How badly did I screw up my hot water heater?
October 1, 2013 11:41 AM   Subscribe

My gas hot water heater's pilot light has been going out regularly. I replaced the thermocouple in an effort to solve the problem, but I seem to have botched it: Not only does the pilot light still go out after the tank is heated, but now the gas continues to flow after the pilot goes out.

Don't panic. After smelling gas in the house, I shut off gas to the heater and called a professional to bail me out. He'll be here in a few days, and in the meantime I will suffer the penance of cold water. But I'm curious: How did I screw up a thermocouple installation in a way that the gas continues to run when the pilot goes out?

If it matters, this is a "new style" water heater with a piezoelectric igniter.
posted by qxntpqbbbqxl to Home & Garden (6 answers total)
I don't know much about this stuff, but for me, that would be one of two expected failure modes from a botched thermocouple installation (the other being that the gas won't run at all). The thermocouple's primary job as I understand it is sensing that the pilot light has gone out via the change in heat, and closing the gas valve when it gets cold. If something in that process is getting messed up, then it won't close the valve incorrectly when the light goes out.

Here's what wikipedia has to say on the thermocouple's role in turning off the gas.
posted by brainmouse at 12:35 PM on October 1, 2013

Gas may be flowing from somewhere else, like a cracked tube or fitting, or the knob that you press in to light the pilot might be stuck somehow.

You might be able to tell where the leak is by spraying soapy water on everything.
posted by yohko at 12:39 PM on October 1, 2013

A few days? Is there only one plumber in your town? I've never had to wait more than 24 hours for gas water heater service.
posted by COD at 12:59 PM on October 1, 2013

Damn, please don't turn the gas back on! I'm not sure where you are, but in the UK working on a gas appliance is a criminal offence without the correct certifications, and quite rightly. See here.

I'd get an electric kettle or something to tide me over for a few days until you can get it professionally sorted.
posted by derbs at 1:14 PM on October 1, 2013 [2 favorites]

Best answer: Sounds to me like a bad gas control valve instead of a thermocouple. Might want to check that the plumber plans to bring one out with them instead of coming out and having to come back.
posted by moitz at 1:19 PM on October 1, 2013

The gas will continue to flow for a short while (maybe 15 seconds) when the pilot light goes out because it takes a while for the thermocouple to cool off. So you can expect to smell some gas as a result of the pilot going out. If the gas valve eventually shuts off, then there may be nothing wrong with your valve, even if you smell gas for a short time.

If the pilot lights and works okay for a while but gets blown out, you may have some sort of back draft problem. Sometimes this can be because of the wind and you have a missing draft cap on your flue pipe outside the house. Or it could be a back draft when the furnace turns on if it shares a common flue with the water heater. Or you may have some loose insulation packed around the draft collar where the flue comes out of the heater. This space should be kept open to prevent back drafts going down to the burner.

Probably time to call in a pro.
posted by JackFlash at 11:08 AM on October 2, 2013

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