AC furnace motor won't shut off.


  #1  
Old 07-26-12, 08:38 PM
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AC furnace motor won't shut off.

I have a Tempstar furnace with AC - Model # NUGS100BH0l.

The blower motor for the furnace runs constantly. The cooling unit outside operates on/off on demand by the thermostat.

I looked at other posts and picked up ideas, but haven't hit on anything for a fix.

So far, I have replaced the thermostat, pulled off the green thermostat wire at the ignitor control module in the furnace, and replaced the fan limit switch. The last time (about two years ago it was doing the same thing and the limit switch fixed it. I used a new switch, but this time it didn't make any difference.

If I take the thermostat off completely it still runs and if I pull off one of the two wires of the heat sensor mounted into the flame box the motor stops for about a 1/2 second, then starts back up with the wire disconnected.

I saw where it may be a stuck fan relay, but I thought that was built into the limit switch.

Open for suggestions and fully appreciative for ideas.

Thanks a lot.
 
  #2  
Old 07-27-12, 11:15 AM
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Oh its an old furnace. Tempstar is carrier so carrier… do you have a circuit board in the furnace? Might be a blown fuse in the circuit board. When thr board loses 24 volts the fan turns on
 
  #3  
Old 07-27-12, 02:19 PM
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Thanks for the response. It's about twenty years old, not a lot of problems. No circuit board that has everything. It has a White Rodgers ignitor box - about 4 x 5 inches, then the limiter switch and the heat sensor in the flame box. Just about everything is individual parts and open wiring.

The thermostat wiring goes into the ignitor box. There's about seven hookups there and seven hookups on the limiter switch.
 
 

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