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Water Heaters · Pilot RepairWater Heater Thermocouple Replacement
Water heater thermocouple replacement costs $150–$300 and is the usual fix when the pilot lights while you hold the button down and dies the moment you let go. The thermocouple proves the pilot flame to the gas control, and when its output falls too low the control shuts off the gas on purpose. If a new one does not hold the pilot, the fault is somewhere else and the diagnosis keeps going.
A thermocouple is a small heat-driven generator. Its tip sits in the pilot flame and produces roughly 25 to 30 millivolts, enough to hold open a tiny electromagnet inside the gas control; lose the flame and the magnet drops out, closing the gas. A thermopile is a bundle of thermocouples in one sheath that makes several hundred millivolts and powers the electronic controls on newer tanks. Both weaken with age, and a weak one lets the pilot drop out at random, often overnight.
Testing comes before parts. The technician reads the output with a meter while the pilot burns and looks at the flame itself: it should be steady, blue and wrapped around the tip. A lazy yellow flame that barely touches the thermocouple is a dirty pilot orifice or a starved air supply, and the thermocouple is fine. Our answer on why a water heater pilot keeps going out covers the causes in order.
On tanks built since about 2003 the burner is sealed behind a gasketed door, with the pilot visible only through a sight glass. The thermocouple cannot be reached in place, so the whole burner assembly comes out as a unit: the burner tube, pilot tube and thermocouple are disconnected at the gas control and the door is unbolted. The door gasket is replaced, not reused, because a leaking door upsets combustion. Many of these tanks take a model-specific thermocouple, some with a built-in thermal cut-off, so the universal part from a hardware store will not fit.
A pilot that still will not stay lit after a new thermocouple has three usual explanations. The magnet in the gas control has failed, which means a gas control valve replacement at $400–$600; the pilot tube or orifice is partly blocked; or poor draft is blowing the flame off the tip, which is a venting repair. On sealed tanks a lint-clogged air intake can also trip the thermal switch on the door. On a tank past 10 to 12 years that needs both a thermocouple and a control, a new tank is the better use of the money.
Call for a thermocouple test when
- The pilot lights with the button held and goes out when you release it
- The pilot stays lit for hours or days, then is out again in the morning
- The status light on an electronic control flashes the low-thermopile code shown on the tank label
- You have relit the pilot more than twice this month
- You smell gas at any point: stop relighting, get everyone out and phone from outdoors
How it works
Output is measured with the pilot lit, the flame is inspected, and the draft at the hood is checked, so the part that is replaced is the part that failed.
The thermocouple or thermopile is changed, the pilot orifice and burner are cleaned, and on sealed tanks the flame-arrestor screen is cleared and the door goes back on a new gasket.
The connections at the gas control are leak-tested, the pilot is relit and the burner is run through a cycle. Gas-side work is done by a TSSA-licensed technician. Book the repair.
Common questions
Can I replace a water heater thermocouple myself?
On an old open-burner tank people do, but in Ontario work on a gas appliance belongs to a TSSA-licensed technician, and the connections you disturb carry gas. On a sealed-burner tank it is not a homeowner job in any sense: the burner assembly, the gasket and a leak test are involved. What you can do safely is relight the pilot once using the lighting steps.
How long does a thermocouple last?
There is no rated life. Many last as long as the tank, and others weaken after several years of sitting in a flame around the clock; a sooty, yellow pilot shortens it. If the pilot has started dropping out on a tank that is otherwise healthy and under ten years old, the thermocouple is the first suspect and the cheapest.
Is it safe to keep relighting the pilot?
Once is fine. The safety design is doing its job: when the pilot goes out, the control closes the gas. Relighting over and over is where the risk comes in, especially if the real cause is a draft problem pushing exhaust back into the room. Wait five minutes for any gas to clear before each attempt, and if it goes out again, book a visit.
How long does the repair take?
About an hour on a tank with an open burner, and closer to two on a sealed-burner tank where the assembly has to come out. The tank does not need to be drained for either. Hot water is back once the burner has reheated the tank, which takes roughly another hour after the relight.
Could it be something other than the thermocouple?
Yes, and that is why we test first. The same symptom comes from a blocked pilot orifice, a failed gas control, a tripped thermal switch, or downdraft from the vent. If the tank is old enough that the repair list is growing, our water heater service page covers replacement, and a straight tank swap runs $1,200–$2,500.