rpm

Paul S_3

can anyone tell me if the newer 90% plus warm air furnaces blower motor run at a higher rpm than the standard 1075 rpm....and if they do why??

Steve Whitbeck

No

I don't think I have seen any that run faster than 1050 to 1075

The only reason for higher RPM would be for higher static pressure.

Paul S_3

fan speed

ok....but why the newer 90 plus furnaces run at a higher fan speed ....ive been replacing older ones with Coleman 90 plus furnaces , in heating mode they are set up for med/hi not low fan speed any particular reason

VictoriaEnergy

RPMs

There are PSC motors rated at 1625 RPM.  These are typically used in very cheaply built furnaces due to them being inherently more noisy.  The 1625 will push a cheaper 11x8 wheel and housing into the airflow range they'd normally get from an 11x10 or 11x11 with a standard 1075 motor.  Probably saves the manufacturer $1.50 per furnace.



On the other end of the the spectrum, some of the recent PWM Variable speed ECM motors can modulate anywhere from 300 to 1300 RPM.  In practice, the Variable speed motors will punish you for installing grossly over-sized equipment by winding up to the necessary RPM to hit the design airflow.  They'll similarly reward you for for sizing the equipment properly and run very quietly, especially in low stage operation. 



The important thing to know is the furnace MUST be set up and running at its intended temperature rise.  The typical complaint is the furnace is too noisy; so the well meaning (but uninformed) tech selects a slower speed tap on the fan.  This will often make the furnace quieter, but the furnace will slowly overheat until it hits the limit, cool of a bit and then re-fire the burner and run it up again.  This continuous cycling on the limit switch will drastically shorten the life of the furnace.  The limit switch's are not sturdy enough to tolerate the continuous work and will fail.  If it fails closed, the house can burn down.



In my opinion all fan limit switches should be manually reset type.



Once you service the furnace, you need to run it with the fan compartment cover on for a good 20 to 30 min and measure the temp rise ac cross the furnace and confirm it is within the specs on the rating label.