[SquashPro]

I have a heating/air conditioning unit in my attic. The air conditioner works fine but the heater doesn't. The main furnace fan/blower does not come on when thermostat calls for heat. I believe that the problem is that the low speed fan setting is not working, and that the low speed fan setting is what is used for heating. Here is why I think this. If the thermostat calls for heat, the furnace burner comes on. The fan doesn't come on in this situation, so the furnace eventually overtemps and shuts down. This cycle repeats once the overtemp is resolved. If I force the fan to operate all the time by using the fan control on my thermostat, then the heater will work properly and heat the house. The air conditioner seems to work OK in "auto fan" mode because the a/c seems to use the high speed fan setting by default.

Other data: I have noticed that while the fan is running continuously at high speed, it does make occasional clunking/thumping type noises. I'm not sure when this started, so it may not be related to the problem that exists above.

It would seem that there are a couple of possible causes for this problem: 1) the control board is defective and is not calling for the fan to turn on in low speed mode when it should. 2) Something is wrong with the low speed electrical components associated with the fan (windings, contactor? capacitor?), or 3) some interference with the fan housing is occurring that might prevent the fan from turning when in low speed mode. I am fairly handy with things mechanical, so I'm considering trying to do this repair myself (after the weather cools). Any suggestions on how to diagnose this further or on fan replacement if necessary? Are there contactors and capacitors on furnace blowers? Are these separate for low speed vs high speed operation? Does this type of diagnosis seem like something that a non-professional should attempt?

[KC13]

Do you know if the fan control on the furnace is mechanical (combination fan/limit control and/or fan relay center) or electronic (circuit board)? In either case a voltmeter would allow you to check if power is being supplied to blower motor. In most (but not all) cases the cooling blower speed is higher than the heating blower speed, so you will have to determine which motor speed tap lead is used for heating. You may also want to check for any faulty electrical connections.