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Water Heater Thermocouple: What You Need to Know

 
 

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A water heater thermocouple is an important component of gas-powered water heaters. If your heater is powered by natural gas or propane, a thermocouple ensures that is operates safely. Whether your water heater is having troubles or you simply want to learn more about this critical component, this article will help you gather the information you need.

Many people think that if the thermocouple malfunctions that it's necessary to replace the entire water heater, but in reality, they are actually relatively easy to repair and not too expensive. We'll explain the role of a thermocouple, how to troubleshoot a problem, and even show you how to replace the part when it malfunctions. 


What is a Water Heater Thermocouple?

A standing pilot water heater uses a pilot flame which burns continuously and ignites the main burner when heat is needed. Next to the pilot is the thermocouple which generates a small electrical current when it's heated by the pilot flame. The electrical current generated is less voltage than a AA battery, but it's enough to power the gas valve.

The thermocouple acts as a safety switch by preventing unburned gas from being released into your home. It does this closing the gas valve when the pilot light goes out. When the pilot light is no longer lit, the thermocouple cools to a point where it is no longer generating the voltage necessary to keep the gas valve open. When the voltage drops, the gas valve closes.

Once the pilot light has been re-lit, the thermocouple will warm and begin generating the necessary voltage to open the gas valve.

Some newer gas water heaters use a flame sensor instead of a thermocouple to monitor and control the flow of gas to the burner. Check your owner's manual to see if your heater is equipped with electronic ignition.

Why Does a Water Heater Thermocouple Go Bad?

When the thermocouple is working properly it uses a phenomenon called the Seebeck Effect. This is where two metal rods create a tiny electrical current, which keeps your water heater's gas valve open. 

If your thermocouple stops working, the most likely cause is there's an issue preventing the two metal rods from generating the necessary voltage. Although, in some cases the problem can be fixed, most homeowners choose to replace the thermocouple since they are so inexpensive.

Thermocouple Location

The thermocouple is part of the water heater's burner assembly and is positioned next to the pilot light. It slides into a bracket and is held in place by a couple of clips. The thermocouple line runs outside the heater and is connected to the gas control valve, which is located on the outside of the tank.

Difference Between a Thermocouple and a Flame Sensor

As we mentioned above, your water heater may use a flame sensor instead of a thermocouple. Although, these terms may be used interchangeably, they are technically different.

A flame sensor is typically found on newer water heaters that use an electronic controller, where thermocouples are found on older heaters. While these differences are worth considering, they're not ultimately all that impactful. Both thermocouples and flame sensors perform the same job, and in the end, that's really all that matters.

Yellow Multimeter with probes

Troubleshooting a Faulty Thermocouple

If your thermocouple is faulty, your first indication may be that you don't have hot water. Since a malfunctioning thermocouple will shut off the gas valve, your water heater won't have the resources it needs to heat the water in the tank.

If your thermocouple is bad, the pilot light will not be lit. If you try to light your pilot and it won't stay lit, it may be an indication of either a faulty thermo cutoff switch or the thermocouple. Some people choose to simply replace their thermocouple when they can't keep the pilot light lit, but here's how to test to see what's actually happening.

How to Test a Thermo Cutoff Switch 

The thermo cutoff switch is located on the front of the combustion chamber. It's designed to basically shut down the thermocouple if temperatures within the chamber reach above 160° to 180°F.

The thermo cutoff switch is wired in series with the thermocouple. If the temperature in the combustion chamber gets too high, the switch will trip and cut power to the valve. Some thermo cutoff switches have a reset button, others will reset automatically, and still others use a one-time fuse.

It's critical that if at any point you notice a strong gas smell to shut off the gas supply and call a professional plumber to help you determine the cause. 

You'll need a multimeter to perform this test. Here's what to do:

  • Remove the cover and check the electrical connections to make sure they are secure.
  • Check the thermal cutoff switch for a reset button. Reset if needed.
  • To test the switch for continuity (to see if a continuous electrical current can pass through it), set the multimeter to the ohms setting.
  • Disconnect the wires to the switch, and place the probs on the terminals.
  • If the multimeter reads OL or open loop, then there's no continuity and the switch will need to be replaced.

If the thermal cutoff switch is good and the pilot still won't stay lit, then test the thermocouple.

How to Test a Thermocouple

To do this, you'll need to light the pilot and test the voltage. You'll need an adjustable wrench and multimeter. Here's what to do:

  • Set the multimeter to the volts DC option.
  • Unscrew the thermocouple from the gas control valve using an adjustable wrench.
  • Set the probes to the copper line and the end of the thermocouple.
  • Light the pilot and hold down the button.
  • Over the next few minutes, the voltage should begin to rise as the thermocouple heats up. A healthy thermocouple should read between 20 and 30 millivolts in an open circuit test.
  • If the reading is below 20 millivolts, the pilot may have difficulty staying lit. A build-up of carbon may have formed on the thermocouple and is acting as an insulator. You can remove the thermocouple and clean it, but many times it's best to simply replace it with a new one.
Emery sandpaper and gloves

How to Clean a Thermocouple

If you've determined that your thermocouple is getting a reading of below 20 millivolts and you'd like to give cleaning a try, here's how:

  • Shut the gas inlet valve.
  • Remove the water heater burner assembly. You'll need to disconnect the thermocouple, main supply tube, and pilot supply tube before pulling the burn assembly out.
  • Clean the thermocouple using emery cloth sandpaper. Sand the ends of the thermocouple's metal rods, then go back over them with a lint-free cloth.
  • Reconnect the burner assembly and open the gas inlet valve before lighting the pilot.

You can use your multimeter again to see if you get a higher millivolt reading, but if the problem continues, your best option is to replace the thermocouple.

Man wearing tool belt

How to Replace a Thermocouple

If you're a DIYer then replacing your water heater thermocouple is in your wheel house. It's not difficult and it's very inexpensive. However, you will be working with a live gas line, so use extreme caution throughout the replacement process. We highly recommend calling a professional to do the work for you if you're at all worried about working with natural gas appliances. 

Before starting, you'll need to find the right size thermocouple for your water heater. The majority of heaters can use a 24-inch thermocouple, but if you're uncertain, it's best to buy a longer one and carefully coil the excess inside the panel.

Honeywell CQ100A1013/U Not Available CQ100A1013 24-Inch Replacement Thermocouple for Gas Furnaces, Boilers and Water Heaters, quot

Honeywell Replacement Thermocouple

Step-by-Step Instructions

  • Shut off the gas supply to the water heater.
  • Remove the bottom plate that covers the burner assembly.
  • On the gas control valve, disconnect the thermocouple ignitor wire, main burner supply, and pilot tubing.
  • Remove the screws to the burner door, and slide out the burner assembly.
  • Remove the thermocouple from the pilot assembly and replace it with a new thermocouple. 
  • Replace the burner assembly in the exact position it was before. Be sure it fits into the slot or bracket if one is provided.
  • Reconnect the thermocouple ignitor wire, main burner supply, and pilot tubing to the gas control valve.
  • Replace the burner door and secure with screws.
  • Reconnect the cut off switch and ignitor wiring.
  • Turn the gas back on and light the pilot.
  • Test for gas leaks with a soap solution at the pilot and main burner connections. 

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