furnace problem


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Old 11-08-08, 08:15 PM
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furnace problem

I have an Arco-Aire Gas furnace with A/C, Honeywell CT3200programmable thermostat and Honeywell ST9120C4016 control board. The exhaust blower and the main fan run non-stop - no heat or AC will start. This problem continued even when I removed the t'stat wiring...

The system has been working fine... Late Sept I shut the system off via the switch on the t'stat.
Last weekI went to change filters and start the heat . The heat came on and everthing appeared to be working fine...I turned the stat back down to 68 and started cleaning up. After a while I noticed that while the heat was off, the fan was still running, so I checked to see if I left the fan in the on
position.

The fan was in the auto position the stat was set on
heat and the setting was well below the room temp. The stat
display show --AC-- that usually means there is a power problem, but the fan was running??? So I changed the batteries in the stat. No change it still showed --AC-- and the fan was still running.

Show I climbed up into the attic to check it out.
That is when I discovered that both motors were running.
Once again, no heat or air would start up...
So I went back to the t'stat thinking that it may have gone bad. That is when I disconnected the thermostat wiring and the fans continued to run. even without the stat...

So I figured the control board went bad.I picked up a Honeywell ST9120U1003 control board which was the exact replacement part for the existing one...I replaced every wire one for one and everything matched up no problem...set the dip switches to match the existing board and figured everything was fine.

I turned the power back on and I had the same exact problem. Disconnected the thermostat wiring and the same problem both fans continued to run...

I don't know what else to look for, what would cause those motors to run continously, yet not allow the heat or AC to work???
 
  #2  
Old 11-08-08, 09:04 PM
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An open limit will cause this problem on some furnaces. Specifically, a manual resettable limit switch. Look for the switch above the burners or very near the burners if it is a rollout switch. If you find that it is tripped, then figure out why. These switches can be reset by pushing in the button in the center.
 
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Old 11-09-08, 06:31 AM
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High Limit

There could also be a manual reset limit in the blower compartment particularly if the furnace is horizontal or downflow.
 
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Old 11-09-08, 06:52 PM
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Thanks for the input

I just finished checking it out...everything seem to be in order
I located the high limit in the blower section. another limit was in the gas section, and the roll out switch, I found the re-set but everthing was OK..All contacts were in the closed position.
The pressure switch was open but did close once the fan got started.

I'll call a service guy in the morning...luckily it isn't quite that cold ...yet.

Thanks for your help.
 
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Old 11-09-08, 07:49 PM
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Service Person

Please let us know what the servicer finds. This long distance troubleshooting can be tough & any help we can get is well, help & may be the answer to the next poster's question. Thanks
 
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Old 10-02-12, 10:46 AM
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furnace fans

Hi.
I know it was quite some time ago, but....
I have the exact same issue. Can you remember what the problem was.
Any advice would be much appreciated.
thanks, Ryder
 
 

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