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I light the pilot on my water heater, turn the knob to on and the burner ignites and heats water. When the burner shuts off, the pilot light goes out.

About two years ago the water line above the water heater was leaking badly. My husband would not fix it nor let me call a plumber. My sons finally replaced the leaking pipe. After that we had this continual problem. One of my sons replaced the thermocouple.

So what we do is light the pilot, turn the knob to on, the burner ignites and heats the water for maybe an hour, depending I think on the temperature of the water. If we want a full tub of hot water or to be able to use the dishwasher, we have to relight the pilot a few times to go through the cycle of the burner flaring to life and heating some of the water. Just one cycle is not enough to get hot enough water to start with to run the dishwasher. So to get a full tub of warm/hot water, if I didn't light the pilot at all that day, I have to light the pilot a few times over an hour or two, I start filling the tub, then go down to the basement and relight the pilot one last time. By the time the tub is full, the water coming out of the faucet is lukewarm.

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    Is thermocouple in the right spot? It should be in the pilot flame. Misplacement could give the behavior you describe. – Wayfaring Stranger Nov 25 '14 at 12:59
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A thermocouple makes 30 mv power by a temperature differential across the tip of the thermocouple which contains 2 dissimilar metals. If the entire tip is hot,there is no temperature differential thus no power made or not enough power will be made.A thousand millivolts = 1 volt,so a thermocouple's 30 mv is about 1/33 of 1 volt,a AA battery has 1.5 volts which is 1,500 mv,as you can see the amount of power made is very small, consequently it does not take much to affect the ability to maintain the pilot light. Many think that putting the entire tip of the thermocouple into the pilot flame makes more power, exactly the opposite will happen. A temperature differential must exist across the tip, in other words the base will be a cold junction and the tip a hot junction and this will create a tiny bit of power (30mv), If the entire tip is hot no power or insufficient power will be made to power the tiny electromagnet in the gas valve that holds open the pilot valve after we have depressed it's button and lit it then held it closed for a minute or so.

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