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Discussion Starter · #1 · (Edited)
I am a HVAC contractor looking for anyone who may have seen this issue before. I was on a service call today for a bryant 80plus 395CAV and the control board was showing error 31 pressure draft safegaurd/ Aux Limit.
Of course this could be a potential problem with either the draft fan,pressure switch or aux limit but all checked ok. Aux limit was not tripped and was closed. Inducer runs and operates correctly. Pressure switch shows open when inducer is off and closed when inducer runs. I checked voltage on the common side of the draft switch and monitored to make sure the draft switch was not droping out while inducer was running and found everything to check fine.I noticed a few things that seemed odd to me. (1) Ignitor would still energize when control board showed 31. (2) when checking common side of pressure switch to ground on initial start of the draft fan,I removed my test leads and the furnace would suddenly shut down almost as if there was a power bump. (VERY ODD). I checked the molex connector at the board and it looked fine didnt see any obvious issues. I consider myself to be very competent and I always refuse to randomly change parts and fix thru process of elimination but Im leaning towards replacing the conrol board and my questions are:
(1) Could this be some sort of intermitent faulty ground issue ?
(2) Has anyone seen anything like this before (a known issue with this board)?
(3) I have never seen ignitor energize when showing draft or aux limit failure, ignition sequence should never start. What do you think? Curious to know.

Thanks,
ALLEG
 

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It sounds as if the board is bad. I hear you as far as refusing to change parts unless you are certain. Did you take the board out and inspect it for signs of excessive heat? There was a recall on bdp boards back around '98. The furnaces were made around the mid 90's. Is the plug on the board orange?
 

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Discussion Starter · #3 · (Edited)
Hi, Thanks for the reply. no, the connector is white and the furnace was manufactured in 99. Problem is intermitent, the homeowner would reset when error would occur. Couldnt pinpoint trouble with any safegaurds.
Thought it was really wierd when checking common side of pressure switch that when I removed my leads the furnace combustion suddenly shut down (happend 2x). Most likey a board issue just curious if anyone has seen before. I didnt pull the board out and inspect for heat damage. Plan on going back 2morrow with a board. Just find it kinda weird.Is it common for some boards to energize the ignitor when there is a potential draft or limit issue? I personally have not seen before?

thanks,
alleg
 

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Have you monitored the draft on the pressure switch and in the flue vent with a manometer during operation?

Lack of combustion air or venting problem can cause the draftguard to operate. Make sure the mechanical room is set up as it would during normal operation. All doors closed etc. Monitor the draft for an extended run time at least 15-20 minutes. If the draft starts to decay open the door to the mechanical room. If the draft re-establishes you have a combustion air problem and need to bring combustion air into the room. If the draft does not re-establish then it is a venting problem. Have seen small dead birds cause this.
 

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Discussion Starter · #6 ·
Hi, Thanks for the tip MechAcc, I had gone back the following day after my original post and replaced the control board. I had checked aux. limit and draft switch, venting etc and found no issues. Control board was the issue all along. Just kept thinking after the fact, that there was a possible faulty ground that I had missed. (night of original posting). Ive seen some control boads do some wierd things but this was very different. One I wont forget! ;)

Thanks.
 
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