How do I know if my gas fireplace thermopile is bad?

A gas fireplace thermopile is an important component that allows your gas fireplace to operate safely and efficiently. The thermopile generates a small electrical current that powers the gas valve to open and allow gas to flow to the burner when you turn your fireplace on. If the thermopile malfunctions, your fireplace may not ignite or stay lit properly. Here are some signs that indicate your gas fireplace thermopile may be bad and need replacing.

The pilot light won’t stay lit

One of the most common signs of a bad thermopile is that the pilot light won’t stay lit after you light it. The thermopile powers an electromagnet in the gas valve that keeps the main gas supply open after you light the pilot. If the thermopile doesn’t generate enough millivolts of electricity, the electromagnet will lose power, causing the gas valve to shut off the gas supply to the main burner and the pilot light will go out.

To test this, light the pilot light as you normally would. The pilot should ignite and stay lit. Then release the control knob. The pilot should remain ignited. If the pilot goes out after you release the control knob, the thermopile is likely failing to keep the gas valve open.

The burner won’t light

If pushing the ignitor or turn the flame height knob has no effect, the thermopile may not be producing enough power for the burner to operate. The thermopile opens the main gas valve by generating millivolts of electricity to power on the electromagnet in the valve. If the thermopile is defective, it won’t generate enough electricity and the gas valve will remain closed, preventing gas from flowing to the burner.

To test this, ensure the pilot is lit, then turn the flame height knob to the ON position. If the main burner doesn’t light, the thermopile may not be providing enough power to open the main gas valve.

Low flame height

A weak, low flame that is difficult to adjust higher could also indicate thermopile failure. The thermopile generates electricity proportional to the temperature differential between the pilot flame and the tip of the thermopile. A defective thermopile with corroded or broken electrical connections can cause low voltage output. This provides just enough power to open the gas valve slightly, but not fully to allow proper gas flow and flame height.

The pilot light keeps going out

If the pilot light will ignite but repeatedly goes out after some time, the thermopile may be on its way out. A failing thermopile will still generate enough electricity to power the electromagnet and open the gas valve, but only for a short time. As the thermopile wears out, it will provide power for shorter and shorter durations before the voltage drops low enough that the valve closes and the pilot goes out.

This gradual failure of the thermopile to provide sustained power to the electromagnet is why the pilot light keeps extinguishing repeatedly after staying lit for some time when the thermopile is failing.

Using a multimeter to test voltage

The best way to confirm a thermopile is bad is to use a multimeter to check the millivolt output. A gas valve requires a thermopile reading of at least 300 millivolts to open properly. Any lower and there may not be enough voltage to power on the electromagnet and allow gas flow.

To check the thermopile voltage:

1. Remove the thermopile wires from the gas valve terminals (note which wire went into which terminal).
2. Turn your multimeter to the “Millivolts” setting.
3. Touch the multimeter probes to the thermopile’s wire terminals.
4. With the pilot lit, note the millivolt reading on the multimeter.
5. If the reading is less than 300 mV, the thermopile is likely faulty and should be replaced.

The pilot flame needs to be touching the tip of the thermopile to get an accurate voltage reading. If the pilot won’t stay lit, you may need someone to hold a lighter to the thermopile tip while taking the measurement.

Thermopile Voltage Reading	Result
Less than 300 mV	Bad thermopile – replace
300 mV or higher	Good thermopile

Visual inspection

You can also visually inspect the thermopile for signs of failure:

• Corroded or cracked ceramic insulators – The ceramic material that insulates the thermopile’s copper and alloy wires can become cracked or corroded over time due to heat and flame exposure. Any cracks or corrosion can allow voltage leaks, reducing output.
• Sooting or discoloration – The thermopile may become covered in soot or show discoloration if it’s exposed to condensed moisture or combustion byproducts, negatively affecting heat transfer.
• Melting of metals – The metals inside the thermopile can melt from extreme heat exposure, damaging the components and electrical connections.
• Damaged pilot hood – The pilot hood focuses the pilot flame on the thermopile. If it’s damaged or misaligned, the thermopile won’t heat up properly.
• Loose electrical terminals – Loose wire connections to the gas valve caused by vibration, thermal cycling, or corrosion can increase resistance and reduce voltage.

Any signs of damage, sooting, deterioration, or loose connections indicates potential thermopile failure or underperformance.

When to replace a gas fireplace thermopile

Here are some general guidelines for thermopile replacement in a gas fireplace:

• The thermopile voltage reading is less than 300 mV
• The pilot won’t remain lit after releasing the pilot valve knob
• The main burner won’t light even with the pilot lit
• The pilot keeps shutting off repeatedly after staying lit for a short time
• Flame height seems low and won’t adjust up with the flame height knob
• The thermopile shows damage, deterioration, overheating signs, or loose connections
• The fireplace is over 10 years old and the thermopile has never been replaced

Replacing your thermopile as preventative maintenance every 5-10 years can help avoid emergency repairs and improve fireplace reliability. If your fireplace pilot won’t stay lit or the burner isn’t functioning properly, replacing the thermopile is usually the solution.

How to replace a gas fireplace thermopile

Follow these general steps to remove and install a new thermopile in your gas fireplace:

1. Turn off the gas supply – Locate the gas supply shutoff valve and turn it off.
2. Disconnect from valve – Unscrew the thermopile’s wire leads from the gas valve terminals. Note which wire goes into each terminal.
3. Remove mounting hardware – Remove any screws, bolts or clips securing the thermopile. It may be threaded into a mounting bracket.
4. Take a picture – Take photos of the old thermopile installation for reference when installing the new one.
5. Remove old thermopile – Gently pull out the old thermopile and set it aside.
6. Inspect pilot hood – Inspect the pilot hood and make sure it is not damaged. Replace if needed.
7. Install new thermopile – Align the new thermopile in place and secure with mounting hardware.
8. Reconnect leads – Attach the thermopile leads to the correct gas valve terminals.
9. Turn on gas – Slowly turn on the gas supply and check for leaks using a gas leak detector solution.
10. Test operation – Attempt to relight the pilot and turn on the main burner to ensure proper operation.

Always exercise extreme care when working with gas appliances. If you don’t feel completely confident, have a professional gas technician replace the thermopile instead.

Thermopile replacement tips

• Purchase a replacement thermopile made specifically for your fireplace make and model
• Check the millivolt rating – Common ratings are 325mV, 500mV, 750mV.
• Note the mounting style – common are threaded, bracket, or tension clamp.
• Take a photo before disassembling for easier reinstallation.
• Don’t forcefully bend or kink the new thermopile.
• Ensure wires are connected to the proper gas valve terminals.
• Test the pilot several times after installing new thermopile.

Troubleshooting thermopile issues

If replacing the thermopile doesn’t resolve your gas fireplace issues, here are some additional things to check:

Pilot won’t light or stay lit

• Low gas pressure – Check line pressure and contact gas company if low.
• Clogged pilot orifice – Clean pilot hood and orifice opening.
• Dirty or obstructed pilot hood – Clean hood so flame hits thermopile.
• Gas valve failure – Valve may need replacing if pilot won’t light.

Burner won’t light

• Gas valve stuck closed – Tap the gas valve lightly to unstick.
• Defective ignitor – Ignitor will need replacing if not sparking.
• Gas line obstruction – Ensure gas is flowing properly.
• Faulty wiring – Check for broken or poor connections.
• Gas valve failure – Replace the valve if it won’t open.

Low flame height

• Low gas pressure – Check line pressure for blockages.
• Partially clogged burner – Clean burner tubes and ports.
• Improper venting – Check vent cap and venting system.
• Gas valve stuck partially closed – Tap valve to loosen.

A gas technician can troubleshoot further by checking for issues like gas pressure, clogged gas lines, wiring problems, vent problems, burner issues, and gas valve failure.

When to call a professional

While do-it-yourselfers can replace a thermopile, it’s best to call a professional gas technician for diagnosis and repairs in these cases:

• You smell gas or the fireplace is leaking.
• The pilot won’t light after multiple attempts.
• Other components like the ignitor or gas valve also need replacement.
• You don’t feel fully confident working on a gas appliance.
• The fireplace has other mechanical or venting issues.
• Diagnosis requires gas pressure testing.
• The thermopile replacement doesn’t resolve the issues.

Gas fireplace repair can be complex and risky. Certified gas technicians have specialized training, access to proper equipment, and the know-how to safely troubleshoot and fix issues the right way. They can also determine if a full fireplace rebuild or replacement is needed for older or damaged units.

Conclusion

A faulty thermopile is one of the most common reasons a gas fireplace fails to operate properly by causing pilot or burner issues. Using a multimeter to check thermopile voltage and visually inspecting for damage are two ways to test if the thermopile is bad and requires replacing. With some care and safety precautions, homeowners can typically replace a gas fireplace thermopile on their own if they feel confident working with gas appliances. However, for any complex troubleshooting or additional repairs needed, it’s best to have a trained professional service the fireplace.