Most existing furnaces have single or multi-speed fan motors that use alternating current (AC). An electronically commutated motor (ECM) converts this alternating current to direct, or DC current. By converting the current, electrical consumption is reduced (usually by half or more), saving you money on your electric bill.
In addition, the motor's operating speeds are either multiple or variable. Variable speed motors gradually speed up on the start, and gradually slow down when shutting off. This minimizes cold and hot drafts, resulting in a more comfortable home, with less stratification.