How Thermocouples Work on Ones Water Heater

Posted by on Dec 9, 2013 in Water Heater Repair San Francisco | 0 comments

Okay, not to get too technical with you today, but we want to share some information on a water heater’s thermocouple.

The Family Handyman correctly concluded that “when thermocouples wear out, the pilot light goes out, the burner won’t light, and you don’t get any hot water.”

Conventional style
Likewise called universal water heater thermocouple. It screws into below the gas valve and a probe on the other end slides into a holding bracket near the pilot light. It appears like a piece of copper wire.

Universal thermocouples are available in lengths from 15 to 36 inches. An 18 or 24 inch will be sufficient for all residential water heaters utilizing universal kits. Practically all older waters heaters and numerous of the newer style systems with a sealed burner compartment and a seeing window will use this thermocouple.
If you wish to use an exact replacement length simply eliminate your old one and measure it.

TCO style

Thermal Cut Off design has a re-settable thermal turned off switch integral into the thermocouple. In the event of a flash fire or blow back from the burner the TOC will close down gas flow to the burner and pilot. In case of a closed down the TOC should reset itself after it cools down and enable you to relight the pilot. The TOC switch appears like a watch battery. You will just find the TOC on more recent style water heaters with seal burner compartments.

TOC non-resettable style

These thermocouples have a small TOC build inside television. They look just like a conventional one other than the diameter is bigger about midways of television. When these TOC’s switch off they will not reset and should be changed.

How Thermocouples Work

A pilot thermocouple sends a small electrical current to the gas valve when the probe is heated to a specific temperature. This electrical current tells the gas valve that the pilot is lit and its ok to send out gas to the burner if the thermostat requires it.  There are some great images to illustrate how this works here.

To test a water heater thermocouple push the pilot button down on the water heater gas control valve. Then light the pilot. If after holding the pilot button down 30 to 60 seconds the pilot light doesn’t stay on when you release the button, the thermocouple is most likely the reason on older systems. The TOC or air flow could be the issue on newer systems with a pilot light viewing window.

If the pilot does not light at all the trouble is gas flow, gas control valve, TOC tripped.

Usual Thermocouple Problems

The most common issue is the idea on the thermocouple probe wears away. It can quickly be changed.

Thermocouple Repair

If the probe looks white in color this is carbon build up. It can keep the probe from getting hot enough to send a signal to the gas valve. Try sanding it with emery cloth and relight the pilot. It needs to work for a while but keep a brand-new thermocouple helpful.

  • The thermocouple probe should be in the pilot light flame.
  • The thermocouple has to be screwed into the gas valve firmly.
  • Inspect the pilot thermocouple tube for kinks.

We hope this has actually been a good introduction to a water heater’s thermocouple for you!

Phil Luther Plumbing Content Writer and Curator Phil is a prolific writer of content for plumbing related websites in the United States. Google Profile, Phil on Twitter, Phil on Home Energy Pros