water heater won't stay lit part II

I have not been able to keep my water heater pilot lit since I turned on my AC. It might stay on for a day at most. The plumber suspected downdraft. The Chimney sweep found nothing. When the heater is heating water there is a powerful updraft through the chimney.
The sweep said I should have the thermostat replaced. I can light it ok, so it must work somewhat. A previous poster suggested taking off the gas lines from the valve and blowing them out, maybe that is next?
Jeff
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The downdraft is when the heater is NOT running. Air can get sucked down any opening.

Let him sweep the chimney, not fix gas appliances.

Blow them out? What do you expect to find in them aside from gas? If the line was clogged so much that a pilot would not operate, surely the main burner would be starved. It works when the AC is not running. Think about the cause and effect of this.
Go back to the original problem again. The pilot goes out when the AC is on.
Does the AC affect the water heater thermostat? NO Does the AC use the gas line in question? NO Does the AC draw air from the same areas the water heater is in? YES Does the blower cause a negative pressure in that area? Very POSSIBLE Would a negative pressure cause a downdraft in the chimney? YES
Check that the return air ducts are functioning properly or if air is being lost elsewhere to create the negative pressure in the AC area and thus, the downdraft.
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I just had another thought on this. Previously, someone mentioned watching the pilot light when you have somebody turn on the blower. Good idea, but carry this a step further. I mentioned negative pressure in the area of the AC causing the down draft.
Watch the pilot light while the AC is turned on. After the blower gets up to speed, have that person open a door upstairs. Opening the door will allow air to leave the house while more is needed to feed the blower, thus it may pull air down the water heater chimney. If there is a door to the basement, close that while the upstairs entry door is opened.
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Edwin Pawlowski wrote:

Yes, and any fans that blow outside the house especially if its a newer house. Dryer fan, kitchen stove fan, bathroom fan. Be sure fireplace duct is closed if u have one. And on that front door thing, try opening it with 'purpose' like you might when coming home from work. Maybe its a quick gust thats blowing it out. I would expect if the door opens inward it might suck air up the chimney and if it opens outward it might suck air down the chimney.
--
Respectfully,


CL Gilbert
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For anyone who is still interested, the problem turned out to be my gas valve. I turned off the AC for a few days during a cool spell and it made no difference, the water heater pilot still went out. The plumber looked at it for a long time and decided the valve was "bursting" the flame, which blew out the pilot. AC is back on, everything is fixed.
Jeff
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Thanks for the update. Glad things are working OK now, especially in these temperatures that you need the AC
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Jeff wrote:

I agree with Ed on this one. I will add that you did miss one thing that might be part of the problem. You may want to try replacing out the thermocouple. I wonder if you get some cold air in the area of the WH when it is on? If the thermocouple is weak it could be the problem.
I once had a problem with a WH that no one could figure out. Thermocouple was replaced several times, venting triple checked finally they replaced the whole heater and no more problems. I never will know what was the problem.
--
Joseph Meehan

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