Quick answer
A boiler pilot light that keeps going out usually has a worn or dirty thermocouple, or a dirty pilot orifice giving a weak yellow flame. On older cast-iron boilers, chimney downdraft or a tripped spill switch can also put it out. Relight once, following the instructions on the boiler. If it goes out again, treat it as a venting and carbon monoxide question for a licensed technician.
When a boiler pilot light keeps going out, something is starving the flame, blowing it out or telling the gas valve the flame is gone. A standing pilot is a small flame that burns all season and lights the main burner on demand. It is watched by a thermocouple, and the gas valve shuts off the pilot gas soon after the thermocouple cools. That safety is why a pilot that goes out stays out.
A worn or dirty thermocouple, or a dirty pilot orifice
The thermocouple is the first suspect. Its tip sits in the pilot flame and turns heat into a tiny electrical signal, roughly 30 millivolts, that holds a magnet open inside the gas valve. A tip that is burned thin, coated in soot or bent out of the flame makes too little signal, and a loose nut where the thermocouple screws into the gas valve loses what there is. The symptom is a pilot that lights while you hold the button and dies when you let go, or one that drops out hours later.
The pilot flame itself is the second suspect. A healthy pilot is a steady blue flame that wraps the top centimetre of the thermocouple. Dust, rust flakes or spider web in the pilot orifice give a small, lazy, yellow flame that wavers off the tip and is easy to blow out. Cleaning the orifice and pilot tube is delicate work on the gas side of the appliance, which in Ontario is done by a TSSA-licensed gas technician.
Some boilers use a larger pilot generator, a thermopile, that powers the whole gas valve instead of a single safety magnet. It fails the same way and is tested the same way, with a millivolt meter.
Chimney downdraft, a tripped spill switch or a failing gas valve on older atmospheric boilers
A pilot that goes out on windy days, or when the dryer, range hood or a bathroom fan is running, is being blown out by a venting problem. An atmospheric cast-iron boiler relies on warm flue gas rising up the chimney with no fan to help. A cold exterior chimney, a flue blocked by a nest or fallen brick, or a house under negative pressure from exhaust fans can reverse that flow and push air down through the draft hood and across the pilot.
Many of these boilers have a spill switch, a small temperature-sensing disc mounted at the draft hood. When hot flue gas spills into the room instead of going up the chimney, the switch opens and shuts the burner down, and on some boilers it has to be reset by hand. A tripped spill switch is evidence that combustion gases entered the house. Resetting it and walking away ignores the one warning the boiler can give.
Chimneys change without anyone touching them. When a furnace or water heater that shared the flue is replaced with a sidewall-vented model, the old boiler is left alone on a chimney now too large and too cold for it, and draft suffers. A properly sized liner is the usual remedy, described under chimney liner installation.
The last suspect is the gas valve: if the thermocouple tests good, the pilot flame is strong and the draft is fine, the safety magnet inside the valve may be weak. It lets go at random, often when the valve is warm. A technician confirms this with a meter before condemning the valve, because the valve is the costlier part. On a boiler old enough to have a standing pilot, that quote is worth weighing against the boiler's age and condition.
Relight once, following the rating-plate instructions
- Find the lighting instructions on the label fixed to the boiler, beside the rating plate or inside the front panel, and follow that label over any general advice, this page included.
- Turn the gas control knob to OFF and wait at least five minutes for any gas to clear. If you smell gas after waiting, do not light anything: leave and call Enbridge Gas or 911 from outside.
- Turn the knob to PILOT, hold it down, light the pilot, and keep holding for the time the label states, commonly up to a minute.
- Release the knob; if the pilot stays lit, turn the knob to ON and set the thermostat.
- If the pilot dies when you let go, or goes out again in the following days, stop relighting.
A pilot lost once to a gust or a gas interruption is not an emergency, a point covered in is it dangerous if a pilot light goes out. A pilot that keeps going out is different, because the two likeliest explanations, a failing safety and a venting fault, both bear on carbon monoxide.
Call a licensed technician now if you see any of these
- The spill switch has tripped, or there is soot, scorching or rust streaking around the draft hood.
- A carbon monoxide alarm has sounded, or people in the house have headaches or nausea that ease outdoors.
- The pilot or burner flame is yellow and lazy instead of blue.
- You have relit the pilot more than once this season.
- The boiler shares a chimney with other appliances and one of them was recently changed.
Ontario law requires working carbon monoxide alarms in homes with fuel-burning appliances. Test yours the same day you relight the pilot.
What a pilot repair costs on a boiler
The visit starts with the $90–$150 diagnostic, credited to the repair. A thermocouple and pilot cleaning sit at the small end of boiler repairs and a gas valve at the larger end; both are quoted in writing once the cause is confirmed, along with a draft and carbon monoxide test when venting is in doubt. For boiler repair by a TSSA-licensed technician in Burlington, Hamilton, Oakville and Georgetown, call ZK Mechanical at (647) 801-1252 or book online.