Quick answer
A garage heater that keeps shutting off is tripping one of its safeties. If the burner cuts out while the fan keeps running, the high limit is opening from a dirty fan, blocked louvres or too little ceiling clearance. If it dies seconds after lighting, suspect a dirty flame sensor. If it quits on windy days or when other gas appliances fire, suspect the vent run or low gas pressure.
When a garage heater keeps shutting off, the timing of the shutdown names the part. A gas unit heater has the same safety chain as a furnace, a pressure switch, an igniter, a flame sensor and a high limit, but it hangs from a ceiling in a dusty, drafty room at the far end of a long gas line. Each of those conditions produces its own pattern.
Burner cuts out but the fan keeps blowing: the high limit is tripping
The high-limit switch opens when the heat exchanger gets hotter than it should, and closes again once the fan has cooled it. You hear the burner stop, the fan run on, and the burner relight a few minutes later. The heater is overheating because air is not moving across it fast enough.
- A dirty fan. Garage air carries sawdust, drywall dust and lint, and it cakes on the leading edge of the fan blades and on the motor.
- Blocked or closed louvres. Louvres angled almost shut, or a heater aimed at a shelf, a stored canoe or the overhead door when it is raised, bounce hot air straight back into the unit.
- Too little clearance. Every unit heater has minimum distances to the ceiling, the walls and anything stored nearby, printed on the rating plate and in the installation manual. Mounted tight to the ceiling or boxed in by storage, the heater recirculates its own hot air.
- A fan motor with worn bearings that starts slowly or runs below speed.
Lights, then dies in seconds: a dirty flame sensor
A burner that lights, drops out after a few seconds and then retries is failing to prove flame. The flame sensor is a thin metal rod in the burner flame; the control board reads a tiny current through it, and a coating on the rod blocks that current. In a garage the coating builds quickly, because paint overspray, aerosol lubricants, solvent vapour and welding fume all bake onto the rod as an insulating glaze.
After several failed tries most heaters lock out for a period or until the power is cycled. Cleaning the rod is a short job for a technician. If the heater shares the space with spray finishing or solvents, expect it to need doing every season, and consider a separated-combustion model that draws its burner air from outdoors. A heater that never lights at all is a different fault, covered in why a gas garage heater will not ignite.
Shuts off on windy days: the pressure switch and the vent run
The pressure switch proves that the draft inducer is pulling exhaust through the heat exchanger and out the vent. If the draft falls away mid-cycle, the switch opens and the burner stops. Wind gusting against a sidewall termination does it, and so does a vent that is longer, or has more elbows, than the manufacturer allows for that pipe diameter.
A garage invites long vent runs because the heater goes where the truck is not. Each model lists a maximum equivalent vent length, and every elbow counts against it. Other causes on the same switch are a cracked or water-filled sensing hose, a nest or ice in the vent cap, and an inducer motor that is slowing down.
Shuts off when other appliances run: low gas pressure on a long line
A heater at the end of an undersized gas line starves when demand rises. The pipe to a detached garage is often a long run added years after the house was built. CSA B149.1, the gas installation code used in Ontario, sizes pipe by the length of the run and the total load on it. A line too small for the distance delivers the right pressure with nothing running and too little with the heater, furnace and water heater all firing.
The symptoms are a flame that shrinks or lifts, a rumble at the burner and a shutdown on flame loss, most often on the coldest mornings. A technician confirms it with a manometer on the heater's inlet tap while every gas appliance on the property is running, and compares the reading with the minimum inlet pressure on the rating plate. The fix is a larger line, which is work for a licensed gas fitter; see gas line upgrades. On propane, common on rural lots in Carlisle, Kilbride and rural Caledon, a tank running low or a regulator icing up gives the same pattern.
Check this yourself, and when to stop
- Thermostat location: beside the overhead door it reads cold drafts, and in the heater's own airstream it satisfies in minutes.
- Louvres open and aimed into clear space, with nothing stored inside the listed clearances.
- Vent cap on the outside wall or roof: look for frost build-up, wasp nests and anything leaning against it.
- The control board light, visible through the sight glass: count the flashes and write the code down before cycling the power.
- Fan blades: with the power switched off, look at them with a flashlight. Leave the burner compartment closed.
Stop and call if you smell gas, see soot or scorch marks on the cabinet, see flame outside the burner area, or have reset the heater more than twice. A safety that keeps tripping is reporting a real condition, and resetting it over and over is how heat exchangers crack.
What garage heater repairs cost
A unit heater uses the same class of parts as a residential furnace, so the furnace repair bands are a fair guide:
- Diagnostic visit to read the fault code and test the safeties: $90–$150, credited toward the repair.
- Flame sensor cleaning or replacement: $150–$300.
- Pressure switch: $250–$500.
- Draft inducer motor: $400–$900.
- Gas line resizing or vent rerouting: quoted in writing after the run is measured.
ZK Mechanical repairs and installs gas unit heaters in attached and detached garages from Burlington and Waterdown to Milton and Georgetown. If the heater is the wrong size, badly placed or past its life, garage heater installation covers the options and what a garage heater costs installed has the numbers. Call (647) 801-1252 or send photos of the heater and its vent.