Thermocouple

Short definition

A thermocouple is the small pencil-shaped sensor in a standing-pilot gas water heater that generates roughly 25 millivolts when its tip is bathed in pilot flame. That tiny voltage holds the gas valve open. When the pilot goes out — or the thermocouple fails — voltage drops, the valve closes, and the burner can’t fire. A failed thermocouple is the most common cause of “my pilot won’t stay lit.”

What it is

A thermocouple is two dissimilar metal wires joined at one end (the “junction”). When the junction is heated, it generates a small DC voltage by the Seebeck effect — about 25 millivolts at full pilot temperature. That voltage feeds the gas-control valve’s pilot magnet, holding the magnet open as long as the pilot is burning.

When the pilot goes out, the thermocouple cools, voltage drops below the magnet’s holding threshold, the magnet releases, and the gas valve closes. Failsafe: no pilot, no gas to the burner.

The relight procedure on every standing-pilot heater is built around this: hold down the pilot button manually for about 60 seconds while the pilot flame heats the thermocouple, release, and the magnet holds itself open from then on.

Thermocouples are cheap ($10–$30 retail), standardized in length and thread, and a 30-minute DIY swap. Lifespan is typically 5–15 years; failure is usually gradual (pilot won’t stay lit reliably) but sometimes sudden (open circuit, no voltage).

Why it matters to a homeowner

If your gas water heater pilot won’t stay lit after holding the pilot button for 60 seconds, the diagnostic is almost always thermocouple replacement. The procedure:

1. Shut off the gas at the appliance shut-off valve.
2. Wait 5 minutes for any residual gas to clear.
3. Pull the burner assembly out (typically 2–3 fasteners or a clip).
4. Unscrew the old thermocouple from the gas valve and from its mounting bracket.
5. Install the new one — finger-tight at the gas valve, then 1/4 turn with a wrench (don’t over-torque, the threads are aluminum on most valves).
6. Position the tip in the inner cone of the pilot flame.
7. Reassemble, restore gas, and relight.

When a plumber’s quote on a no-hot-water call says “thermocouple replacement,” that’s a $100–$200 fix. When the same call leads to “you need a new water heater,” push back if the heater is under 12 years old — the thermocouple is rarely the heater itself failing.

Common variants and what a thermocouple is not

• Thermocouple vs. thermopile. A thermopile is multiple thermocouples in series; generates about 750 millivolts. Used on millivolt-only gas appliances (common in fireplaces, rare in water heaters).
• Thermocouple vs. flame sensor (HSI heater). Hot-surface-ignition heaters use a flame sensor based on a different principle (rectified AC current). Not interchangeable with a thermocouple.

Common failure modes

• Won’t keep pilot lit even after holding pilot button 60+ seconds. Thermocouple worn or contaminated. Replace.
• Loose connection at gas-valve threading. Torque too low. Tighten by another 1/8 turn (don’t over-torque).
• Junction contaminated with carbon or soot. Clean carefully with fine sandpaper or replace.
• Bent in the wrong position relative to pilot flame. Tip must sit in the inner flame cone. Reposition.