Thermopile — millivolt flame generator for gas heat

A thermopile is a flame-heated sensor that generates millivolt power to operate certain gas valves and controls without household electricity.

Thermopile diagram — labeled parts, dimensions, and installation context

What It Is

A thermopile works like a stronger version of a thermocouple. It sits in a pilot flame and produces a small but usable electrical output that can power a millivolt gas valve or wall switch circuit.

This design is common in gas fireplaces and heaters that continue working during a power outage. If the thermopile output drops, the pilot may stay on while the main burner no longer opens reliably.

Types

Most residential thermopiles are millivolt generator assemblies used in fireplaces, wall heaters, and older space-heating equipment. Universal replacements vary by mounting style, lead length, and output rating.

Where It Is Used

Thermopiles are used in gas fireplaces, decorative gas logs, wall furnaces, and some older gravity or millivolt heating appliances. They are especially common where a simple on-off wall switch controls the burner without line voltage.

How to Identify One

Look for a probe assembly mounted in the pilot flame that is slightly larger than a basic thermocouple and connected by two wires or terminals to the gas valve. Fireplace manuals often refer to it as a generator or millivolt generator.

Replacement

Replacement is needed when millivolt output falls too low, the burner stops responding to the wall switch, or the pilot assembly has deteriorated. The replacement must match the appliance and be positioned correctly in the pilot flame.

Thermopile — FAQ

What is the difference between a thermopile and a thermocouple?

A thermocouple mainly proves the pilot flame is present, while a thermopile produces more voltage and can help power the gas valve circuit itself. In many fireplaces, the thermopile is what lets the burner work without house power.

How do I know a thermopile is failing?

A common symptom is a pilot that stays lit but a main burner that will not turn on or stay on. Weak millivolt readings, corroded connections, and a lazy pilot flame can all point to thermopile trouble.

Can a dirty pilot flame affect the thermopile?

Yes. If the flame is not hitting the generator properly, output drops and the valve may not get enough power to open. Cleaning the pilot assembly is often part of the diagnosis.

Do thermopile fireplaces work in a power outage?

Many do, as long as the unit uses a true millivolt control system and not an add-on blower or electronic ignition requirement. The pilot flame generates the power the basic gas valve needs.