How To Fix It

Gas Water Heater Thermocouple Possible Issues

A thermocouple is a sensor that is used to measure temperature and detect whether or not the water heater’s pilot light is lit. You certainly wouldn’t want this element to fail as it’s an important safety device.

The most common gas water heater thermocouple issues include a pilot light that keeps going out, a burner that won’t light, and the absence of hot water. This might happen, in case the thermocouple is dirty, faulty, worn down, or improperly positioned.

How Do You Know If You Have a Bad Thermocouple?

To find out for sure whether your gas water heater has a bad thermocouple or another problem, you can perform this test:

  1. Unscrew the thermocouple.
  2. Set your multimeter to millivolts.
  3. Start the pilot (you might want to ask someone to help you as the gas control knob needs to be held to keep the flame burning while you’re testing the thermocouple).
  4. Give the unit a minute and then place the lead on the shaft and the other one – on the connector.
  5. The reading has to be over 25 millivolts. If it’s less, then your thermocouple had gone bad and you would have to replace it. 
  6. You can try reversing the leads if you are not getting a reading. Give the device around 30 seconds; if the thermocouple is not giving at least the minimum rated millivoltage, then you need a new one. 

Bear in mind that some thermocouples do not have dual wire leads. You are going to need special equipment to test those, that’s why calling a professional might be the best idea. 

What Happens If You Have a Bad Thermocouple?

If the thermocouple had worn out, then:

  • The pilot light will go out
  • The burner won’t light
  • The water heater won’t be able to produce hot water

An obstruction in the pilot tube and some other issues might lead to such problems as well. However, if the flame goes out once you release the gas control knob (after holding it for 20-30 seconds), then the chances are high that there’s an issue with the thermocouple.

Read: What Is Water Heater Heat Pump? Everything You Need To Know

What Causes a Thermocouple to Fail?

The thermocouple or the ‘flame sensor’ is a safety device that will shut the gas off, if the pilot light goes out. The element is, basically, a metal rod that should be touching the flame when the water heater is operating.

The main things that can cause the thermocouple to fail include:

  • General wear and tear – this important component has to deal with extremely high temperatures on a regular basis. Throughout the years, it can wear down.
  • Damage – the thermocouple can easily get damaged as the component is quite fragile.
  • Soot buildup – even a slight residue can make the thermocouple malfunction. 
  • Improper positioning – the element has to be lined correctly, otherwise, it won’t be able to sense the flame.

Read: Does HVAC Include Water Heater?

Will a Pilot Light Stay Lit with a Bad Thermocouple?

The thermocouple is the component in your gas water heater that allows gas to flow to the burner. If the thermocouple had gone bad, it will shut off the gas to the burner and pilot, so the pilot won’t be able to stay lit.

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How Do You Fix a Thermocouple?

At times, the simplest thing that you can do is replace the thermocouple. It is a relatively easy job that won’t take you too long. 

Moreover, thermocouples are not expensive, so getting a new one won’t make a huge hole in your wallet.

If you have confirmed that the thermocouple has not gone bad, but it is still failing to sense the flame, then you can try cleaning the element or adjusting it.

To adjust a bent thermocouple or move it closer to the flame, you should:

  1. Shut off the gas.
  2. Locate the thermocouple (under the burner). 
  3. Gently pull or push the probe.

Hint: if the pilot flame is too small and is not able to reach the thermocouple, then try turning the pilot adjustment screw counterclockwise. Also, do make sure that there are no obstructions in the pilot tube.

Read: Ideal Temperature Of The Water Heater

Can You Clean a Thermocouple?

You can certainly clean a thermocouple as this element will get covered in soot and/or dirt over time.

Cleaning the thermocouple every 6 months or so should be enough.

How to Clean Thermocouple on Water Heater?

  1. Prepare a multiway wrench, a screwdriver, an emery cloth, a lint-free fabric, and fine-grit sandpaper (or steel wool).
  2. Turn off the gas supply.
  3. Remove the burner assembly from the control center. 
  4. Sand the end of the thermocouple and then wipe it with a lint-free cloth. If you have to – detach the thermocouple from the assembly.
  5. Reconnect the burner assembly.
  6. Open the gas valve and light the pilot.

Hint: before removing any part of the unit, take a picture of how it was assembled. 

Read: The Best Temperature For The Water Heater And Why?

How Long Should a Water Heater Thermocouple Last?

If properly installed and maintained, thermocouples can last for up to 10 years. 

Gas water heaters, in their turn, have a general lifespan between 8 and 12 years, so if you manage to take great care of your unit, you might not have to replace the thermocouple at all.

Water Heater Thermocouple Replacement

Make sure to disconnect all the connections to the gas control valve before you begin (disconnect the thermocouple, the burner supply tube, and the pilot supply tube). 

Turn the gas control valve to ‘off’ and close the shutoff valve (on the gas supply pipe).

If the water heater has been running, give the unit around 30 minutes to cool down. 

If you have an electronic ignition water heater or a standing-pilot unit:

  1. Remove the burner assembly cover and then remove the actual burner assembly. 
  2. Around the manifold cover plate, you’ll find the cover plate gasket – remove it.
  3. Locate the thermocouple’s mounting bracket and carefully remove the thermocouple by pulling and twisting the element.
  4. Insert the new thermocouple. Make sure that the tip is positioned right – it should get enveloped in the pilot flame by around 3/8-1/2 of an inch.
  5. Reinstall the rest of the water heater’s components. 
  6. Reattach the gas tubes. Bear in mind that the mounting nut on the new thermocouple shouldn’t be more than a ¼ of a turn beyond hand-tight.
  7. Turn on the gas and test the water heater.

For an electronic ignition unit, you would have to remove and then reinstall the igniter.

For a standing-pilot water heater, you would have to relight the pilot before setting the gas control valve back to ‘on’.  

How Much Is a Thermocouple for a Gas Water Heater?

Thermocouple replacement is one of the most common repairs needed for gas water heaters. It will cost you anywhere between $100 and $325 to have the element replaced by a professional.

If you have the necessary skills and equipment, you might want to replace the thermocouple on your own. In such a case, you would have to pay only for the actual part – that’s between $5 and $20.

Are Water Heater Thermocouples Universal?

Generally, thermocouples are interchangeable as the majority of them have standard connectors. Universal components are compatible with clamp-type, push-in, and screw-in pilot burners.

By the way, even the length of the thermocouple doesn’t really matter as it has no effect on the element’s accuracy. 

However, do bear in mind that older water heaters with standing pilot lights utilize a thermocouple, while the more modern units with electronic ignition have a flame sensor. 

The devices are pretty much the same, but you should still take the model of your water heater into consideration.

Read: How Many Amps Does Water Heater Use?

Can You Bypass a Thermocouple on a Water Heater?

You should not bypass a thermocouple as it is an extremely important safety device.

Without this element, your water heater might continue supplying the unit with gas even if the pilot light is out. In such a case, the gas is going to escape into your house – and that is a very serious safety hazard.

If you need to urgently use the water heater, the chances are high that you’re not going to be next to the unit all the time to keep an eye on the pilot light. So, in the worst-case scenario, gas might end up accumulating close to your floor.

In any case, cleaning the thermocouple or getting a new one and replacing the faulty element would require less effort (and, of course, is a much safer option).