Quick answer
A water heater pilot light that keeps going out is usually a worn thermocouple or thermopile that can no longer hold the gas valve open, a clogged flame-arrestor screen starving the burner of air, or a draft pulling the flame off the pilot. Relighting is a stopgap. The fix is a new sensor, a cleaned screen, or a venting correction by a TSSA-licensed technician.
If your water heater pilot light keeps going out, the tank is doing exactly what its safety controls are built to do: the moment the pilot sensor stops feeling heat, the gas valve closes. The real question is why the sensor keeps losing the flame. In the Burlington, Oakville and Hamilton basements we work in, it comes down to a short list of causes, and how the pilot behaves before it dies tells you which one.
The thermocouple or thermopile is worn out
The most common cause is a tired pilot sensor: older standing-pilot tanks use a thermocouple that makes a tiny voltage when heated, and tanks with electronic gas controls use a thermopile, which does the same job with more output. Both weaken with age. When the signal drops below what the valve needs, the pilot dies a minute or two after you let go of the knob, or the burner cuts out partway through a heating cycle. A thermocouple replacement runs $150–$300 installed and is almost always worth doing on a tank under 10 years old.
A quick tell: if the pilot flame looks strong and blue but still will not hold after you release the knob, suspect the sensor. If the flame itself is small, yellow or flickering, keep reading — the problem is air or gas, not the sensor.
The flame-arrestor screen is clogged
Tanks built since the mid-2000s draw their combustion air through a fine mesh screen at the base, designed so that if flammable vapours such as gasoline fumes get drawn in, the flame cannot escape the chamber and ignite them in the room. That screen collects dryer lint, pet hair and drywall dust, and once it plugs the pilot starves for air, the chamber overheats, and the thermal cutoff on the burner door trips and shuts everything down. A tank sharing a room with the dryer, or a house that has just finished its basement, is the classic case. Vacuuming the screen from outside is a homeowner job, but on some models the cutoff is a one-shot fuse that has to be replaced along with whatever tripped it.
A draft is blowing the pilot out
An atmospheric-vent tank relies on natural draft up a chimney, and that draft is easy to disturb. The classic case in 1970s–90s subdivisions like Millcroft and Glen Abbey is the orphaned water heater: the furnace got replaced with a high-efficiency unit that vents through PVC, leaving the tank alone on a masonry chimney sized for both appliances. The flue gases cool too much, the draft stalls, and on a gusty day the chimney backdrafts and snuffs the pilot. A backdrafting flue also spills combustion gases into the basement, so a pilot that dies on windy days is a reason to check the CO alarm and have the draft tested, not just to relight.
Homes along the escarpment brow — Tyandaga, parts of Ancaster and the Hamilton Mountain — get the same effect from wind alone, and tightly sealed houses can do it to themselves when a range hood, bathroom fans and the dryer all run at once and pull air back down the flue. The lasting fixes are a properly sized chimney liner, or a switch to a power-vent water heater that pushes exhaust out a side wall with a fan. Our comparison of power-vent and direct-vent tanks explains which suits which house.
When the pilot tube, the gas pressure or the gas valve is the problem
- Pilot orifice partly blocked — the flame turns small and lazy and no longer wraps the sensor tip. Cleaning the tube is a small job for a technician, but it is not a place to poke a pin yourself.
- Gas pressure sag — if the pilot only drops when the furnace and the range fire together, the supply line may be undersized for everything on it. In freezing rain, the regulator at the Enbridge Gas meter can also ice up and starve the house; that is an Enbridge call, not a plumber.
- Gas control valve failure — the electromagnet inside the valve that the thermocouple holds open weakens with age too, so a pilot that still will not hold after a new thermocouple points at the valve. A gas valve is strictly a licensed gas technician's job, and at $400–$600 for a new one the repair only makes sense on a younger tank.
What to do tonight, and when to stop relighting
- Relight once, following the label on the tank — our step-by-step on relighting a water heater pilot covers the knob sequence and the status light.
- Note the pattern: dies as soon as you release the knob (sensor), dies after a few minutes (sensor or air), dies at random on windy or laundry days (draft or screen).
- If you smell gas at any point, stop, leave the house and call Enbridge Gas from outside before anyone relights anything.
- If the tank is rented, the rental company is responsible for the repair — check the agreement before you pay a contractor.
- If the tank is past 10 years and the diagnosis is a gas valve, put the money toward a new one instead; see what a water heater replacement costs in Ontario.
A pilot that keeps failing is a symptom, not a nuisance: the safety circuit is telling you the sensor, the air supply or the draft is marginal, and a draft problem in particular can put combustion gases in the basement. ZK Mechanical's TSSA-licensed technicians diagnose the sensor, the screen and the draft in one visit, with the price written down before the work starts. Book water heater repair in Burlington or across the GTA and Hamilton, or request a visit and we will aim to have your hot water back the same day.