There are Manufacturers that do use constant torque motors in residential furnaces though. Trane S8X2 S8X1 and S8B1, Carrier Comfort and Performance series, York TM9Y, Bryant Preferred and Legacy Line are just a few that use constant torque motors for the blower fan. Basically every Manufacturer offers a lineup that consists of different series that use VCA (Variable Speed Constant Airflow), VCT (Variable Speed Constant Torque) and FCT (Fixed Speed Constant Torque).