room layout and airflow matter more.”

(Reference: Residential HVAC Load Calculation Standards)

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2. Tony’s Real BTU Sizing Chart (Based on Field Experience, Not Manufacturer Optimism)

Manufacturers often undersize recommendations to make units look more efficient.
Tony sizes based on real cooling needs, not marketing numbers.

Tony’s Through-the-Wall AC Sizing Chart

These BTUs assume:

• normal ceiling height

• average insulation

• standard number of windows

If your room has ANY special conditions, keep reading.

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3. Sun Exposure: The #1 Factor Homeowners Ignore

Two rooms with the same square footage can need totally different BTU levels.

South- or west-facing rooms need more BTUs.

If your room gets heavy afternoon sun:

• add 1,000–2,000 BTUs

• expect longer run cycles

• expect warmer corners

Rooms in full shade can size slightly lower.

But Tony never sizes down unless the insulation is outstanding.

(Reference: Regional Climate and Temperature Zone Guidelines)

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4. Ceiling Height: Anything Over 8 Feet Changes Everything

A 500 sq ft room with 8’ ceilings is easy work for a 12,000 BTU unit.
A 500 sq ft room with 12’ ceilings acts like a 750 sq ft space.

Tony’s rule:

Add 20–25% BTUs for every extra 2 feet of ceiling height.

High ceilings = more cubic feet = more heat load.

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5. Kitchens Always Need Extra BTUs

Tony always sizes up for kitchens because:

• ovens add heat

• refrigerators add heat

• extra lighting adds heat

• cooking steam adds humidity

Add +4,000 BTUs for kitchen cooling.
No exceptions.

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6. Open-Concept Rooms Are NOT the Same as Closed Rooms

Wall ACs cannot push air around corners or down long hallways.

If your room has an open layout:

Add 15–25% BTUs.

If you expect the AC to “help cool the next room”:

Add 10–20% BTUs, but manage expectations.

A 12,000 BTU wall unit will NOT cool:

• the whole apartment

• two bedrooms

• far corners

• long hallways

You need realistic expectations with wall units.

(Reference: Air Distribution and Duct Sizing Reference)

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7. Insulation Quality — The Silent BTU Killer

Bad insulation =

• hotter summers

• colder winters

• higher bills

• harder cooling loads

Tony adjusts BTUs as follows:

✔ Good insulation (newer home, foam, tight envelope):

No BTU increase.

✔ Average insulation (standard drywall, moderate windows):

Standard BTU chart.

✔ Poor insulation (older home, drafty windows, no wall foam):

Add 10–30% BTUs.

Poor insulation makes any wall AC work harder.

(Reference: Home Insulation and Envelope Performance Manual)

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8. How Many People Use the Room? (Yes, This Matters)

People generate heat — a surprising amount.

Add +400 BTUs per additional occupant.

(After the first person)

Bedrooms don’t change much.
Living rooms do.

If you regularly have family, friends, or kids in the space, size up.

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9. Wall Placement & Airflow Direction — Where the Unit Goes Changes BTU Needs

Wall units blow the air in one direction.
If the layout works AGAINST the unit, Tony sizes up.

Bad wall placement examples:

• Behind furniture

• Across from a tall bookshelf

• Facing a hallway

• Back corner with restricted throw

• Near the ceiling with hot air pocketing

Good wall placement examples:

• Centered on the longest wall

• Across from open space

• With strong return path

• Pointed toward hotter zone

• Away from sunlight

Airflow direction determines whether a room feels evenly cooled.

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10. Climate Zone Adjustments — Tony Sizes for Reality, Not Labels

Your state changes your tonnage requirements — period.

Hot climates (South, Southwest, Gulf, Desert):

Add 1,000–3,000 BTUs
Expect long afternoon cooling cycles
Use higher EER/CEER units

Mild climates:

Use standard BTU chart
Humidity matters more here

Cold climates:

Wall ACs work GREAT for cooling —
but heating mode (if included) needs extra consideration.

Tony never sizes wall units without considering climate.

(Reference: Regional Climate and Temperature Zone Guidelines)

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11. Sizing for Heat Pump Through-the-Wall Units

If you buy a wall AC/heat pump combo, you’re sizing for:

• cooling BTUs

• heating BTUs

• defrost cycles

• cold-weather performance

Heat pumps lose capacity as temperatures drop.

Tony’s rule:

If you need real heating, size the BTUs based on HEATING, not cooling.

Heat pumps should never be undersized.

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12. When You Should Size UP (Tony’s Red Flags You Can’t Ignore)

Tony always sizes up when he sees:

✔ Big windows

✔ South/west sun
✔ High ceilings
✔ Open layouts
✔ Poor insulation
✔ Kitchen usage
✔ Multi-person rooms
✔ Expectation to cool an adjacent space

If you see two or more of these → size up 1,000–4,000 BTUs.

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13. When You Should NOT Size Up

Oversizing wall ACs leads to:

• short cycling

• terrible humidity control

• uncomfortable cold blasts

• increased wear

• more noise

Don’t size up if:
✔ room is shaded
✔ small enclosed room
✔ new insulation
✔ low occupancy
✔ low ceilings

Humidity is the main casualty of oversizing.

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14. Real Examples Tony Sees in the Field

Example 1: Small Bedroom, 180 sq ft, shaded

→ 6,000 BTU

Example 2: Large living room, 420 sq ft, open concept, west-facing

→ 12,000–14,000 BTU

Example 3: Studio apartment, 620 sq ft, high ceilings

→ 14,000–15,000 BTU

Example 4: Kitchen/dining combo, 350 sq ft

→ 12,000 BTU

Example 5: Office space with 4 people

→ Add +1,200–1,600 BTUs

Real rooms. Real loads. Real BTU numbers.

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15. Tony’s Final Verdict on Wall-Unit Sizing

Through-the-wall ACs cool beautifully when sized correctly.
But they are unforgiving when sized wrong.

Here’s Tony’s rule:

✔ Size based on room layout

✔ Add BTUs for real-world heat load

✔ Never undersize

✔ Never oversize

✔ Consider sun, ceilings, insulation, and people

✔ Pick the right wall placement

If you size your wall AC like Tony does, you’ll enjoy:

• faster cooling

• quieter operation

• lower bills

• longer equipment life

• steady, even comfort

Get it wrong?
You’ll fight that unit every summer.

In the next blog, Tony will discuss the wall sleeve fitment.