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Heating · Older FurnacesFurnace Pilot Light & Thermocouple Repair
Furnace pilot light and thermocouple repair applies to standing-pilot furnaces, mostly built before the 1990s, where a small flame burns all the time and a thermocouple proves it is lit. A pilot that will not stay lit is nearly always a worn thermocouple, a dirty pilot orifice or a draught, and those are inexpensive repairs at the low end of the $150–$800 that covers most furnace work. The decision rule: fix the pilot, but once a furnace of that age needs a gas valve or shows heat exchanger damage, put the money toward a new one.
A standing-pilot furnace has no electronics deciding whether it is safe to open the gas: the pilot flame heats the tip of the thermocouple, the thermocouple generates a tiny voltage, and that voltage holds a magnet open inside the gas valve. If the pilot goes out, the voltage collapses and the valve shuts, which is why a dead pilot on a healthy valve is an inconvenience and not a gas leak. These furnaces run at roughly 60–80% AFUE, so a fifth to two-fifths of the gas goes up the chimney. They turn up mostly in pre-war brick houses, century farmhouses and 1950s–60s bungalows where the furnace has never been replaced.
Three parts cause nearly every pilot call. The thermocouple wears out: a healthy one produces about 25–30 millivolts with nothing connected, and when the output sags the magnet lets go, so the pilot lights while you hold the button and dies when you release it. The pilot orifice clogs with dust or a spider web, leaving a small, wavering yellow flame that does not wrap the thermocouple tip. And the pilot tube, a thin aluminum line from the valve, gets kinked or its fitting works loose.
The fourth cause is not a part at all. A pilot that lights, holds, and is out again the next morning is often being blown out: a missing burner compartment door, a downdraft in the chimney on a windy night, or a dryer or exhaust fan pulling air down the flue of a house that has been tightened up. We check the draft at the hood and the shape of the flame before changing anything. Many furnaces from the 1980s and 1990s have no standing pilot at all — they use a spark-lit pilot or a hot-surface igniter, and that is ordinary furnace repair.
Thermocouples are a standard part and will be available for years; gas valves, burners and blower assemblies for a 35-year-old furnace often are not, or only as universal substitutes. A new furnace must meet the 95% AFUE federal minimum, so it burns far less gas for the same heat, and it has no pilot burning through the summer. Replacement runs $4,500–$7,500 installed and is not a drop-in: the new furnace vents through plastic pipe out a side wall, and the water heater left on the old chimney usually needs a liner — see furnace installation.
Call for pilot light repair when
- The pilot lights while you hold the button and goes out when you let go — the classic weak thermocouple
- The pilot flame is small, yellow or flickering instead of a steady blue cone around the thermocouple tip
- It stays lit for hours or days, then is out again, especially after windy nights or laundry days
- You have relit it twice following the instructions on the furnace and it will not hold
- You smell gas near the furnace at any point — stop, leave the house and call Enbridge Gas from outside
How it works
The technician looks at the pilot flame's size, colour and position on the thermocouple, and checks for spillage at the draft hood and air movement across the burner compartment.
The thermocouple is disconnected and its millivolt output read with the pilot burning. A low reading means a new thermocouple; a good reading with a pilot that still drops out points to the valve's magnet.
The pilot orifice and tube are cleaned or replaced, the new thermocouple is seated in the flame, and the furnace is run through full cycles. While the burners are on we look over the heat exchanger and flue so you know what shape the rest of the furnace is in.
Common questions
Is it safe to relight the pilot myself?
Yes, if you follow the lighting instructions on the furnace's label exactly and there is no gas smell. Turn the control to off and wait the stated time for any gas to clear before lighting. If you smell gas, or the pilot will not stay lit after two attempts, stop. Whether a pilot light going out is dangerous covers the safety side.
Why does my pilot light keep going out?
In order of likelihood: a thermocouple that no longer makes enough voltage, a dirty pilot orifice giving a weak flame that misses the thermocouple tip, a draught blowing the flame off, and last, a failing magnet in the gas valve. The first two are quick, low-cost repairs. A draught takes more detective work, because the cause is the house, not the furnace.
Does my furnace even have a pilot light?
If it was installed after the early 1990s, almost certainly not. Mid-efficiency furnaces of that era use a spark or hot-surface igniter, and every condensing furnace does. When one of those will not light, the suspects are the igniter and the flame sensor, not a thermocouple. A control board with a flashing diagnostic light behind the lower door means electronic ignition.
How much gas does a standing pilot use?
A small but constant amount, every hour of the year, including all summer unless you shut it off. It is a minor part of the bill beside the furnace's real inefficiency: at 60–80% AFUE the burner wastes far more than the pilot does. That gap, not the pilot, is the argument for replacing a furnace of this age.
Do you repair pilots on boilers, water heaters and fireplaces too?
Yes. The thermocouple principle is the same on an older boiler or a tank water heater, and gas fireplaces use a related part called a thermopile. Tanks have their own page, water heater thermocouple replacement; for a boiler or fireplace pilot, call the service line or use the contact form.