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Thermopile Voltage Low: What You Need to Know

Testing for thermopile voltage drops is crucial for keeping your water heater functioning properly.

So, your thermopile replacement went well, and you managed to save a packet by doing it yourself. However, there’s a problem because the water heater is telling you that the thermopile voltage is low.

We explain what a thermopile is and show you how to check for low thermopile voltage.

Key Takeaways

  • A thermopile is a group of thermocouples that control the electronic heating system in water heaters.
  • Low thermopile voltage means it’s not generating the required current, causing issues like pilot light going out and water not heating.
  • Normal thermopile voltage range is between 650 and 850. If it’s below 400, consider replacing the thermopile.
  • Common causes of low thermopile voltage include pilot light problems, dirty water heater system, faulty gas control valve, and air drafts.

How a Thermopile Works

A thermopile (sometimes called a powerpile) is the collective name given to a group of thermocouples. Thermocouples control the electronic heating system to keep your water nice and hot.

Because your electronic system consists of circuit boards, it operates at a particular current level, and the thermocouples provide that current.

When the thermopile is faulty, your water heater stops working and flashes an error code on your appliance.

What Does Thermopile Voltage Low Mean?

Thermopile low voltage means that the thermopile fails to deliver the correct voltage, and the water heater blinks a red light at you.

Not all systems alert you in the same way. Some show a continuous blinking red light, while others work in light sequences of three or four.

Essentially, your thermocouples (as a collection) are not generating the required current, so the system shuts down. You will notice the pilot light going out multiple times and the water failing to heat.

Normal Thermopile Voltage Range

For your Rheem, A.O. Smith, or Honeywell system to work well, the thermopile voltage needs to be between 650 and 850. If you get a reading below 400, you should consider replacing the thermopile rather than attempting a fix.

How to Test Your Thermopile

Before you dive in with both feet and rip out your thermocouples on your water heater, it’s good advice to check the thermopile. It could make a massive difference to costs if you can repair it rather than replace it.

Other factors could be at play here. You might have a faulty thermal switch or even a loose wire, triggering the “thermopile voltage low” warning.

What You’ll Need

1. Set Up Your Multimeter

Turn the dial on your multimeter to direct current (DC) and set the range to one volt or millivolts.

2. Remove Thermopile Wiring Connector

Wiggle the thermopile wire connector free from the gas supply valve. If the connector is too stiff, use pliers to give you extra grip. It might be an idea to use a flashlight if you are working in limited light.

3. Connect the Probes

Place the red probe onto the red wire and the black probe onto the white wire.

4. Light Your Pilot Light

You will need to read your water heater manual to see how your specific manufacturer recommends how to do this. Typically, you turn the gas valve control to the pilot light setting and ignite the pilot light.

Watch as the voltage rises on your multimeter as it registers the current.

5. Hold the Thermostat

Press the gas control thermostat and hold it for two or three minutes while it reaches maximum voltage. As it does, the readings on the multimeter will start to slow.

If your thermopile is healthy, you’ll get a reading between 650 and 850. Below 400, and it needs replacing. A healthy reading tells you that the problem does not lie with your thermocouples, and you can switch your focus elsewhere.

How to Replace a Thermopile

So, your thermopile is dead, and the only remedy is to replace it.

Keep In Mind

You should only attempt this if you have some knowledge and skill in electrics. Calling in the professionals may be expensive, but at least the work is done properly.

Still happy to do the work yourself? Let’s take a look at the tools you’ll need.

What You’ll Need

  • Adjustable wrench.
  • Screwdriver (flathead).
  • Wrench set.
  • Pliers (optional).
  • New thermopile.
  • Flashlight.

1. Turn Off the Gas

Isolate the gas supply to the heater and turn the gas control knob to the off position. Now, use the screwdriver to remove the protective cover that conceals the wires.

2. Disconnect the Wires

Disconnect the thermocouple and ignitor wires. You will need to wiggle the connector to get it free or use the pliers if it proves stubborn.

Next, remove the manifold (water pipe) and pilot tubes using the adjustable wrench. Do this carefully because you could strip the connections and kink the tubes in the process.

3. Remove the Manifold Door

The thermopile lives inside the burner chamber, behind the burner assembly. Removing the manifold door with the screwdriver gives you access to the chamber and the burner. Once removed, you can access the thermopile.

Pro Tip

Take a photo on your smartphone to record how the wires are connected.

4. Remove the Burner and Thermopile

First, remove the retaining manifold door clip and unscrew the anchor that secures the pilot assembly and shield. Disconnect the thermal switch. You should be able to feed the assembly through the manifold opening to expose the thermopile.

If the light is limited, use the flashlight. Disconnect the wires connected to the thermopile and slide it out.

5. New Thermopile Wire

Thread the new thermopile cable, ensuring it is attached to the manifold door. Secure the wire in place with the retaining clip. Feed the new wires and the pilot tube through the manifold doors.

Take Note

Take extreme care not to damage the pilot tube. Even the slightest kink could sabotage your efforts.

6. Reattach the Manifold Component Block

Attach the shield and pilot assembly using the screwdriver. Take care not to catch any of the manifold block wiring and ensure the wiring insulation is flush with the block.

7. Connect the Thermal Switch

Connect the shortest wire on the left-hand side. Check the manifold door for gasket damage. Replace the gasket if it is damaged in any way.

8. Attach the Manifold Door

Locate the small tab on the base of the burner plate and slide it into the bracket at the bottom of the chamber. Wiggle the burner left and right until it connects with the bracket.

Now attach the two screws back into the manifold door.

9. Connect the Pilot Tube

Reattach the pilot tube and the manifold tube, and turn the gas back on. Also, turn the thermostat knob to the “pilot” position. Depress the ignitor switch and wait while the pilot relights.

Safety Note

Stop immediately if you smell gas. It could mean your pilot tube is incorrectly attached.

10. Set the Temperature

If you can’t smell gas, adjust the water heater’s temperature. To do this, hold the knob for a full minute, and the gas control light will start to blink. Now, you can adjust the temperature.

Thermopile Voltage Still Low After Replacement

So, you’ve followed the above steps, and still, your thermopile voltage is low. There has to be another source of the problem, so let’s see what it could be.

Pilot Light Problems

Because the pilot light heats the thermopile, sending a current to the gas control valve, many issues are related to the pilot light. Here’s a handy at-a-glance troubleshooting chart:

Problem Likely Causes
Pilot flutters or goes out when the gas valve moves to heat. Bad gas control valve.
Pilot light shrinks when the gas valve moves to heat. Low gas pressure/ bad gas control valve.
Pilot light is small and blue. Dirty pilot tube and combustion chamber.
Pilot light is yellow. Check venting/ low gas pressure/ dirty pilot tubes and combustion chamber.
Pilot light keeps going out. Check venting/ sporadic gas supply/ low gas pressure/ insufficient air supply.
Pilot light will not ignite. Check venting/ dirty pilot tube and combustion chamber/ low gas pressure/ insufficient air supply/ bad gas control valve/ replace thermocouple/ backdraft down the flue.
Pilot light goes out. Poor thermocouple connection/ misaligned thermocouple/ defective thermocouple/ defective gas valve.

Clean the Water Heater System

Your water heater gets dirty, just like any other appliance within your home. If the pilot tube becomes clogged, it will cause your heater to malfunction. You should schedule a yearly water heater deep clean to keep debris and dust at bay.

The easiest way to tackle dirt is to open the combustion chamber and scrub with a toothbrush to remove the debris. Use a pin to unblock a clogged gas supply and flush the system through to remove limescale from the heating elements.

Replace the Gas Control Valve

While the pilot light may cause the gas control valve to shut down, it could also be a faulty gas control valve. If there is a fault, it tricks the system into shutting off the supply. The electromagnet holds the supply open when the pilot light heats the thermocouple.

But if the pilot light is on yet the supply is closed, your gas control valve is probably to blame.

Check for Air Drafts

Pilot lights extinguish for many reasons, but excessive drafts are a common cause. There is no issue if the pilot light goes out in a force nine gale and relights when the storm passes.

Backdrafts are the most significant cause of the pilot light going out. As part of your maintenance inspection, pop outside to see if the flue is intact. If the vent cover is missing, it could expose the system to excess wind.

It could also allow rainwater in, which will extinguish your pilot light. Also, check that the vent isn’t blocked. Often, birds nest in the vents, causing blockages.


How Much Does a New Thermopile Cost?

Thermopiles are pretty cheap, costing between $5 and $10.

How Long Does a Thermopile Last?

There is no official lifespan for a thermocouple. Lifespan depends on how the component is used and whether it is well-maintained.

Is a Thermopile the Same As a Thermocouple?

A thermopile is a collection of thermocouples of opposing materials.

What Makes a Thermocouple Go Bad?

The most common issue is when the pilot light goes out and stops sending a charge to the gas control valve. When this happens, the electromagnet shuts the valve and ceases the gas supply.

Thermocouples are like any other water heater component that wears out over time. It could also be a faulty wire causing the thermocouple to malfunction.

The Final Word

Thermopiles are complicated bits of kit; they perform the vital role of keeping the gas supply flowing to your water heater. When they go wrong, your water heater stops working. Luckily, they are cheap to buy and can be DIY installed.

However, if you lack the skills for this task, don’t take chances and call in the pros. It could be the best money you spend!

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About the Author

Mark Weir

Mark spent 24 years working in real estate, so he knows his way around a home. He also worked with contractors and experts, advising them on issues of planning, investments, and renovations. Mark is no stranger to hands-on experience, having renovated his own home and many properties for resale. He likes nothing better than seeing a project through to completion.