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Old 12-26-2007, 06:35 PM   #1
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Gas furnace just barely does not work.


The thermostat calls for heat. The first motor starts. A few seconds go by and the pilot clicks, igniting the pilot light. About 30 seconds goes by and the blower turns on, blowing air throughout the house that is only being heated by the pilot light, the burners never turned on.

I had a problem before, no gas was making it through the smart valve. I disassembled the valve and found that somehow a small, hair-thin thread of copper wire had become detached from it's terminal. I soldered it back on and we had heat for about a week. Then this begins. I already checked the smart valve, tested all wire terminals for continuity and made sure the limit switch wasn't thrown. Does anybody have an idea why the burners wouldn't turn on even though everything else does?

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Old 12-26-2007, 11:14 PM   #2
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Gas furnace just barely does not work.


It's a Honeywell GUJ100D12-1B, and when everything is running, I'm not getting a voltage reading fro the blue mv/pv (from the smart valve) at it's destination on the circuit board.

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Old 12-26-2007, 11:49 PM   #3
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Gas furnace just barely does not work.


And am I supposed to be reading 24 volts from both terminals of the limit switch when everything is running? I may have misunderstood when it was explained to me before, but I got the impression I shouldn't see anything.
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Old 12-27-2007, 05:42 AM   #4
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Gas furnace just barely does not work.


If you are reading voltage across the limit switch it's open, holding the heat off. Replace it.
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Old 12-27-2007, 09:55 AM   #5
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Gas furnace just barely does not work.


edit: he answered it, I just didn't read it that closely.

Last edited by Halcyon; 12-27-2007 at 10:46 AM.
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Old 12-27-2007, 11:03 AM   #6
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Gas furnace just barely does not work.


Well, I bypassed the limit switch to make sure that was it, but the problem is still happening. It's got to be something else. Thank you for our help, though.
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Old 12-27-2007, 11:19 AM   #7
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Gas furnace just barely does not work.


Sounds like the dumb valve to me. Do not try to repair as you might blow yourself up. Replace it. Unless you have a tester its a guess between the valve and the board. Usually the valve.
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Old 12-27-2007, 11:36 AM   #8
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Gas furnace just barely does not work.


I haven't been reading any current across the mv/pv wire from the valve. Should the valve be sending current to the board through that line or should the board be sending current to the valve? Or am I way off and looking in the wrong direction? (This valve has three wires, one MV, one MV/PV/ and one PV. The single MV & PV wires show readings of 24v across them when everything is running)

I know everyone likes to say that they're competent and the warnings the pros give them don't apply in that scenario, but if it is the valve; I've repaired them before, even had to install a rebuild kit in a valve once(which was a lot easier than fixing broken solder points).

So I know my way around the valve, but that doesn't make a pro, and I would never say as much, that's why I'm here! I only bring that up because if you can offer some guidance as to how to go about repairing the valve, I want you to be comfortable telling it to me and knowing that I won't get myself killed.
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Old 12-27-2007, 10:40 PM   #9
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Gas furnace just barely does not work.


Mv is power to the main valve from the board, Pv is power to the pilot valve from the board, Mv/Pv is the common grounded side of the low voltage transformer. You should have 24vac on Pv, Mv/Pv first and then Mv, Mv/Pv after pilot is proven.
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Old 12-27-2007, 11:28 PM   #10
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Gas furnace just barely does not work.


Awesome. Thanks bigMike, that gives me something to work with.

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