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Old 04-13-2009, 06:01 PM   #1
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Gas Furnace Main Gas Valve not igniting


I am trying to troubleshoot this problem; My furnace fan kicks on when the thermastat call for heat, its a hot surface igniter(ceramic plate that heats up) but the main gas valve won't open(most of the time), it was intermittent but now its about 90% of the time it won't ignite. Also when it does ignite it will run then turn off, but the fan will keep running then it will call for heat again and the blower kicks in and it will basically run all night if the gas does turn on?

So I think it is in the control box/panel but I am not sure what to troubleshoot in there as I don't know which control is for the main gas valve.

I checked the pressure switch, seems ok considering it lets the igniter engage.

I checked the flame sensor looks good, but it doesn't have anything to do until the main gas comes on correct?



ps the unit is about 20 years old and is a model;
Rheem "Imperial high efficiency"

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Old 04-13-2009, 09:27 PM   #2
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Gas Furnace Main Gas Valve not igniting


Test the hot wire ( when it calls for heat ) on the g.v. with a volt meter. red lead from v.m. to hot on the g.v. black lead on v.m. meter to ground You should get around 24-28 volts a/c . perform this test to see if the g.v. is loosing it's power O,r if power stays there but valve isn't staying open

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Old 04-14-2009, 07:33 AM   #3
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Gas Furnace Main Gas Valve not igniting


Problem could still be at the ps. It could be a problem the vaccum at the switch...could also be flame sensor
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Old 04-14-2009, 07:46 AM   #4
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Gas Furnace Main Gas Valve not igniting


Sounds like it might be time to retire the old beast. It could be the gas valve, but I've only run across three valves in the last 20years that have actually been bad. It could be the flame sensor, but a lite sanding should make it ready to go again. It could be a poor ground between the sheetmetal and the control board. It could be dirty burner tubes. It could be an old tired control board itself. Finally after 20 years you could fix everything on the unit and find out the heat exchanger is cracked.
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