In short
A thermocouple is the safety probe in the pilot flame of a gas water heater, fireplace or older boiler. Heated by the pilot, it makes about 25 to 30 millivolts, enough to hold the gas valve open. If the pilot lights but dies when you release the knob, the thermocouple is the first suspect, a $150–$300 repair.
Two different metals joined at a tip produce a tiny electric current when the tip is heated. A thermocouple puts that effect to work as a flame-proving device: no flame, no current, no gas. It needs no household electricity, which is why appliances built around it keep working in a power outage.
How a thermocouple keeps the gas valve open
The thermocouple powers a small electromagnet inside the gas control. Holding the pilot knob down opens the pilot gas by hand; after the probe has sat in the flame for 30 to 60 seconds it is producing enough voltage for the magnet to hold the safety valve open by itself, and you can let go. Lose the pilot and the probe cools, the magnet releases, and gas to both the pilot and the main burner stops.
A healthy thermocouple reads roughly 25 to 30 millivolts on a meter with the pilot lit and the lead disconnected from the gas control. As the tip erodes, output falls until the magnet can no longer hold, and the pilot starts dropping out at random.
Where thermocouples and thermopiles are found in GTA homes
- Tank water heaters with a standing pilot, the atmospheric vent type above all
- Older cast-iron boilers and standing-pilot furnaces
- Gas fireplaces, and unit heaters in garages and shops
A thermopile is several thermocouples wired in series inside one probe, producing roughly 500 to 750 millivolts. Newer sealed-combustion (FVIR) water heaters use one to power an electronic gas control with a blinking status light, so the pilot both proves flame and runs the electronics. Many gas fireplaces use a thermopile too, which is how a wall switch can work with no power to the unit.
How a thermocouple fails and what you will see
The classic symptom is a pilot that lights but will not stay lit once the knob is released.
- Pilot dies the moment you let go of the knob, every time: failed thermocouple or a loose connection at the gas control
- Pilot holds for hours or days, then drops: weak thermocouple, or a pilot flame too small to wrap the tip
- Blinking status light on an electronic gas control: low thermopile voltage or another fault the flash count identifies
- Pilot will not light at all: not the thermocouple; look at the gas supply, the pilot orifice or the igniter
Not every outage is the probe. A dirty pilot orifice, a clogged air-intake screen under an FVIR heater or a downdraft in the vent produces the same complaint, which is why a pilot light that keeps going out gets a diagnosis and not just a part.
What thermocouple replacement costs and who should do it
Replacing a water heater thermocouple runs $150–$300. The part is inexpensive; the visit covers removing the burner assembly, resealing the combustion chamber door on an FVIR model, setting the pilot flame and leak-testing afterwards. If the pilot lights and holds but the burner never fires, the gas control is the likelier fault, at $400–$600.
Relighting a pilot by the label on the tank is a homeowner job; the steps are in how to relight a water heater pilot light. Opening the burner compartment and disconnecting gas components is gas work, and it belongs with a TSSA-certified technician. If you smell gas, do not relight anything: leave the house and call Enbridge Gas from outside.
On a tank past ten years old, weigh the repair against the heater's age before spending on it. ZK Mechanical quotes either route under water heater service; the HVAC glossary explains the other parts named on a quote, and you can reach us here.
Questions people ask about Thermocouple
Can I replace a thermocouple myself?
The part is sold at hardware stores, but fitting it means opening the burner compartment and disconnecting parts of a gas appliance, with a leak test afterwards. That is gas work and belongs with a TSSA-certified technician, and a rented heater must be serviced by the rental company. On FVIR water heaters the burner door gasket also has to seal again, or combustion suffers. Relighting the pilot is the homeowner's part.
What is the difference between a thermocouple and a flame sensor?
Both prove that a flame exists, by different means. A thermocouple generates its own voltage from pilot heat and holds a gas valve open with no outside power. A flame sensor is a plain metal rod in the main burner flame of an electronic-ignition furnace; the control board passes a tiny current through the flame to it and closes the gas valve if that current disappears.
How long does a thermocouple last?
Many last as long as the appliance and some fail after a few years, so there is no replacement schedule. Life depends on how clean and steady the pilot flame is: a sooty or oversized flame overheats the tip, and a weak one barely heats it. On a tank water heater near the end of its 8 to 12 year life, a failed thermocouple is often the prompt to price a new tank as well.