Geyser Thermocouple: What It Does and When to Replace

Geyser thermocouple: what it does and when to replace it

The thermocouple is a thin copper probe that sits in the pilot flame. Heat makes it produce a tiny electric signal that holds the gas valve open. If the flame goes out, it cools and the valve shuts within a minute or so. A worn or misplaced thermocouple is the most common reason a pilot lights but will not stay lit.

Signs of a weak thermocouple

The pilot lights while you hold the knob but goes out when you release it after a full minute.
The pilot stays lit sometimes, then fails for no clear reason.
The tip is heavily corroded, bent or not in the flame.

Check positioning first

The thermocouple tip should sit in the upper part of the pilot flame. If the bracket has moved or the flame is small, it may not get hot enough. Cleaning the pilot jet often fixes what looks like a thermocouple fault.

Replacement

Thermocouples are inexpensive and screw into the gas valve. Use the correct length and fitting for your valve, and do not overtighten. If you are not comfortable working on gas connections, a technician can replace it quickly.

Why it matters for safety

The thermocouple is what makes a pilot going out safe: without it, gas would keep flowing unburnt. Never bypass or jam it yourself. An automatic ignition device must bring its own safe gas control: Autopilot has a gas controller that opens and closes gas automatically and an emergency shut-off.

Frequently asked questions

How long does a geyser thermocouple last?

Several years in normal use, less where the pilot flame is dirty or the geyser is exposed to weather.

Does Autopilot need a working thermocouple?

Not by itself. Autopilot bypasses the pilot function and brings its own gas controller with automatic shut-off, so a weak pilot or thermocouple no longer stops the geyser starting. Without Autopilot, replace a worn thermocouple.