This post is for people in here who have heard about thermocouples having to be replaced on water heaters, boilers and furnaces, but don't know what they are or what they do.

Anyone with any kind of gas fired appliance with a standing pilot should have a spare thermocouple (or thermopile) on hand. For the $10 to $15 a spare thermocouple is going to cost, it's likely to pay for itself because it'll save you having to pay a heating contractor $40 for a service call and another $40 for him to sell you one of HIS thermocouples, the one he bought for $10.

Thermocouples produce a small voltage when they're placed in the pilot light, and it's that voltage that energizes an electromagnet that holds a valve in the gas valve of the appliance open. As long as the thermocouple is producing sufficient voltage, that "safety" valve will remain open and gas will flow through that safety valve to the pilot light. If the thermostat on the appliance calls for it to fire up, a second valve called the "main valve" in series with the safetly valve opens and allows gas to flow to the burner trays of the appliance. So, the gas has to flow through the safety valve first, and then through the main valve to get to the burner trays. If, for whatever reason, the pilot light goes out on an appliance, the thermocouple would cool down and not produce enough voltage to keep the safety valve open, and all gas flow to the appliance would stop. Neither the pilot light nor the burner trays would get any gas until you opened the safety valve on the gas valve manually, typically by depressing a button or turning a knob. That's to prevent your house being filled with gas and creating the conditions that could lead to a house explosion.

Thermocouples gradually go bad because the heat generated by the pilot light results in the metal grains growing larger and the thermocouple producing less and less voltage. Once the voltage is barely sufficient to keep the safety electromagnetic valve open in the gas valve, an aging thermocouple will result in the pilot light going out by itself every so often with the result that you discover that you have no heat or no hot water until you relight the pilot light manually. Cleaning the thermocouple will help, but the fix is to replace it. The first time you find you have to relight the pilot light manually is the time you should be planning to change that thermocouple.

Thermocouples come with different mountings and in different lengths. Some just push in, some have a collar that screws in, and some are different still. The best thing to do is to buy your first thermocouple from the factory authorized service depot so that the person selling it to you can explain how to replace it if it's not obvious. The length of the thermocouple doesn't really matter. In that regard, they're like extension cords. You can't use an extension cord that's too short, but you can always use one that's too long. So, if it's friday night and your wife tells you the water heater's pilot light has been going out by itself lately, and the hardware store only has a 48 inch long thermocouple when you only need a 24 inch long one, buy and install the 48 inch thermocouple (if the mounting is suitable). It'll work fine.