How to Heat a Garage Workshop Without Rusting Your Tools

To heat a garage workshop properly you need three things in this order: insulation, the right size of heater, and a heater that does not dump its exhaust into the room. Size it with square footage multiplied by your temperature rise multiplied by an insulation factor, then add 25 percent. And skip the unvented propane and kerosene units, however cheap they look, because a 30,000 BTU unvented heater releases roughly a third of a gallon of water into your shop air every hour it runs. That is about seven gallons a day, landing on cold cast iron.

Which is the part most guides leave out. Heating a shop badly does not just cost money. It rusts everything you own.

In this article

• How many BTUs do you need?
• Why unvented propane and kerosene are a trap
• Option 1: 120V electric
• Option 2: 240V electric forced air
• Option 3: Vented gas or propane unit heater
• Option 4: Infrared radiant
• Option 5: Mini-split heat pump
• Which should you actually buy?
• Insulate first, or you are heating the street
• FAQ

How many BTUs do you need to heat a garage workshop?

The working formula is straightforward:

Square footage × temperature rise × insulation factor, then add 25 percent.

Temperature rise is the target shop temperature minus the coldest outdoor temperature you actually work in. The insulation factor runs from about 0.45 for a well-insulated space up to 1.10 for a bare uninsulated garage.

Run it on a typical 24 by 24 foot two-car garage, targeting 60°F when it is 10°F outside, and the difference is stark:

• Uninsulated: 576 sq ft × 50 × 1.10 = 31,680, plus 25 percent = roughly 39,600 BTU
• Well insulated: 576 sq ft × 50 × 0.45 = 12,960, plus 25 percent = roughly 16,200 BTU

Insulating the space more than halves the heater you need to buy, and then more than halves what it costs to run, every winter, forever. This is why the insulation section is not filler.

Why unvented propane and kerosene heaters are a trap

The torpedo heater is tempting. It is cheap, it is portable, it produces an absurd amount of heat for the money, and every second garage has one.

Here is the problem nobody mentions at the register. Burning a hydrocarbon produces carbon dioxide and water. A vented heater sends both up a flue. An unvented one sends both into the room you are standing in. As Howard Environmental puts it, a 30,000 BTU unvented gas heater produces around a third of a gallon of water vapour per hour, which works out near seven gallons a day of moisture added to your shop.

That warm wet air then meets your cold table saw top, your cold hand planes and your cold garage door. You already know what happens next. If you missed it, we went through the mechanism in how to prevent rust on tools in a garage shop. Running an unvented heater is the single most efficient way to undo all of that work.

Then there is the part that matters more than your tools. Carbon monoxide is colourless and odourless, and in sufficient concentration it kills without warning. The Insurance Information Institute is blunt about the conditions for using these things: a constant source of outside air, a window or door open at least an inch, never refuel while the unit is hot, and use only 1-K grade kerosene.

And one line in that guidance should stop any woodworker or finisher cold: never operate them in a space containing flammable solvents, aerosols or gasoline. That is a description of most workshops. Thinners, spray cans, finish rags, a fuel can for the mower, plus a fine airborne dust load. If you run one anyway, a carbon monoxide alarm is not optional.

Option 1: 120V electric (spot heat only)

A standard plug-in electric heater is capped by the circuit, not by ambition. 1,500 watts is the practical ceiling on a normal 15 amp outlet, which is about 5,000 BTU.

That will not heat a garage. It will take the edge off the three feet around your bench, which is occasionally all you want for a half hour of sharpening. Treat it as a personal heater that happens to live in a shop, and do not run it on the same circuit as anything with a motor.

Option 2: 240V electric forced air

Move to a 240V hardwired unit and the ceiling lifts to roughly 34,000 BTU. This is a genuine garage heater. No flue, no combustion, no added moisture, no carbon monoxide, and nothing to service.

The trade is running cost and installation. You need a dedicated circuit, which usually means an electrician and a panel with room in it. Resistance heat is also the least efficient electric option: every watt in gives you one watt of heat, no more.

Option 3: Vented gas or propane unit heater

The ceiling-hung unit heater with a flue through the wall is the traditional answer for a shop that gets used seriously. Output runs from 30,000 to 125,000 BTU, and on natural gas it is typically the cheapest per hour to run of anything here.

The flue is the entire point. Combustion by-products, including all that water, go outside. You get the cheap heat of gas without the swamp.

It is the biggest install job on this list: gas line, venting, clearances, and in most places a permit and an inspection. Worth it if the shop is your main workspace, hard to justify for occasional weekends.

For whoever installs it

Comfort Restored, Patience Expired

The HVAC tee. Garment-dyed 100% ring-spun Comfort Colors cotton, relaxed fit, printed to order.

From $29.99 Have a look →

Option 4: Infrared radiant

Radiant heaters do not warm the air. They warm the objects the energy lands on, the way sun through a window warms you on a cold day. Output ranges from 5,000 to 60,000 BTU depending on the unit.

That makes them the best answer for a drafty or poorly insulated shop, because there is no point heating air that leaks straight out. Mount a tube heater over the bench and the bench, the tools and you get warm within about a minute, while the far corner stays cold. If you work in one or two fixed spots, that is a feature.

The limitation is the same thing: line of sight. Step behind a machine and you are out of the beam.

Option 5: Mini-split heat pump

A ductless mini-split is the efficiency answer. Because it moves heat rather than making it, it delivers roughly two to three BTU of heat per BTU of electricity consumed, which no resistance heater can approach. Output typically runs 9,000 to 36,000 BTU.

It also does the other three seasons. It cools in July, it dehumidifies, and dehumidifying an unheated shop is worth real money in saved tools on its own.

The catches: highest up-front cost of anything here, it needs a properly insulated space to make sense, and capacity falls off as it gets very cold unless you buy a cold-climate rated unit. If you are in a genuinely cold region, check the rated output at your design temperature, not the headline number.

Which should you actually buy?

Running costs move with your utility rates, so treat any per-hour figure you read online as an order of magnitude rather than a quote. The ranking rarely changes though: natural gas cheapest to run, mini-split close behind and far cheaper than resistance electric, 240V resistance the most expensive per hour of the permanent options.

Insulate first, or you are heating the street

Look again at the two BTU numbers at the top of this article. Same garage, same target temperature, and insulation is the only variable between a 16,000 BTU problem and a 40,000 BTU one.

The order that gives the most back per dollar:

1. The garage door. Usually the single largest uninsulated surface in the building, and a rigid foam kit is a couple of hours of work.
2. Air sealing. The gaps around the door, the service door, the rim joist and any wall penetrations. Cheap, unglamorous, and it kills the drafts that make radiant heat necessary in the first place.
3. The ceiling. Heat leaves upward. If there is a habitable space above, you also stop heating your own bedroom floor.
4. The walls. Most work, most disruption, do it last unless they are already open.

One caution specific to shops: if you insulate and seal a garage tightly and then heat it intermittently, you can make condensation worse rather than better, because you have built a sealed box that swings between warm damp and cold. Steady low heat beats blasting it for four hours on a Saturday.

Three weekends later

Started As A Quick Fix

For everyone who set out to insulate the door and ended up rewiring the panel.

From $29.99 Have a look →

Frequently asked questions

How many BTUs do I need to heat a 2-car garage?

For a typical 576 square foot two-car garage, expect roughly 16,000 BTU if it is well insulated and closer to 40,000 BTU if it is not, based on a 50 degree temperature rise. Calculate your own with square footage times temperature rise times insulation factor (0.45 insulated to 1.10 uninsulated), then add 25 percent.

Is it safe to use a propane heater in a garage?

A vented propane unit heater is safe because combustion products go outside. An unvented portable propane or kerosene heater needs constant outside air, a carbon monoxide alarm, and should never run in a space holding flammable solvents, aerosols or fuel, which rules out most workshops. It also dumps large amounts of water vapour into the room.

What is the cheapest way to heat a garage workshop?

Cheapest to install is a 120V electric heater, but it only spot-heats. Cheapest to run over a season is usually a vented natural gas unit heater, with a mini-split heat pump close behind and far cheaper than resistance electric. Cheapest overall is almost always insulating first so you can buy a smaller heater.

Will a mini-split heat pump work in a cold garage?

Yes, but capacity drops as outdoor temperature falls. If your winters get seriously cold, buy a cold-climate rated model and check its rated output at your local design temperature rather than the headline BTU figure. A mini-split also needs a reasonably well insulated space to make sense.

Does heating a garage cause rust on tools?

Heating itself does not, but two common patterns do. Unvented combustion heaters add gallons of water vapour to the air. And heating a shop intermittently repeatedly warms damp air against cold metal, which condenses on it. Steady moderate heat, or a dehumidifier, protects tools better than occasional blasts of heat.

The short version

Insulate the door and seal the gaps first. Size the heater with the formula rather than by guessing. Pick anything on this list except an unvented one, unless the door is open. Then run it steadily rather than in bursts, and your tools will thank you in March.

Related reading: How to prevent rust on tools in a garage shop