Actually, to my knowledge their is no "cfm" sensor in any of the residential furnaces that use a VS motor. The only thing a VS motor does is attempt to maintain its preprogrammed power useage based on a external signal given to it, which usually comes from the air handler microprocessor and/or the thermostat.Originally posted by jeffw_00
I get it now. So there's a cfm sensor as part of the system./j
As a by-product only the VS motor tends to maintain a more constant CFM as reasons to maintain less or more CFM are created. This includes a dirty air filter. But that perprogrammed power usage which can equal a certain CFM is only within the boundries of the programming. Once these factors are exceeded then the VS motor is instructed to shut itself down or slow down to a minimun speed for it's own protection.
They are a good thing but like everything else they must be applied properly and understood to properly trouble-shoot.