Before the morning of December 31, 2016, I don’t think I’d ever heard the word “thermocouple” and I certainly didn’t know what one was. By noon, I’d replaced the thermocouple in my house’s water heater and in doing so repaired it and put it back into service.
The previous evening, I noticed there wasn’t much hot water coming out of the faucet. I went downstairs and took a look at the water heater and noticed that the pilot light was out. That was unusual. (The pilot light, btw, is a small flame that burns 24/7 that serves to ignite the thermostatically-controlled main burner when it comes on, as necessary, to heat the water in the tank.) Before that day, the only time the pilot light ever went out was when someone used a fan to dry a wet spot on the floor (caused by a leaking washing machine) and positioned it so that it blew toward the water heater. The fan, I think, blew out the pilot light. When I re-lit the water heater pilot that time, it stayed lit. On December 30, I re-lit it the pilot light, saw the main burner come on, but the next morning … no hot water. I tried lighting it again, but it wouldn’t stay lit. For some reason, it was repeatedly going out. Some internet investigation made me suspect that the problem was the thermocouple.
A thermocouple is a very clever device. Gas water heaters have a useful safety feature: the gas turns off and won’t come on if the pilot light isn’t lit. In other words, if the water in the tank gets cold enough to trigger the main burner, but there’s no pilot light, the gas won’t come on. The gas to the pilot light itself turns off when it’s not burning. Thus, if it’s not going to be properly burned immediately in water heater, the gas will not flow. This prevents gas from accumulating and possibly causing an explosion or fire. That’s pretty useful, preventing your house from catching on fire or blowing up. If you wonder how this can be made to work in a machine that is not connected to household electricity, the answer is the thermocouple. A thermocouple is a device made from two metals which, when heated, produce a small electric current. This small current can be made to keep open an electrically-controlled valve such that when the current stops, the valve closes. (So, in this way a gas water heater with a pilot light does make use of electricity, even though it’s not connected to household electrical system.)
Like every other useful device, thermocouples eventually (like maybe after 10 years of continuous use) stop working for various reasons. And so it happened that on the last day of 2016, I had no hot water for my morning ablutions. The internet told me that spending $12 for a new thermocouple, available from the nearby big-box home improvement supplies store, and installing it myself was a good bet. Youtube showed me how to do it. The DIY fix is working just fine and (a penny saved being a penny earned) probably earned me a couple hundred dollars for an hour’s work.
Note: If you do this yourself, you must be comfortable with your ability to disconnect and reconnect the gas lines that go from the water heater’s thermostat to the main burner and pilot light. Research and learn how to do this before you start. Be sure to follow all safety precautions such as turning off the gas to the water heater, etc. Handy hint: Use noncorrosive gas leak detection liquid (not soapy water) to test the gas connection for leaks when you’re finished.