Last night I heard a rumbling noise coming from the closet in the hallway where our Trane XE80 gas furnace (April 1996) is located. With the door open the noise appeared to be coming from the blower area.

We are in the heat of summer so, to stop the system, I set the temperature on the thermostat up a few degrees and the blower turned off. As the fan was stopping I heard squeaking noises from the fan area.

I removed the front covers and an examination of the fan and motor revealed the following:

1) The fan turned freely but had some resistance. Spinning it as hard as I good resulted in only 1-2 revolutions.

2) There was dust on the motor and the fan housing so I used compressed air to clear this.

I returned to the thermostat and set the fan switch to On and heard the usual humming noise, except this time, the fan did not start spinning. The noise continued with the fan not spinning so I set the fan switch to Auto and waited a few minutes. After a few minutes I set the fan switch to On again, the humming noise started, the fan slowly began to turn, and appeared to function normally with no rumbling noise.

I ran this scenario several times with the same result. It appears that the motor has difficulty starting after running for some time. However, once the fan has been stopped for some time then the motor will slowly start and run normally.

Since the problem appears to be starting the fan, I have set the fan switch to On to allow the unit to run continuously and have left the temperature setting higher than normal to minimize use of an 'a/c' speed if the motor has such.

Here are the BLW0593 DIRECT motor specs:

Diameter: 10 X 7
No. used: 1
Speeds (No.): 4
Motor HP: 1/3
RPM: 1075
Volts/Ph/Hz: 115 / 1 / 60

I read the posts and am inclined to replace the fan motor but I'm concerned that this is a mis-diagnosis since some posts referred to a "starting"/"running capacitor?" and 1 post lead me to believe that the problem could be the fan since it said that if the bearings in the fan are bad then the motor can't turn the fan.

Also, please let me know if running the system with the fan switch to On is bad since I don't want to make the problem worse by doing something stupid.

Thanks in advance for your responses.

Update on this.

I was not home this morning and stopped by @ 12:30 to check on the furnace / fan. Fan was running and all seemed normal. Left.

Just arrived home @ 1:45 pm to discover the following:

1) Fan was stopped.
2) Humming noise from fan area.
3) Thermostat displayed "Cool" indicating A/C mode.

At the thermostat, I immediately moved the switch from Cool to Off to disengage the compressor, set the Fan switch to Auto and checked the furnace. Here's what I found at the furnace:

1) Small copper line was very warm to touch. I think this line is usually cold.
2) Although Fan was cool to the touch, the shaft and motor were warm.
3) Fan resistance is very significant now compared to earlier.

Update:

Called neighbor that has more experience with such things and he referred me to a friend in the business.

A/C guy said that problem is motor and capacitor must be replaced as well. Made 4:00 appointment... it's 4:15 and work is in progress.

Will update with results.

Done... A/C guy installed new motor and capacitor and we're back in business.

Cooling is better since new motor is more efficiently moving air.