Quick answer
A gas garage heater that won't ignite is stalling at one step of its start sequence: inducer fan, pressure switch, igniter, gas valve, then flame proving. After a summer idle, suspect a closed gas shut-off, dead thermostat batteries, a spider-blocked orifice or pilot, a dirty flame sensor or a blocked vent. On propane, check the tank gauge first; under about 20 percent in deep cold can starve the heater.
If your gas garage heater won't ignite, watch one start-up from beginning to end before touching anything. A ceiling-hung unit heater runs the same sequence every time, and the step where it stops points at the part that failed. Turn the thermostat up, stand where you can see and hear the heater, and count what happens.
The unit-heater ignition sequence, and where each step stalls
- Step 1, the thermostat calls. Nothing happens at all: no power to the heater, a dead thermostat, a tripped breaker or GFCI on the garage circuit, or a blown fuse on the control board.
- Step 2, the inducer (the small venter fan) starts. If it hums or stays silent, the inducer motor has seized, often from sitting in damp air all summer.
- Step 3, the pressure switch proves the vent is clear. Inducer runs on and on with no spark or glow: most often the switch has not closed, because the vent cap is blocked, the rubber hose is cracked or holds water, or the switch has failed.
- Step 4, the igniter fires: a rapid ticking spark on most unit heaters, a glowing hot-surface element on others. If the fault light says the pressure switch is satisfied and there is still no spark, the igniter, its cable or the board is at fault.
- Step 5, the gas valve opens. Spark or glow with no flame: no gas is arriving (shut-off closed, air in the line, empty propane tank) or the orifice is blocked.
- Step 6, flame is proven. Burners light and drop out in a few seconds: the flame sensor is dirty. That fault has its own page, why a garage heater keeps shutting off.
After a failed try most boards purge and try again, typically three times, then lock out. Cutting power to the heater for 30 seconds clears the lockout but also clears the fault memory, so read the light first. Infrared tube heaters follow a slightly different sequence; see why a radiant tube heater will not ignite.
What a summer of sitting does: the off-season causes
A heater that ran in March and refuses in November has usually not broken; something changed while it sat. Check these before calling.
- Gas shut-off: the quarter-turn valve on the drop to the heater, handle in line with the pipe. A detached garage has a second valve where the line leaves the house.
- Thermostat: set to Heat and above garage temperature. Battery thermostats fade in an unheated garage, so put in fresh cells.
- Power: the service switch near the heater, the breaker, and any GFCI outlet upstream.
- Vent cap outside: look for a wasp or bird nest, leaves or ice in the termination.
- The heater itself: sawdust, drywall dust and paint overspray on the burner and air intake. A garage workshop coats a flame sensor far faster than a basement does.
Two off-season causes are not homeowner fixes. Spiders and mud-dauber wasps build inside burner orifices and pilot tubes, and a web is enough to stop gas reaching the igniter. A flame sensor coated in oxide or dust needs to be pulled and cleaned. Both mean opening the burner compartment, which is licensed gas work in Ontario.
Propane heaters in rural Flamborough and Caledon
Where gas mains do not reach, in rural Flamborough, Carlisle, Kilbride and much of rural Caledon, the garage heater runs on a propane tank, and the tank is the first suspect. Propane has to boil off as vapour inside the tank, and the colder and emptier the tank, the less vapour it makes. A tank under about 20 percent on a −20 °C morning can show fuel on the gauge and still not hold pressure at the heater.
Regulator freeze-up is the other propane fault. Moisture in the fuel, or freezing rain over the regulator vent, ices the regulator and gas flow drops or stops. Leave it alone and call the propane supplier, who usually owns the tank and regulators.
A heater converted between natural gas and propane also needs the correct orifice kit, so a unit that has never lit properly since a fuel change should be checked. Our post on propane heating in rural Flamborough covers the supply side.
Reading the LED fault code on a unit heater
Most unit heaters have a diagnostic light on the control board, visible through a sight hole or behind the access panel, and its flash count names the failed step. The legend is printed on the wiring diagram inside the panel. Typical entries separate a pressure switch stuck open, a pressure switch stuck closed, an ignition lockout, an open high-limit and flame sensed with the gas valve off. Flash counts are not standard between makes, so go by the label on your heater, and write the count down before you reset anything.
What a garage heater repair costs, and when to replace instead
A garage heater diagnostic visit is $90–$150, folded into the repair if you go ahead. Unit heaters use the same class of parts as a furnace, so the published furnace bands are a fair guide:
- Flame sensor cleaning or replacement: $150–$300
- Igniter: $200–$450
- Pressure switch: $250–$500
- Inducer motor: $400–$900
- Gas valve: $500–$1,000
The exact figure for your heater is quoted in writing before work starts. Replace instead of repairing when the heat exchanger tubes have rusted through or the control board for an older unit is discontinued; garage heater installation covers sizing and venting a new one.
- Call now: you smell gas or propane in the garage. Leave, and do not operate the door opener or any switch.
- Call now: the heater lights with a bang, or flame is visible outside the burner box.
- Call now: soot streaks on the cabinet, or a carbon monoxide alarm.
ZK Mechanical's TSSA-licensed technicians work on natural gas and propane garage heaters across Burlington, Milton, Hamilton, Flamborough and Caledon. Phone (647) 801-1252 or describe the fault online, and have the flash count ready.