Thermocouple

A thermocouple is a safety sensor used in gas water heaters, boilers, and furnaces. It consists of two dissimilar metals joined at one end (the hot junction) and connected to a gas valve at the other. When the pilot flame heats the junction, it generates a small electrical voltage (typically 15–30 millivolts) that holds the gas valve open. If the pilot goes out and the junction cools, the voltage drops, and a spring in the gas valve snaps it shut — stopping all gas flow to the burner.

Why it matters

Without a working thermocouple, a gas water heater would release unburned gas into your home every time the pilot blew out. The thermocouple is what makes standing-pilot appliances safe. It's a simple, passive device with no power source — pure physics keeps the gas valve in check.

Signs of thermocouple failure

• Pilot lights but won't stay lit after releasing the control knob
• Appliance works for a few minutes then shuts off
• Water heater produces no hot water despite the pilot appearing to light

Testing and replacement

A multimeter set to DC millivolts can test a thermocouple: a good one reads 25–35 mV when the tip is in flame. Under 20 mV means the thermocouple is weak and should be replaced. Replacement thermocouples cost $10–$25 at hardware stores and take 20–30 minutes to swap — shut off gas, disconnect the lead from the gas valve, unscrew the bracket, insert the new unit, reconnect, and relight the pilot.

Thermopile vs. thermocouple

Some appliances use a thermopile — a series of thermocouples wired in series, generating more voltage (250–750 mV). Thermopiles can power electronic controls and remote thermostats, while a single thermocouple only produces enough current to hold open a simple gas valve. Both devices fail the same way and are diagnosed similarly.