What is the difference between a variable speed and a two-speed furnace?
Both the two speed and variable speed furnaces have a two stage gas valve to create two levels of heat and the blower motor will operate at a low or high speed operation to move air and extract the heat at the right rate. The difference is that the blower speeds of a two speed furnace are set at fixed speeds while the blower motor of the variable speed furnace will adjust itself within the low range or the high range of its operation to overcome any resistance to move the air from changes in the duct work static pressures. The changes in static pressures (resistance to move air) in the ducting system can develop if the return air openings are blocked by furniture, if the supply air vents are closed in a particular room, or as the air filter of the system gets dirty. As an example: if the low speed/heat operation runs the blower motor to create 600 CFM (cubic feet per minute) of air movement and the air filter has not been changed and has collected a lot of dirt/dust. The resistance to
