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Old 10-10-2012, 08:09 AM   #1
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Is gas valve is series with safeties?


My furnace schematic shows all connections to the control board but I've got no functional sequence description. My gas valve never opens though everything seems fine (no error messages etc). It just cycles between purge and ignition forever.

The odd thing (to me anyway) is I see about 14v at the gas valve at all times (leads pulled off so I'm measuring open circuit voltage) until the time I think the valve should open, then it goes to 1V for a second or two before the ignitor goes off and it starts a purge again.

I'm thinking perhaps the contacts on a safety have welded (hence the partial voltage at all times) but the bad connection and low voltage prevents the gas valve opening.

Or perhaps the gas valve is just on a seperate relay and that relay's contacts have welded.

Or perhaps seeing 14V all the time is normal? I just don't work with furnaces enough to be sure what I should see at the gas valve. I guess it's possible the controller is keeping a low current on the gas valve to check for continuity before it goes full voltage to open the valve and since I'm checking open circuit voltage that test fails and it just cycles again.

Anyone more familiar with how these work or know where I can find a good description of how the controls usually work?

Thanks!

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Old 10-10-2012, 11:47 AM   #2
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Is gas valve is series with safeties?


Your voltage to the gas valve would normally be 24V (ac) or nothing.
Removing the gas valve leads and measuring 14V just says you've got a faulty board.

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Last edited by how; 10-10-2012 at 11:49 AM.
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Old 10-11-2012, 10:04 AM   #3
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Is gas valve is series with safeties?


Thanks. I was thinking it should be 0V until the gas valve call but with all the "smart" controllers out there I thought it might also be possible the board was checking for continuity (there is a shorted valve error in the error listings). I didn't want to shoot in the dark and just start replacing parts based on half-arsed guesses on my part - that'd get expensive real fast!
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