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Discussion Starter #1
I am trouble shooting a furnace. When there is a demand from the stat, the burner fires long enough to get the blower to turn on then runs for several minutes. the burner turns off (voltage to the gas valve dropsout to zero). After about 15 seconds the burner refires and runs for about 20 seconds then drops out again. This cycles over and over with the burner turning off until it will sometimes just turn off immediately after the ignitor turns off. I suspected the flame sensor, but it made no difference after I cleaned it off gently with a mild paper. With it unplugged, the burner extinguishes immediately after ignition timout (simulating no flame sense) as it should. Limit switches are all in series (3) and have continuity so I don't think it is that either. I have checked the vacuum sensor as well by uplugging it and it seems to be operating as well (wont allow ignition when disconnected). Exhaust blower is running fine. Gas valve seems to be operating fine when it has voltage and only shuts off when voltage drops out. It seems like something is telling the control board to drop the voltage but I can't figure out what it is. Anyone have any ideas?
 

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Is this an 80% or condensing furnace.
 

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Also post the make model and serial #. Sounds like a pressure switch tripping. There may be a LED on the circuit board with a viewing window in the door which will give you an error code when it happens, code should be on the back of one of the doors.
 

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please can someone help me with this problem,I have a lennox gas furnace with a lennox vr8204h gas valve and a lennox sp 745 ignitor module.the pilot will lite up,but when it calls for heat,you can hear a short click from the valve,the pilot light goes out and the burners will not ignite.need help bad.
 

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Discussion Starter #6
This is a Ducane 80% furnace 75kBTU. It does have an LED indication on the circuit board which only gives two indication during the problem I described above. Slow hartbeat when power is on, waiting for call, then fast heartbeat when there is demand for heat. Again, normal according to the error codes shown on the inside of the panel. I have noticed that the heartbeat "skips" a beat when the voltage drops to the gas valve, but does not revert to any of the fault codes shown on the panel; still fast heartbeat. I have seen the other codes (4-pulse of 5-pulse etc.) when conduction tests of the limit and vacuum sesors though. It almost seems like the system needs to warm up before the problem occurs because it runs normally for the first several minutes before it cycles off and on. I may look at the flame sensor again to try to see if there may be a hairline fracture in the ceramic or something like that, and also inspect the solder joints on the back of the control panel. Beyond that I would love to hear any more suggestions - Thanks!
 

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post 2 pics, 1 with the top door off and 1 with the lower door off so we can see the burner section etc. Does it have a Honeywell Smartvalve? Should have a model # with SV 8500xxx or similar stamped on it.. If so that may be the problem.
 

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I jumper R to W and see if it does it.
 

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Discussion Starter #9
I dont have a photo but here is a link to the manual attached. It is installed in a crawlspce horizontally so instead of running upstairs to mess with the thermostat, I've just jumpered a switch across the R&W to demand heat.

https://docs.google.com/file/d/0B8F5puDAZgpBWlRWaDlNRXB6Y1k/edit

The burner unit comes out through the grate in the side and includes four burners, roll-out sensors, ignitor, and flame sensor. No smart valve on this - but it is a Honeywell.
 

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look for a crack in the heat exchanger.
 
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