The Virtual Repairman's Appliance Help

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Goodman Manufacturing Co.
"CAT 1"
Model: GMP050-3 REV B
Serial:0004606054
115 volt, 1 PH, 60HZ
Blower 10X6, 1/3 HP

The blower fan came on 4 times yesterday morning on my second floor central air unit. The strange part is the thermostat was set to OFF. I switched the thermostat with the thermostat on the first floor (I have two units in my house) and the problem continued to present itself. I'm quite sure the problem isn't in the thermostat.

Note that the AC compressor is is shut down at the breaker. So I'm not sure if the compressor would turn on as well.

In the area where the blower fan is, it appears there is a large circuit board. Could that be the culprit??

Any light you can shed on this problem would be helpful.

[Virtual Repairman]

This could be a glitch in the furnace where the fan control is located. Some newer furnaces use a circuit board, some use a mechanical switch. I am not familiar with your model, but if it has a circuit board just iside the main cover panel,chances are its faulty. Only a faulty thermostat on the wall could create the same problems, but is not so common.

The fact that the thermostat was set to "off" indicates a problem withing the furnace fan control. Only if the attic got above 140 degrees for an extended time would the fan be automatically energized.

[mikep153]

It definitely has a circuit board.

I'll look into replacing that since switching the thermostats didn't help.

The latest incident happened at 4:00 am this morning. The temperature outside was in the 50's. So excess heat wasn't a factor.

Any idea what the furnace fan control looks like?? Is that usually on the circuit board or separate from it??

[Virtual Repairman]

Not being totally familiar with your brand, I would say that most of the modern furnaces which have circuit boards DO control the fan, along with a temperature sensor somewhere in the blower cavity to tell the fan to come on upon heat rise.

The older, more archaic units in my time has a small steel coil probe which mounted to the side of the furnace and actually poked into the heat exchanger itself to monitor heat rise.

I would expect that you can find the wiring diagram to your unit just inside or on the back of a blower door which will show how the fan circuit runs. A quick search on Google turned up some more info on your model and the problems that others have experienced:
http://www.google.com/search?hl=en&q=goodman+furnace+GMP050

[mikep153]

I replaced the circuit board and all is well.

Many thanks for your input!!