reply to lutful
Re: noisy furnace fan Are you absolutely certain the motor is the source of the noise? Many furnace-related noise complaints can be traced to vibration. Carefully check all hardware on the furnace for proper torque. Set your thermostat mode to "off" and fan to "on" to start the blower motor. Using an open palm and firm pressure, place your hand on various metal surfaces of the furnace and surrounding duct work to see if anything is vibrating (make sure everything has sufficiently cooled off before beginning).
If it does turn out to be the motor, they are generally inexpensive and relatively easy to replace yourself.
lutful... of ideasPremiumReviews:
The metal duct just above the furnace makes a clang sound when the blower stops which is really annoying at 5AM.
However, it is the HIGH setting of the blower motor which causes the continuous grinding sound. The MED setting seems to be tolerable because I used the manual fan setting many times.
If I disconnected the black wire coming from IR-1 relay on the controller and tied it to the blue wire from motor, it should run on MED speed during heat call too. But I will wait until less cold weather to take that risk.
P.S. The schematic actually shows just the blower. It is a simple old furnace but worked OK all these years.