What Size Heater Do You Need for a Garage?

The quickest way to read that table: an insulated 2-car garage wants 5,000W of electric or around 17,000 BTU, and an uninsulated one in a cold climate wants a gas unit heater. The gap between those two columns is the argument for insulating.

Cross-check: what the makers publish

Published coverage figures vary widely between products, and the variation is informative rather than random.

Two things fall out of that. The vented unit heaters assume about 30 BTU per square foot, which is an insulated-space assumption. The portable propane heaters assume 40-43, which is more conservative — appropriate, because they are designed for construction sites with doors open.

Dura Heat's 20 BTU per square foot is the outlier and reads as the most conservative figure in the category, which fits a product that also publishes a real BTU number rather than just watts.

The four variables that change the answer

1. Insulation — the big one

The difference between the first and last columns of the table above is roughly a factor of two. An uninsulated garage needs twice the heater of an insulated one, and runs it twice as long.

The garage door is usually the single biggest loss — a large uninsulated panel with gaps around it. Insulating the door and sealing its perimeter is the highest-return work available, and it is cheaper than the difference in heater size. See how to insulate a garage for heating.

2. Ceiling height

The table assumes 8-9 feet. A 12-foot ceiling is roughly 40% more air volume for the same floor area, so scale up — and note that heat stratifies, so a tall garage wants either a ceiling fan to push warm air down or a radiant heater that warms surfaces rather than air.

Heatstar's radiant unit publishes a 7-9 ft mounting height range, so in a tall garage it is simply the wrong product. Layouts for tall garages are on the high-ceiling garage page.

3. Climate

The bands above assume a moderate-to-cold winter. In a genuinely cold climate — sustained sub-zero — go up a column. In a mild one, down.

4. What you want from it

Holding a garage at 55°F all winter is a different job from raising it from 20°F to 60°F in thirty minutes. The table sizes for holding temperature; for fast recovery, size up or accept waiting.

This is also the argument for multi-level output: Mr. Heater's 2VAHG5000 offers 5,000 / 4,000 / 3,000W, so you can size for the cold snap and run low the rest of the time.

Why oversizing a gas heater is a real mistake

With electric, oversizing mostly wastes capital — the thermostat cycles it off and a multi-level unit runs lower.

With gas it is worse. An oversized unit heater short-cycles: it fires, satisfies the thermostat quickly, shuts down, and repeats. Each cycle is an ignition event, and ignition is where these units wear out. It also feels drafty, because you get blasts of hot air rather than steady warmth.

So for gas, size it properly rather than generously. Mr. Heater's 50,000 BTU unit publishes 1,667 sq ft — in a 450 sq ft two-car garage that is substantially oversized, and the 45,000 BTU Modine or a smaller unit is the better fit.

The right heater for each size

Next

Having sized it, price the running cost on garage heater running costs — the figure that often changes the fuel decision — and then pick from the best garage heaters.

And if the table has told you that your uninsulated garage needs a gas unit heater, read how to insulate a garage for heating first. Insulating down a column is cheaper than heating up one.