hot air furnace problem - http://www.hvacmechanic.com/ Forums
Original message
| geokyl
 |  "hot air furnace problem"
, posted Sat 8 Nov 20:21  
I have a horizontal combo hot air furnace with air conditioning controlled via a honeywell programable t'stat. It worked fine all summer and some time in Sept I shut the unit off via the system switch on the t'stat. I went to change filters and start the heat last week.
The heat came on and everthing appeared to be working fine...I turned the stat down to 68 and started cleaning up. After a while I noticed that 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.
I discovered that the fan AND the gas blower motor were running. But nothing else. So I went back to the t'stat thinking that it may have gone bad. I disconnected the stat only to discover that 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 everthing 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 again and the same problem both fans continued to run...
While trying numerous things to figure out what the hell the problem could be, I took the old board and simply hooked up the 24 volt power nothing else, and that energized the relay!!??
Why would that relay pull in??? That is obviously why the fans are running but what could be causing it???
I am a 35+ year electrician by trade and considered myself very good at troubleshooting control circuits. Maybe I'm losing it in my old age but I am completely baffled by this one...
Thanks in advance for your input...
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Replies:
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| Jojo98
| "Re(1):hot air furnace problem" , posted Sat 8 Nov 23:07
Sounds like you have a low voltage short in the t-stat wire bundle.
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| | geokyl
| "Re(2):hot air furnace problem" , posted Sat 8 Nov 23:20
Thanks for your responce...I'm not quite sure I understand what "bundle you are refering to. Even when I remove all of the t'stat wires from the control board the fans motors continue to run...
That was my first thought, that there may be a problem with the either the thermostat or the thermostat wiring.
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| | oldmangobles
| "Re(3):hot air furnace problem" , posted Sun 9 Nov 12:27
blower fan will energize when the furnace is in lock out without the t-stat calling for it.check continuity thru safety (24v)ciruit.look for roll out or limit switch open
old man mike
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| | oldmangobles
| "Re(3):hot air furnace problem" , posted Sun 9 Nov 12:26
blower fan will energize when the furnace is in lock out without the t-stat calling for it.check continuity thru safety (24v)ciruit.look for roll out or limit switch open
old man mike
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| | geokyl
| "Re(4):hot air furnace problem" , posted Sun 9 Nov 20:58
quote:
blower fan will energize when the furnace is in lock out without the t-stat calling for it.check continuity thru safety (24v)ciruit.look for roll out or limit switch open
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Thanks I'll give that a try...
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| | geokyl
| "Re(5):hot air furnace problem" , posted Sun 9 Nov 21:54
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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