A cold detached or attached garage in Hamilton or Burlington in February is not just uncomfortable. It kills the EV battery overnight, freezes wiper fluid solid, makes weekend work impossible, and means every paint can on the shelf is a sludge until April. The plug-in 120V garage heater you bought at Canadian Tire ten years ago is not going to solve it. Here is what an actual hardwired garage heater install looks like, the sizing math, and why 240V is the right answer for almost every garage worth heating.
The BTU math: sizing the heater correctly
The right heater size depends on garage volume, insulation, target temperature, and outdoor design temperature. Rules of thumb for Ontario:
- Poorly insulated garage (uninsulated walls, uninsulated overhead door, single layer of drywall on the ceiling): ~30 W per square foot of floor area to maintain +10°C at -20°C outdoor
- Well-insulated garage (R-20 walls, insulated steel overhead door, R-40 ceiling): ~15 W per square foot of floor area for the same target
- Two-car garage, ~440 sq ft, well-insulated: ~6,500 W heater = 22,200 BTU/hr
- Two-car garage, ~440 sq ft, poorly insulated: ~13,200 W heater = 45,000 BTU/hr
The first take-away: insulation matters more than heater size. Adding R-20 batt in the walls of a previously uninsulated garage often cuts the heating load by half. A homeowner who insulates before installing the heater spends less on the heater, less on the circuit, and less on operating cost forever after.
Why 240V beats 120V for any serious heater
A 120V garage heater is limited by the 15A or 20A circuit it plugs into:
- 15A circuit at 120V → 1,800 W max → 6,100 BTU/hr — barely takes the edge off a 200 sq ft garage
- 20A circuit at 120V → 2,400 W max → 8,200 BTU/hr — single-car garage, well-insulated, in shoulder season
That is not enough heat for any double garage in Ontario winter. The 240V options:
- 20A at 240V → 4,800 W → 16,400 BTU/hr
- 30A at 240V → 7,200 W → 24,600 BTU/hr — the right answer for a well-insulated two-car garage
- 40A at 240V → 9,600 W → 32,700 BTU/hr — uninsulated two-car or insulated three-car
- 50A at 240V → 12,000 W → 41,000 BTU/hr — large or poorly-insulated three-car
The cost difference between a 240V hardwired install and a 120V plug-in install is meaningful but not enormous — typically a half-day of electrical work, a dedicated circuit from the panel, and a higher-rated heater. The performance difference is night-and-day. Every owner who has done both regrets starting with the 120V.
Brands we install
Common units we wire in residential garages in Hamilton, Burlington, and Oakville:
- Dimplex Pro Series (DGWH4031, DGWH4051) — ceiling-mount fan-forced, hardwired, 240V, with built-in thermostat
- Stelpro Oasis Plus — wall-mount fan-forced, well-built, made in Quebec
- Modine Hot Dawg HD — natural-gas-fired alternative for owners who want to skip electric entirely; we wire the 120V control and venting fan
- Mr. Heater Big Maxx — gas-fired, similar to the Modine, popular on shop installs
- Cadet Com-Pak — for smaller garages with limited mounting space
Circuit layout: what we run from the panel
For a typical 30A 240V heater install:
- Dedicated 30A 240V double-pole breaker at the main panel
- 10/2 NMD90 or NMD90-equivalent wiring (gauge depends on run length; we size for voltage drop on runs over 25 m)
- Run from panel to garage — typically through the basement ceiling, up through the rim joist, and out to the garage
- For detached garages, the run goes underground in NMWU or through conduit, depending on length and access
- Disconnect within sight of the heater (the integrated thermostat on the heater unit often counts; if not, a wall switch at the unit)
- Junction box at the heater for the final connection, sized to the unit
For detached garages with multiple loads (lights, receptacles, heater, EV charger, compressor), the right architecture is often a small sub-panel in the garage, fed from the house main panel. Detail on sub-panel sizing is similar to the boathouse case in our dock and boathouse post — the same logic applies on land.
Thermostat options
Three tiers:
- Integrated thermostat on the heater. Cheapest, easiest, set-and-forget. Works fine for most owners.
- Line-voltage wall thermostat. Mounted on a wall away from the heater outflow, giving better temperature regulation across the space. The thermostat itself switches the full heater load — must be sized to the breaker rating.
- Smart thermostat with relay or contactor. A low-voltage smart thermostat (Honeywell T6 Pro, Sinopé TH1124ZB) drives a high-current relay that switches the heater. Owner can schedule, remote-control, and integrate with the smart-home system. Best option for EV-charging garages where you want the space warm an hour before you go out.
Operating cost: the conversation
A 7,200 W heater running 4 hours a day at Ontario time-of-use rates is real money over a winter. Insulation pays back first; smart scheduling pays back second. The order of priority for a cold garage:
- Insulate the overhead door (an uninsulated steel door is a 100-square-foot single-pane window)
- Insulate the ceiling above the garage (most of the heat loss in a two-car garage)
- Insulate the walls (if the garage is finished, often already done)
- Install the right-sized heater with a smart thermostat
- Schedule the heater to run only when needed
The EV charger pairing
If you are heating the garage and also installing an EV charger, the right approach is a small sub-panel in the garage fed by a 60A or 100A feeder from the main panel. The sub-panel houses both the heater breaker and the EV charger breaker (and any future loads — compressor, welding receptacle, additional lighting). This is cleaner than running two separate dedicated circuits from the main panel and gives you headroom for future additions. Detail on the EV charger side is in our EV charger install page.
When to call us
If you have a garage you want comfortable this winter (Hamilton bungalow with a detached one-car, Burlington two-storey with an attached double, Oakville new build with a heated shop wing), we do residential electrical work across the cluster. The right scope often pairs with a panel upgrade if the existing service is tight. Request a garage heater install quote with a photo of your panel.