If you have been troubleshooting a gas fireplace that will not stay lit or will not ignite the main burner, you have probably come across the terms thermopile and thermocouple. These two components are often confused because they look similar, are located in the same area of the fireplace, and both involve generating voltage from heat. However, they serve different purposes and fail in different ways.

Understanding the difference between a thermopile and a thermocouple is essential for diagnosing fireplace problems correctly and ordering the right replacement part. This guide explains what each component does, how they work, how to test them, and how to know which one has failed.

What Is a Thermocouple?

A thermocouple is a safety device found in virtually all gas fireplaces with a standing pilot light. Its sole purpose is to detect whether the pilot flame is lit. If the pilot goes out, the thermocouple signals the gas valve to shut off gas flow, preventing unburned gas from accumulating in your home.

How a Thermocouple Works

A thermocouple consists of two different metals joined at one end (the sensing tip). When the pilot flame heats this junction, the difference in temperature between the hot end and the cool end generates a small electrical voltage, typically between 20 and 30 millivolts. This voltage holds open an electromagnetic safety valve inside the gas valve body.

When the pilot goes out, the thermocouple cools, voltage drops to zero, and the electromagnetic valve snaps shut, cutting off all gas flow.

Signs of a Failing Thermocouple

• The pilot light will not stay lit after you release the control knob. You can light the pilot while holding the knob down, but it goes out within seconds of releasing.
• The pilot used to stay lit but now takes longer and longer to "catch" before you can release the knob.
• The thermocouple tip looks corroded, bent, or is not positioned in the pilot flame.

What Is a Thermopile?

A thermopile is essentially a series of thermocouples wired together to produce a higher voltage output. While a single thermocouple generates 20 to 30 millivolts, a thermopile typically generates 300 to 900 millivolts. This higher voltage is what powers the main gas valve and allows it to open and close on demand.

How a Thermopile Works

The thermopile is positioned in the pilot flame alongside the thermocouple. As the pilot heats the thermopile, it generates enough voltage to power the gas valve circuitry. When you flip the wall switch, press the remote, or the thermostat calls for heat, this voltage is what opens the main gas valve and allows gas to flow to the burner.

In a millivolt fireplace system, the thermopile is the only power source. There is no external electrical connection. This is why millivolt fireplaces continue to work during a power outage, as long as the pilot light is burning. For a deeper comparison, see our guide on millivolt vs electronic ignition gas fireplaces.

Signs of a Failing Thermopile

• The pilot light stays lit (thermocouple is fine), but the main burner will not ignite when you turn on the fireplace.
• The fireplace works intermittently. Sometimes the burner lights, other times it does not respond.
• The remote or wall switch seems unresponsive, but the pilot is burning strong.
• The thermopile voltage reads below 300 millivolts under load.

Key Differences at a Glance

Feature	Thermocouple	Thermopile
Purpose	Safety: detects pilot flame	Power: operates the main gas valve
Voltage output	20-30 millivolts	300-900 millivolts
Number of junctions	Single junction (two wires)	Multiple junctions in series
Connection to gas valve	Threaded nut on a single lead	Two leads (spade connectors or screw terminals)
What happens when it fails	Pilot will not stay lit	Pilot stays lit but burner will not ignite
Typical lifespan	5-10 years	5-10 years

How to Test a Thermocouple

You will need a multimeter set to the DC millivolts range:

1. Light the pilot and let it burn for two to three minutes to fully heat the thermocouple.
2. While holding the control knob in the pilot position, disconnect the thermocouple lead from the gas valve.
3. Touch one multimeter probe to the thermocouple lead and the other to the outer casing of the thermocouple connection.
4. A healthy thermocouple should read between 25 and 35 millivolts.
5. Below 20 millivolts, the thermocouple should be replaced.

How to Test a Thermopile

Testing a thermopile is similar but you need to measure under load (with the thermopile connected to the circuit) for an accurate reading:

Open Circuit Test (No Load)

1. Light the pilot and let it burn for three to five minutes.
2. Disconnect the thermopile leads from the gas valve.
3. Set your multimeter to DC millivolts.
4. Touch the probes to the two thermopile leads.
5. A healthy thermopile should read 650 to 850 millivolts open circuit.

Closed Circuit Test (Under Load)

1. Reconnect the thermopile leads to the gas valve.
2. Use the multimeter probes on the thermopile terminals at the gas valve (leave everything connected).
3. Turn the fireplace on (call for main burner ignition).
4. The reading under load should be at least 300 millivolts. The gas valve needs a minimum voltage to open, and each additional device in the circuit (remote receiver, wall switch) adds resistance.
5. If the reading drops below 300 millivolts under load, the thermopile is too weak and should be replaced.

Can You Replace Just One?

Yes, thermocouples and thermopiles can often be replaced independently. However, in many gas fireplaces, both components are integrated into a single pilot assembly. In these cases, replacing the entire pilot assembly is more practical and ensures all components are new.

Our thermocouple replacement guide walks through the process step by step. If your fireplace uses an ODS (Oxygen Depletion Sensor) pilot assembly, the thermocouple is built into the assembly and must be replaced as a unit. Learn more in our ODS pilot assembly guide.

Extending the Life of These Components

While thermocouples and thermopiles will eventually wear out, you can maximize their lifespan:

• Keep the pilot assembly clean: Dust and debris on the pilot burner can affect flame quality and how well it heats these components.
• Ensure proper pilot flame position: The pilot flame should fully engulf the sensing tips of both the thermocouple and thermopile. If the pilot flame has shifted, adjust the pilot assembly so the flame makes solid contact.
• Annual maintenance: During your yearly fireplace inspection, have the pilot assembly checked and cleaned.
• Consider turning off the standing pilot in summer: Running the pilot year-round exposes the thermocouple and thermopile to constant heat, which accelerates wear.

Shopping for Replacements

When ordering a thermocouple or thermopile, you need to know:

• Your fireplace make and model (see our model number guide)
• The length of the current component (measured from tip to connection)
• The connection type (threaded nut, spade connectors, or specific plug)
• Whether it is part of a complete pilot assembly

Browse our selection of fireplace replacement parts including thermocouples, thermopiles, and complete pilot assemblies for all major brands.

When to Call a Professional

Replacing a thermocouple or thermopile is within the ability of most handy homeowners, but consider calling a professional if:

• You are not comfortable working around gas appliances. Even though you are not disconnecting gas lines to replace these components, working near the gas valve and pilot assembly requires care.
• The replacement does not fix the problem. If you install a new thermocouple and the pilot still will not stay lit, or a new thermopile does not restore burner operation, there may be an issue with the gas valve itself.
• You smell gas at any point during the process. Stop work immediately, ventilate the area, and call your gas utility.
• Your fireplace uses an electronic ignition system. IPI systems do not use traditional thermocouples and thermopiles. If you have an electronic ignition fireplace with ignition problems, the issue is likely the control module, flame sensor, or igniter, which may require professional diagnosis.