Thermopile Voltage Low: Causes & 3 Efficient Solutions

Your HVAC technician has informed you that your thermopile voltage is reading low. But what does that mean, and more importantly, what can you do about it? 

While you don’t need to go in-depth regarding the functioning of your HVAC system, it is recommended to be familiar with its basic processes to recognize issues such as this one. 

In this blog post, we’ll discuss the causes of low thermopile voltage and three solutions that will help you get your system up and running again. Let’s get started!

Why is the thermopile voltage low?

Before we get into the different reasons behind low thermopile voltage, it is essential to understand the role of the thermopile in your gas fireplace or another heating appliance.

A thermopile is a device that converts heat into electricity. It is made up of several thermocouples connected in series. 

Thermocouples are devices that produce a voltage when the temperature is different between the two conductors. The voltage produced by the thermocouples is proportional to the temperature difference.

The thermopile produces a voltage that is used to operate the gas valve. The gas valve controls the flow of gas to the burner. 

When the thermopile produces a voltage below a certain threshold, the gas valve will not open, and gas will not flow to the burner.

If the thermopile voltage is low, it will affect the operation of the gas valve and may prevent the appliance from igniting altogether. 

The temperature differential between the two conductors produces the voltage in a thermopile. 

The voltage will also be low if the temperature difference is too low. The thermopile must be at the correct temperature for the appliance to operate correctly.

Let’s look at some of the most common reasons your thermopile voltage is low.

Thermopile cold junction issue

One of the possible reasons why your thermopile voltage is low is a cold junction. What this means is that the temperature difference between the two junctions of your thermopile (the “hot” junction and the “cold” junction) is not significant enough.

A thermopile comprises two junctions: the hot junction and the cold junction. The hot junction is the part of the thermopile that is in contact with the object whose temperature you want to measure. 

The cold junction is the part of the thermopile that is not in contact with the object; it is usually kept at a constant temperature (usually room temperature).

The voltage output of the thermopile is proportional to the difference in temperature between the two junctions. 

Therefore, the voltage output will be inaccurate if the cold junction is not at the same temperature as the object you are trying to measure. Inaccuracies of this kind are called “cold junction errors.”

Several factors can cause the cold junction issue:

  • The environment around the thermocouple is too cold
  • The thermocouple itself is not long enough
  • Something is blocking the flow of heat from the hot junction to the cold junction. This could be a piece of insulation, dirt, or even something as small as hair.

Loose or corroded thermocouple wires

Loose or corroded thermocouple wires may cause low thermopile voltage. The thermocouple is a device comprised of two distinct metals joined together at one end.  

The junction of the two metals produces a small voltage proportional to the temperature difference between the two junctions.

Thermopiles are devices that use multiple thermocouples to measure more significant temperature differences. They are often used in gas-fired appliances to measure the temperature of the flue gas.

If the thermocouple wires are loose, they may not make good contact with the terminals. This can cause a low voltage reading.

Corroded thermocouple wires may also cause a low voltage reading. The corrosion can prevent the proper flow of electricity through the wire.

Faulty or malfunctioning thermopile

Finally, if the thermopile is severely damaged or simply malfunctioning, it is no wonder this is affecting the voltage it produces.

In this case, the only solution is to replace the faulty thermopile.

How to fix low thermopile voltage

As long as the thermopile isn’t defective, there are methods you can try to solve the voltage issue

However, if you’re dealing with a faulty thermopile, the only solution would be to replace it, and we will break down the process step-by-step

1. Fix the cold junction issue

The cold junction issue is often the reason behind low thermopile voltage. Fortunately, you can solve this problem permanently using a few simple solutions.  

You can use a standard operational amplifier if the cold junction is the underlying issue. The role of the operational amplifier is to take the voltage difference between the two inputs and amplify it. 

The amplifier’s non-inverting input is connected to the thermocouple’s positive terminal, while the inverting input is connected to the thermocouple’s negative terminal. The amplifier’s output is the amplified voltage difference between the two inputs.

If you want to use a low-noise operational amplifier, you will have to consider the noise performance of the device. The reason why this type of amplifier is used is to reduce the overall noise of the system. 

In case the environment around the thermopile is too cold, the device will not be able to function as intended. Additionally, if there is moisture in the environment, it will also interfere with the performance of the thermopile.

To ensure that the thermopile functions correctly, keeping it in a dry and temperature-controlled environment is essential. If possible, try to avoid exposing the thermopile to extreme changes in temperature, as this can also damage the device.

It is also worth noting that the thermopile is a sensitive piece of equipment and should be handled with care. 

What’s more, if the thermopile is too long, it could also affect its voltage. To solve this issue, you can try to cut the thermopile down to size using a sharp knife but take extra care not to damage the device.

Another possibility is that something is blocking the heat flow from the hot junction to the cold junction. In this case, you should check to see if anything is blocking the thermopile, and if so, remove it.

To unclog the pathway between the hot and cold junction, you can also try using a small wire or toothpick. Gently insert the wire into the pathway and clear any debris blocking the flow of heat.

If you’re still having trouble getting the thermopile to work correctly, it’s possible that the device is damaged and will need to be replaced.

2. Check the thermocouple wires

Low thermopile voltage may be a consequence of loose or corroded thermocouple wires. If you see readings far below what they should be, this is the first thing you should check. 

If everything looks tight, you may have a problem with your thermocouple

A faulty thermocouple should be dealt with as soon as possible. To replace a malfunctioning thermocouple, you’ll need to:

  • Remove the old thermocouple from the furnace;
  • Install the new thermocouple in the correct position;
  • Recalibrate the furnace so that it recognizes the new thermocouple. 

If you’re not comfortable dealing with a furnace on your own, it’s always best to call in a professional. They’ll be able to quickly diagnose the problem and get your furnace up and running again.

3. Replace a faulty thermopile

If none of the above-mentioned methods seem to work, the low voltage may be caused by a faulty thermopile. In this case, it is necessary to have it replaced – or replace it yourself.

Here’s a detailed guide on removing the old thermopile and installing a new one:

  1. First, shut off the power to your furnace at the breaker box. Then, remove the access panel to expose the furnace’s inner workings.
  2. Locate the thermopile (it will be near the gas valve), and disconnect the wires from it.
  3. Use a wrench to loosen the mounting nut, and pull the thermopile out.
  4. Take the old thermopile with you to a hardware store or HVAC supply store so that you can purchase an identical replacement.
  5. To install the new thermopile, reverse the steps you took to remove the old one. First, attach the new thermopile in the same position as the old one, and then tighten the mounting nut.
  6. Reconnect the wires to the new thermopile and replace the access panel. Finally, turn the power back on at the breaker box.

You will need to test the new thermopile to see if it is functioning correctly. To do this, turn on your furnace and observe the voltage reading on the multimeter. 

The thermopile is working correctly if it is within the normal range (between 750 mV and 950 mV). 

However, if the voltage is still low, another issue may be at play. In this case, you will need to consult a professional HVAC technician for further diagnosis.

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