I've recently went to a call where the blwr mtr runs nonstop. Its an old 90% Carrier furnace. I pulled the Tstat off the subbase, disconnected the wire off the G terminal on the Tstat, but the motor continued to run. I concluded that the relay on the circuitboard was stuck closed. I replaced the circuitboard, but its still doing the same thing. There's no fan limit control on this furnace, so ii really dnt know What else it could be. Can someone help?
Some times if the hi limit switch is open or any other safety switch on the heating circuit is open the board will keep the fan running on some boards don't no about carrier specifically. Also check to make you have the fan wired correctly.
A dirty plugged filter can sometimes cause this, as well as a filthy unit. I think i had that happen years ago, unit was filthy, dirty filter,
probably coil dirty too, then they set the temp high, and it trips the limit, and limit was so dirty that if and when it reset, it didn't make
contact. So it kicks the fan on in "safety" mode to cool it down... until the limit resets.
We are assuming this is not an ECM motor since you said there's a relay on the circuit board. Correct?
I haven't a clue. I was scratching my head, called the boss and he told me to change the transformer and pow! Worked like a charm. The wiring diagram had turned to yellow dust long before. So I can't confirm or deny how it worked.
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Originally Posted by garageink
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