How to Test the Thermocouple on a Gas Tank Water Heater

It’s often said that “little things can make a big difference.” This is true in many areas of life, but the phrase makes us think about water heaters. A typical residential tank water heater can hold 40 or 50 gallons of water and weighs well over 100 pounds when filled. However, this large appliance can abruptly stop working due to a small part called a thermocouple.

How to Test the Thermocouple on a Gas Tank Water Heater

If the thermocouple of the gas water heater fails, it will cut off the gas supply to the unit. Using a multimeter device, you can check the electrical current of the thermocouple to determine if it needs to be replaced.

What does the thermocouple do?

The thermocouple is a safety component that deals with the pilot burner and gas control valve. When the pilot flame is ignited, the thermocouple uses a small electrical current to open the gas valve and fuel the main burner of the water heater. However, if the pilot light is off, the thermocouple shuts off the supply of natural gas coming from the gas valve.

Why is this important? If the pilot is off (or faulty) and the gas valve is allowed to stay open, natural gas can escape the water heater and enter the home, which is a major health concern.

Over time, the thermocouple can become faulty or loose. This prevents the gas valve from opening, even if the pilot light is working fine. Without gas, the water heater doesn’t have fuel to heat up water. That’s why it’s important to diagnose a faulty thermostat so the part can be replaced and your water heater can do its job.

How to test a thermocouple

Testing a thermocouple requires a multimeter (also called a multitester), a small instrument that can perform several electrical measurements such as voltage, current, and resistance. You will also need to know what type of gas valve you have, either White-Rodgers or Robertshaw.

Once you have this device, carefully follow these steps.

  • Hold down the pilot button
  • Set the multimeter to millivolts
  • Place one probe of the multimeter on the copper part of the thermocouple and the other probe on the gas valve
  • If the reading on the multimeter is below 10 millivolts for a White-Rodgers valve or 13 millivolts for a Robertshaw valve, the thermocouple needs to be replaced

Installing a new thermocouple

Unless you have extensive plumbing experience, we don’t recommend trying to install a new thermocouple yourself. This small, delicate part deals with electricity and natural gas, so any small mistake can be very dangerous. Also, if the thermocouple is not installed correctly, it could cause damage to your water heater.

The smarter and safer option is to call the pros at Emergency Plumbing & Solar. We proudly serve the entire island of Oahu. To schedule an appointment for your water heater, call us today at (808) 691-9309.


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