Quick answer
If your furnace igniter glows but the burners won't light, the igniter is working and the gas is not arriving or not catching. The usual causes are a gas valve that does not open (failed coil, or no 24 volts from the control board), a closed shut-off or interrupted gas supply, or blocked burner orifices. If the stove and water heater still work, the fault is inside the furnace.
A furnace igniter that glows while the burners won't light has already told you a lot. The thermostat called, the inducer ran, the pressure switch closed and the control board sent power to the igniter: every step before the gas is proven. What is left is short: the gas valve, the gas supply behind it, the burners in front of it, and one exception where the igniter glows without getting hot enough.
Listen for the click: is the gas valve opening?
After the igniter has glowed for several seconds to half a minute, the gas valve should open with a soft click and the burners should light at once. That click is the dividing line. No click means the valve was never powered or its coil has failed. A click and a faint hiss with no flame means gas is flowing and not catching, which points to the burners, the pressure or the igniter itself.
A technician settles it with a meter on the valve terminals during the ignition trial. If 24 volts arrives and no gas comes out, the valve's coil or internals have failed and the gas valve is replaced. If no 24 volts arrives, the valve is fine and the fault is upstream: a failed relay on the control board, a burnt connector or a broken wire. A valve also has its own on-off switch or knob, and one left off after earlier service gives the same symptom.
Gas supply: check whether the stove or water heater still works
Before suspecting furnace parts, find out whether gas is reaching the house. Light a burner on a gas stove, or run hot water until the water heater fires. If neither works, the supply is off and the furnace is not the problem.
- Furnace shut-off: the quarter-turn valve on the pipe beside the furnace. Handle in line with the pipe is open; crosswise is closed.
- Other gas appliances: all dead means the meter. Enbridge Gas shuts or locks a meter after a leak call, meter work or an account issue, and getting the gas turned back on has its own steps.
- Propane homes in rural Caledon and Flamborough: read the tank gauge.
- Timing: if the furnace lights fine alone but fails while the water heater, dryer or range is running, the line cannot supply everything at once.
- One reset: cut power for 30 seconds and watch a single start. Do not keep cycling it.
Low inlet pressure under load is measured, not guessed. The technician puts a manometer on the valve's inlet tap and fires every gas appliance in the house. A reading that sags points to undersized piping, a partly closed valve or a regulator, and the fix is on the gas line.
Blocked burner orifices and an igniter that glows too cool
Each burner is fed through an orifice, a brass fitting with a hole about the size of a pinhead. Rust flakes and spider webs block them, most often over the summer. If the orifice beside the igniter is blocked, gas reaches the other burners but never passes the hot element, and nothing lights.
The exception to 'a glowing igniter is a good igniter' is age. Older silicon carbide igniters gain resistance as they wear and glow a dull orange instead of a bright yellow-white. That looks like it is working and is not hot enough to light gas reliably, especially on a cold start. An igniter knocked out of position, so the gas stream misses it, does the same, which is why a technician checks resistance and placement instead of judging by eye; what a furnace igniter costs covers the part.
Why the gas train is G2 or G1 licensed work in Ontario
Everything between the shut-off and the flame is the gas train: valve, manifold, orifices and pressure settings. In Ontario, work on it has to be done by a TSSA-certified gas technician holding a G2 or G1 licence, working to the CSA B149.1 code. A homeowner can check a valve handle, the thermostat, the filter and the other appliances, and should stop there.
- Call now, from outside: you smell gas. Leave first; what to do if you smell gas has the steps.
- Call now: the burners light late with a whoomp. Gas is pooling before ignition.
- Stop resetting: after two failed starts, leave the furnace off. Every trial releases a little unburnt gas into the burner box.
- Can wait until morning: no smell, the furnace is simply locked out, and the house is holding temperature.
What it costs to fix a furnace that glows but will not light
- Diagnostic visit: $90–$150, folded into the repair if you proceed
- Hot surface igniter: $200–$450
- Gas valve: $500–$1,000
- Control board: $500–$1,200
- Burner and orifice cleaning: part of a full tune-up at $120–$250
On a furnace past 15 years, weigh a gas valve or board against the age of everything else in the cabinet before approving it. ZK Mechanical's TSSA-licensed technicians test the valve, the board and the gas pressure on the same visit and quote in writing before any part is replaced. To arrange furnace repair in Burlington, Oakville, Milton, Hamilton or Mississauga, phone (647) 801-1252 or book a visit online.