How Do Variable Speed Furnace Blower Motors Work?

Traditional Furnaces

• Traditional forced air HVAC systems, which are single-stage, single-speed units, deliver heated air at a set temperature and speed. The blower’s main purpose is to deliver heated air throughout a building. The same purpose occurs during the summer, when cooled air is delivered via air conditioning.

  For heating, temperature must reach a critical level for the unit to begin operation, which occurs with a full blast of hot air. These blasts of air create wide temperature differences among different areas of a building as well as in the same room within minutes during extreme outdoor weather conditions.

Variable Technology

• Blowers with variable speeds are an option on newer, more energy-efficient furnaces with two-stage heating. These furnaces operate at two levels -- low with partial burner operation when outside temperatures necessitate a minimal heating level, and high when frigid temperatures call for full-capacity heating. Variable furnace blowers contribute to this efficiency by delivering the correct amount of airflow. Sensors inside the unit detect the amount and force of air required to maintain comfortable air temperature and adjust delivery accordingly.

Ductwork Factors

• Resistance to airflow increases proportionately with the amount of ductwork in a building. Rooms in a building that are farthest away from the furnace receive the least amount of airflow with a single speed blower, resulting in uneven distribution of heat. Other factors, such as dirty air filters, can also restrict airflow. Variable speed blowers, particularly when used in conjunction with zoning, which opens and closes ductwork automatically to direct heating and cooling where needed most, compensate for these differences by constantly monitoring.

Constant Operation

• Variable speed blower motors operate year-round to provide constant air circulation. This means that the blower is operating at low speed during mild conditions. Although such operation does result in increased electrical expenses at these times, overall energy consumption is reduced, because constant circulation results in fewer temperature swings and less need for heating. Continual air movement also removes more humidity from buildings during warmer months, which necessitates less need for air conditioning. It also results in cleaner, more healthful indoor air, because more harmful particles are continually trapped through the system's air filters.