The fantastic news: your standard forced-air furnace includes 2 rates. The bad news: the rates are “on” and “off” Permanent split capacitor (PSC) motors in conventional furnace blowers don’t adapt variable speeds. More advanced, high-efficiency furnaces incorporate electronically commutated motor (ECM) technology programmed to operate at variable speeds and output. Most existing PSC furnaces can be retrofitted with ECM technology to boost performance.
Though they run longer blower cycles, variable-speed ECM blowers consume up to 75 percent less power. Because the furnace blower is thought to be a separate part, its electrical consumption isn’t included in the calculation of the manufacturer’s published efficiency evaluations. Therefore, a new furnace with a high annual fuel-utilization efficiency (AFUE) rating may still deliver unsatisfactory efficiency results if it is installed in conjunction with an old-school, on/off blower motor.
Variable-speed blowers ramp up gradually to complete output as the thermostat setting is achieved, then incrementally reduce output down to what is known as a “soft halt.” Variable-speed operation is quicker, creates less wear and tear and prolongs air flow. Extended blower cycles in lower output heat consistently by removing the abrupt fever spikes and sags connected with conventional on/off blowers. Since proper air filtration is dependent upon duct flow, variable-speed blowers that circulate air longer also improve indoor air quality.