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Understanding the Importance of Furnace Motor Efficiency

Furnace motor efficiency is an important part of the overall efficiency of your heating system. Conventional forced-air systems used to rely exclusively upon single-speed, permanent split capacitor (PSC) blowers to convey hot air through the ductwork of your home. In these units, the blower was either on or off. When the blower was running, it was producing 100 percent output of air as well as using the maximum amount of electricity. Anyone who's lived with a conventional forced-air system is familiar with this boom and bust cycle. A large volume of hot air blasts from the vents and fills living spaces when the blower is on, followed by periods of no heat or air circulation whatsoever when the blower cycles off.

However, homeowners may not be so familiar with the high electrical costs associated with the PSC blower motor. Because furnace efficiency is rated solely by the AFUE (annual fuel utilization efficiency) percentage that expresses its efficiency of combustion, the excess electrical consumption of an inefficient blower motor isn’t even figured into the calculation.

The electronically commutated motor (ECM) began to replace the PSC motors in high-efficiency units more than 20 years ago. The ECM is a very efficient brushless DC motor that can be programmed by the controller unit in the furnace to run at variable speeds. Even at a single speed, an ECM motor is inherently more efficient than the older PSC units. The efficiency range of PSC motors is usually less than 45 percent while the furnace motor efficiency of an ECM unit is as high as 75 percent. Installed in a forced-air furnace, the ECM blower motor usually runs nearly all the time, with little cycling on and off.

Here are some of the most notable features of a variable speed ECM blower:

  • Though it generally moves a lower volume of air than a PSC blower, because an ECM motor runs continuously the heating throughout the home is more consistent with fewer cold spots.
  • The on/off cycles of a PSC motor allow hot air to accumulate at the ceiling of rooms while the blower is off. This causes rooms to cool excessively and requires excess energy to return the room temperature to thermostat settings when the furnace cycles on again. The continuous movement of air provided by an ECM motor disrupts the stratification of hot air at the ceiling of rooms, circulating the warm air back down into the living area to maintain consistent room temperature.
  • Effective filtration in any forced-air system is related to the rate of air circulation through the filter in the air handler. Continuous air movement provided by an ECM unit produces constant filtration, resulting in less stale air and improved air quality.
  • High airflow through a forced-air system tends to dry interior air during the winter, resulting in uncomfortably low relative humidity inside the home. The continuous lower rate of air circulation produced by an ECM blower maintains humidity levels in the home closer to the ideal 35 to 50 percent.
  • ECM motors incorporate sealed ball bearings that require no lubrication or other routine maintenance. The programmed starts and stops of an ECM motor are very gradual and reduce wear and tear on internal motor components.
  • Unlike the duct-blasting volume of a single-speed PSC blower, the continuous low-volume operation of an ECM is whisper quiet.

An ECM variable speed blower controlled by a microprocessor in the furnace responds to the changing heating requirements of the home. Operation generally advances through several stages. The ECM begins running at a very low speed, rising gradually to a designated peak output calculated by the controller and appropriate to the thermostat setting. The speed will automatically range between the lowest output and the designated peak as the needs of the home change, while maintaining maximum furnace motor efficiency.

If the controller signals the blower to stop, the ECM will perform a "soft stop," very gradually slowing down. This purges all hot air from the ducts so energy is not wasted on air that's stranded in the ductwork, as when a PSC motor abruptly cycles off.

Nicor Home Solutions keeps the Chicago and Northern Illinois comfortable all year long. Ask us about upgrading to an ECM unit for optimum furnace motor efficiency.

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*Nicor Home Solutions is an affiliate of Nicor Gas and is not regulated by the Illinois Commerce Commission. Customers are not required to buy products or services from Nicor Home Solutions in order to receive the same quality of service from the gas utility.

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