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Cycling Explosions !!!  Rate Topic 
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 Posted: Sun Aug 12th, 2018 09:10 pm
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will8sm
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Pulled and checked the burner assy and found the pilot flame dispenser to be at the proper distance from the burner. Cold start yields a silent/smooth burner ignition. However, upon reaching the preset temperature, the burner shuts off (as expected); followed shortly by an explosion from the burner, as it apparently tries to re-ignite?!?! The pilot light can be observed to be lit during the entire ignition recycling process, as expected. The process will continue, apparently incessantly; until someone physically interrupts the cycle by turning the thermostat temp control knob to the "OFF" position. I think it's more than an adjustment; I think something has failed... I suspect the gas valve/thermostat controller assy. Additionally, the exhaust flue pipe does not appear to be obstructed. Any feedback would be deeply appreciated... thank you.

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 Posted: Sun Aug 12th, 2018 10:04 pm
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geno03245
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Sounds like the gas valve is not shutting off completely,
and a seep of gas is building up in the combustion chamber

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 Posted: Sun Aug 12th, 2018 10:38 pm
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will8sm
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So then... your conclusion is the same as mine? IE gas valve/thermostat assy failure?

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 Posted: Mon Aug 13th, 2018 02:25 am
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eleent
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Hello,  I agree with Geno.  Still, I'd like a bit more information about the heater and its age.  Also, is there a dirt leg on the gas supply serving the heater?  A photo of the lower part of the tank would be nice. ;)

Yours,  Larry

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 Posted: Mon Aug 13th, 2018 01:23 pm
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will8sm
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The heater is an A.O. Smith model GUC 50 400, LO-NOX, naturally vented (not power vented); which doesn't seem to show up when I google search it or search it on the water heater sites. Produced in Feb. 2016, installed in May 2016. It's a 50gal. 400K BTU nat. gas heater in Calif. If a "dirt leg" is simply a branch at an elbow on the gas supply line designed to capture any debris flowing in the gas line; then, NO, there is not one. However, when I pulled the burner assy I removed the gas line from the valve to the burner and inspected the orifice for debris or spider web obstruction... NONE found. Again, the heater lights perfectly when cold, and only starts the cycling explosions when it reaches the preset temp. I apologize, I have an appointment to get to this morning, so, I don't have time to get you the photo right now... I'll post one when I return. Thanks for your interest and help.

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