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Question DetailsAsked on 7/6/2017

burner on a gas hot water heater will not stay on and knocks out the pilot light

I noticed the cold water 3 nights ago, and have taken it apart, blown it all out and cleaned everything up. about 6 months ago I replaced the thermocoupling, Now I light the pilot light, it stays lit, I turn the valve to ON and the burner ignites and stays on for a couple minutes and then goes out and knocks the pilot light out with it. What could be the cause of it doing this and not staying on for long at all?

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1 Answer


Since everything seems to work for a short time, probably most common causes would be:

1) a blockage in the flue system or blocked air inlet filter (sometimes stupidly put on the bottom of the firebox where you can't get at it reasonably) causing the firebox to fill with hot gases so a pressure or overtemp sensor cuts out.

2) with high efficiency water heaters (direct vent, through plastic pipe to outside) if the flue eductor (vent fan) does not come on or the flue damper at that fan does not open, a sensor will detect the lack of flue operation (commonly with a pressure sensing sensor) so will shut down the gas because the exhaust gases are not being vented. Though should occur within 10-20 seconds, not several minutes - but perhaps it just seems like several minutes - time it.

Diagnosis by a Heating and A/C or Plumbing contractor would normally be to first check the eductor fan and damper are operating right, then if they are OK to cause the shutdown, then check all the safety sensors for continuity - because when they trip out they cut the power to the gas control valve, without which power it shuts off the gas to both pilot and burner. Because both the pilot and biurner are going out after having lit and burned OK, almost certain to be a deliberate safety shutdown by the system - properly, or maybe improperly due to a failed sensor, though the way it is operating and repeating the same thing I suspect the shutdown is legit - eductor fan or damper sensor triggering, firebox pressure sensor reading a partial vacuum due to blocked air inlet screen, or firebox overtemp or flame rollout sensor tripping out. (Or with some fancier units - other sensors too).

Also - if this is a high-efficiency heater with a heat pump mounted on top, failure of that to work right could also trigger a shutdown.

Here are links to a few similar previous questions with answers which might help too -

Answered 2 years ago by LCD

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