Quick answer
Most attached two-car garages in southern Ontario need a 30,000 to 45,000 BTU/h gas heater: the smaller size if the walls, ceiling and door are insulated, the larger if they are not. A one-car garage runs 20,000 to 30,000 BTU/h and a three-car 45,000 to 75,000. Holding the space just above freezing takes about a third less heat than holding workshop temperature.
What size garage heater you need comes down to four things: the floor area, how well the shell is insulated, how many overhead doors there are, and whether you want the space frost-free or warm enough to work in. For an attached two-car garage in this part of Ontario the answer is usually a 30,000 to 45,000 BTU/h gas heater. Heat output is measured in BTU per hour, and the figures below are inputs, the number printed on the heater's box.
BTU by garage type: one, two and three-car
Use these as starting sizes for a gas unit heater holding workshop temperature, about 15 to 18 °C, on a design day near −18 °C.
- One-car, roughly 12 by 22 ft, insulated: 20,000–30,000 BTU/h, or a 5 kW electric heater
- One-car, uninsulated: 30,000 BTU/h
- Two-car, roughly 20 by 22 ft, insulated: 30,000 BTU/h
- Two-car, uninsulated: 45,000 BTU/h
- Three-car, insulated: 45,000–60,000 BTU/h
- Three-car, uninsulated: 60,000–75,000 BTU/h
- Ceiling above 3 m, a second overhead door, or a wall facing open fields: go up one size
Target temperature moves the number more than people expect. Holding 5 °C to keep the slab dry and the car thawed is about a third less load than holding 18 °C on the same night, because the heater is fighting a smaller gap to the outdoors. Insulation moves it further: a two-car garage with insulated walls, ceiling and door loses less than half of what the same bare-block garage does. The smallest gas unit heaters start around 30,000 BTU/h, which already exceeds the steady loss of a well-insulated two-car garage; the surplus is what brings the temperature back after the door has been open.
Input is not output. A standard gas unit heater delivers roughly 80 percent of its input as heat, so a 45,000 BTU/h model puts about 36,000 into the room. Electric heaters deliver all of it: 1 kW is 3,412 BTU/h.
Unit type changes the answer
- Forced-air unit heater: sized as above. It warms the air fast and loses that air every time the door goes up, so heavy door traffic argues for the larger size.
- Infrared tube heater: warms the slab, the vehicle and the bench instead of the air, so comfort survives a door opening and the input can often be a step smaller for the same space. It needs ceiling length and a generous clearance below the tube, which a standard-height garage with a truck in it may not have. Our comparison of infrared and forced-air garage heaters goes deeper.
- Natural gas or propane: the BTU sizing is identical. Rural lots beyond the gas main in Kilbride, Lowville, Carlisle and rural Flamborough run the same heaters on propane with the manufacturer's conversion kit and a regulator set for the tank.
- Electric: a 5 kW heater is about 17,000 BTU/h and needs a 30-amp, 240-volt circuit. A 10 kW unit, about 34,000 BTU/h, needs roughly 60 amps, more than many 100-amp house panels can spare.
- Ductless heat pump: a single-zone system at $4,000–$7,000 installed heats and cools a finished, insulated garage used as a gym or shop; see garage mini-split installation.
Install limits that can change the size you can use
The heater that the heat-loss math picks still has to fit the garage legally. CSA B149.1 and the manufacturer's manual set the limits, and they sometimes force a different model.
- Height above the floor: in a garage where vehicles are parked, the burner and ignition source must sit at least 450 mm (18 in.) above the floor, and the unit has to be protected from being struck.
- Clearances to combustibles: the manual lists distances above, beside, below and in front of the heater. Check them against storage racks, a parked vehicle's roof and the overhead door in its open position.
- Venting: most residential unit heaters power-vent through a side wall, and the terminal has to clear windows, doors and the gas meter. A separated-combustion model draws its air from outdoors, the right choice for a dusty woodworking shop.
- Gas line capacity: pipe is sized from the total connected load and the longest run. A 45,000 BTU/h heater at the end of a long half-inch branch can starve, and a full meter means an upsizing request to Enbridge Gas.
- Thermostat location: on an inside wall, away from the door and out of the heater's discharge.
- Solvent or paint spraying: tell the installer, because an open-burner heater is the wrong appliance for a space with flammable vapours.
How a garage heater is sized on a site visit
We measure the garage, note the wall, ceiling and door construction, count the doors and ask what temperature you want and how often the door opens. That gives a heat loss in BTU/h, and the heater is the next catalogue size above it, not two sizes above. Oversizing a unit heater makes it short-cycle and swing the space between hot and cold. If the ceiling and door are not insulated yet, do that first; the heater you need afterwards is smaller.
There is no flat price for these jobs because the gas run and the vent route differ in every garage; what a garage heater costs installed explains the drivers. For a sized, written quote, see garage heater installation or tell us about your garage.