Goodman Fan Runs When AC/Heat is OFF

Furnace Model: GMP100-3. Thermostat: White-Rodgers 1F86-244.
When Stat is in OFF position and Fan is in AUTO position, the fan blower comes on (maybe 3-5 times per day) and runs for a few minutes and then turns off. The fan blower operates normally when either heating or cooling or when the stat Fan switch is in the manual ON position.
I suspect either the limit switch, aux limit switch, the stat or possibly the HSI module.
How do I determine which (if any) is faulty?

 

Is your blower on a fan timer or on a fan switch that only comes on when the heat exchanger comes up to a determined temperature.

If the limit switch was at fault, the furnace would not allow the ignition sequence or fire. You said your heat works. So that can't be it.The HSI circuit would have nothing to do with it, especially if no call for heat and the blower comes on anyway.

 

By fan blower, are you talkibng about the large fan that circulates air around the house or the inducer motor, the small fan in the burner compartment that circulates the combustion gasses?

 

The manual indicates that "30 seconds after the main (heater) valve is energized the heating speed of the circulating air blower is activated". After the burners go off, I notice that it takes time for the blower to shut down. I do not know if that is controlled by time or temp, however.

 

I am talking the large fan.

 

Most likely time, since the manual gave a definitive time. If by temperature, the blower takes longer to come on if the stat has been turned way down, for example. They would not have been able to say 30 seconds. It might vary from 30 seconds to 2+ minutes, depending.

 

Does the furnace fan turn on when the thermostat is set to the Fan On setting? It sounds like it does. If so, my GUESS would be that something is energizing the wire that turns on the fan at the furnace when that feature is selected.

I'll say that again since I may not be clear:

You have a wire that provides 24 VAC from the furnace, and other wires that will switch on the furnace, AC or furnace fan if that 24 VAC is used to energize those particular wires.

I'm guessing that the wire that switches on the fan is being energized when you don't want it turned on.

The simplest way to test this THEORY would be to disconnect that wire at the furnace and see if the problem continues or goes away. If it continues, the theory is wrong.

This could also be some erratic operation of the circuit board, but I'd test the wire theory first since it's easy to do.

 

How woud he know which 24 volt wire to disconnect?(Just trying to help - not being testy.) The only thing obvious to one staring at an open furnace compartment might be how you can trace the 120 volt blower wire back to the board, or fan switch. But when it comes to low voltage wires and then how they involve the board, with relays on the board, with nothing to actually disconnect....what does he try to disconnect? Anything (any of the skinny low voltage wires)he can get his hands on, to see if it shuts off?

 

I wasn't able to find a circuit digarm for the furnace.

It wouldn't be the wires to R or W. He can post the connections made to the thermostat wires here. Also, checking with a multimeter to see which one is energized when the fan is switched on would identify the correct wire.