I have an AO Smith Promax gas hot water heater model XCVL40. It's about 7 years old and the pilot went out the other day. This has never happened before but I was able to get the pilot re-lit. While the burner comes on, it will not stay on for long before everything shuts off (including the pilot).

Per the manufacturers recommendations, I did three things:

1. Made sure the air intake screens were clean.
2. Took a brush (and vacuum) to clean the flame arrestor filter underneath. I cleaned it several times.
3. Gave a “smoke test” to the hood opening. The draft seems fine.

The problem persists. In fact, I can get the burner going for about 30 minutes first thing in the morning before the water heater "scrams" (everything shuts down, including pilot). After that, I can restart the pilot but can't get the burner to stay on for more than a couple of minutes at a time before the system scrams again.

Any idea of what could be causing this? Thanks!

Sounds like whatever it uses to verify flame is failing. You need to know the specific procedure for testing the thermocouple/ flame-rod.

A bad thermocouple was my inital thought. But if it was bad, wouldn't it prevent the pilot light and burner from functioning?

To me, the whole situation seems like an overheat is being detected. Does the thermocoupler measure for this too?

yes

no

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

If the burner impacts on the thermocouple and overheats it, that could cause complete shut down, but a low gas pressure/volume could also do it.

There is a gentleman named Larry Weinstein that frequents heatinghelp.com. He has a water heater museum. He might be willing to offer assistance. He also has produced DVDs on water heater service and repair, so I don't know how willing he will be to give info away. If he does, be gracious, and buy a course. They're not that expensive.

Thanks for the replies. Paul - your suggestion was helpful. I was able to locate some great info on that water heater forum.

After reading over several similar accounts, it became apparent that I needed to remove the sealed burner unit to get a better look at the problem. I was avoiding this because of my lack of confidence in handling a more complicated disassemble/reassemble of a gas appliance. However, I found a nice tutorial on Youtube that showed me how to do it and it wasn't that hard. You just need to take it slow and be very careful not to damage anything.

Once I removed the burner unit, I immediately saw a small pile of debris on top of the burner. I gently cleaned it off, used a fine steel wool pad to clean the thermocouple, and a soft brush (and compressed air) to make sure the burner was all clear. I also vacuumed out the inside of the burner chamber and blew out the flame arrestor pad (from the top).

I put it back together, checked carefully for any gas leaks, and the pilot stayed lit a few seconds after releasing the button (this previously took 2-3 minutes). The burner fired and I completed a heating cycle without the unit shutting down.

I need to keep an eye on it, but it looks like everything is working okay. Keeping my fingers crossed because some have reported that this may only be a temporary fix.