🧰 How to Size a Through-the-Wall Heat Pump: BTUs, Room Size & Climate Tips
Persona: Jake – Your straight-talking, measure-twice-install-once kind of guy. I’ve been burned by buying the wrong size HVAC before, and I’m here to make sure you don’t...
Best Through The Wall Heat Pumps
☕ Intro – My “Measure Twice, Buy Once” Rule
When I bought my first through-the-wall heat pump, I eyeballed the size based on “looks about right” instead of calculating. Big mistake. That unit short-cycled like crazy in summer, cost me more than it should have in energy bills, and never felt quite comfortable.
If you’re shopping for one now, here’s my hard-earned advice: sizing matters just as much as brand or efficiency rating. Oversize it, and you’ll waste money while sacrificing comfort. Undersize it, and you’ll run it into the ground trying to keep up.
This guide will walk you through how to get the right BTU rating for your through-the-wall heat pump—taking into account room size, ceiling height, climate, insulation, and a few things most people forget.
📏 Understanding BTUs — Your HVAC Unit’s Muscle
BTU stands for British Thermal Unit—basically, it’s how much heat energy your system can move in an hour.
-
In cooling mode: BTUs measure how much heat the unit can remove from your space.
-
In heating mode: They measure how much heat it can add.
Think of BTUs like engine horsepower: more isn’t always better. A Ferrari engine in a go-kart is overkill; a lawnmower engine in a pickup truck won’t get you far either.
For more background, the U.S. Department of Energy has a clear explanation of BTUs and their relationship to cooling capacity.
📐 Step 1 – Measure Your Room Size
The base number you need is square footage:
-
Measure length × width of the room in feet.
-
Multiply the two numbers.
Example:
A bedroom that’s 15 ft × 12 ft = 180 sq ft.
Here’s a basic BTU-per-square-foot chart for average insulation and an 8-foot ceiling in a mild climate:
| Room Size (sq ft) | Recommended BTUs |
|---|---|
| 100 – 150 | 5,000 |
| 150 – 250 | 6,000 – 8,000 |
| 250 – 350 | 8,000 – 10,000 |
| 350 – 450 | 10,000 – 12,000 |
| 450 – 550 | 12,000 – 14,000 |
| 550 – 700 | 14,000 – 18,000 |
If you want a quick digital calculation, use the ENERGY STAR® Room AC Calculator
🌍 Step 2 – Adjust for Your Climate Zone
The same room size in Arizona and in Maine will need different BTU levels because climate changes the load.
Here’s a rough guide using the U.S. Climate Zone Map:
-
Hot, humid climates (Florida, Texas Gulf Coast) → Add 10% BTUs.
-
Hot, dry climates (Arizona, Nevada) → Add 5% BTUs.
-
Cold climates (Minnesota, northern New England) → Add 15% BTUs for heating mode.
-
Mild coastal climates (Pacific Northwest) → No major adjustment needed.
Example:
Our 180 sq ft room (base 6,000 BTUs) in Houston (hot/humid) → 6,600 BTUs recommended.
🏠 Step 3 – Factor in Ceiling Height
Most BTU charts assume an 8-foot ceiling. If your room has a cathedral or vaulted ceiling, you’re cooling/heating more air volume.
Rule of thumb:
-
Add 10% BTUs for each foot above 8 ft.
Example:
180 sq ft × 10 ft ceiling = +20% BTUs.
Base 6,000 BTUs → 7,200 BTUs needed.
🔍 Step 4 – Check Insulation & Windows
Poor insulation and large window areas both increase load.
-
Poor insulation: Add 15% BTUs.
-
Multiple or large sun-facing windows: Add 10%–20% BTUs depending on exposure.
If you’re unsure about insulation quality, the DOE’s Home Energy Efficiency Guide is worth a read.
👥 Step 5 – Account for Occupancy
People give off heat—roughly 230 BTUs/hour at rest. For small rooms, extra occupants matter.
-
Add 600 BTUs for each person over two in the space.
Example:
Small home office for 3 people → +600 BTUs.
⚠ Step 6 – Avoid the Oversize/Undersize Trap
Oversizing risks:
-
Short cycling (turning on/off too often).
-
Poor humidity control.
-
Higher electricity bills.
-
Shorter equipment life.
Undersizing risks:
-
Unit runs constantly.
-
Can’t reach set temperature on extreme days.
-
Accelerated wear.
HVAC.com has a great breakdown of why “bigger” isn’t better.
📊 Through-the-Wall Heat Pump BTU Chart by Room Size & Climate
| Room Size (sq ft) | Mild Climate | Hot/Dry | Hot/Humid | Cold Climate (Heating) |
|---|---|---|---|---|
| 100 – 150 | 5,000 | 5,250 | 5,500 | 5,750 |
| 150 – 250 | 6,000 | 6,300 | 6,600 | 6,900 |
| 250 – 350 | 8,000 | 8,400 | 8,800 | 9,200 |
| 350 – 450 | 10,000 | 10,500 | 11,000 | 11,500 |
| 450 – 550 | 12,000 | 12,600 | 13,200 | 13,800 |
| 550 – 700 | 14,000 | 14,700 | 15,400 | 16,100 |
🧮 Step-By-Step Real-World Example
Let’s size a through-the-wall heat pump for my buddy Mike’s workshop:
-
Dimensions: 20 ft × 15 ft = 300 sq ft.
-
Base BTUs: ~8,000 (from chart).
-
Ceiling Height: 10 ft → +20% = 9,600 BTUs.
-
Climate: Hot/humid (Florida) → +10% = 10,560 BTUs.
-
Windows: Two large west-facing windows → +15% = 12,144 BTUs.
Recommendation: Go for a 12,000–12,500 BTU model.
🛠 Tools & Resources for Accurate Sizing
✅ Jake’s Final Sizing Checklist
-
Measure your room in sq ft.
-
Adjust for ceiling height.
-
Add or subtract BTUs based on climate zone.
-
Consider insulation and window exposure.
-
Factor in number of people regularly in the space.
-
Choose a model with a slight capacity margin—never wildly oversized.
💬 Jake’s Wrap-Up
A through-the-wall heat pump can be a fantastic solution—if you size it right. The difference between a “just right” fit and a “good enough” guess can mean hundreds of dollars in bills, years off the unit’s life, and daily comfort issues.
When in doubt, run the numbers twice. And if you want to go deep, a pro can do a Manual J calculation to give you surgical-level accuracy.
Trust me—your wallet, your comfort, and your future self will thank you.
In the Next Topic we will know more about: Through-the-Wall Heat Pump Installation Guide: What Jake Learned Doing It Himself
