4-Speed Vs. Variable Speed Furnace

Four-Speed Furnace

• A furnace with a four-speed motor provides energy efficiencies that single, two-speed and three-speed furnaces do not. Unlike a single-speed motor, which blows continuously, a four-speed utilizes off, low, medium and high speeds. This allows a four-speed furnace to run efficiently, using the right level of speed to blow the proper amount of air to reach the desired temperature for the rooms in the home.

Variable Speed Furnace

• The motor on a variable speed furnace will typically have two or three speeds -- high, medium and low. The furnace starts in high-speed mode; then as the room temperature warms, it reduces the speed and operates at a medium or low speed -- based on the temperature set on the thermostat. As a result, the room reaches the desired temperature quickly because the motor is running in the high-speed mode. The temperature of the room remains constant, without the motor having to exert more speed than necessary when the furnace changes over to the medium speed.

Similarities

• Four-speed and variable speed furnaces are much more efficient than single-speed furnaces, utilizing what the furnace industry calls "intelligent technology." What this describes is how the thermostat "speaks" to the furnace system and tells it what speed the motor should use. For example, on a cold winter morning, a four-speed and a variable speed furnace will start at a high speed to rapidly distribute warm air to rooms. As the rooms reach the desired temperature, the thermostat will tell the furnace to reduce the motor speed.

Differences

• When its motor speed is reduced, a four-speed furnace will automatically slow down to medium or low. You also have the option to turn the motor off by a switch on the thermostat. However, if you are replacing an older four-speed furnace, you may find that repair pros will steer you toward switching to a three-speed replacement motor for your furnace. This is because most homeowners rarely use the "off" function. On the other hand, a two-speed furnace will reduce in speed from high to medium, and a three-speed furnace will reduce to medium or low. You can only turn a variable speed motor -- whether it's a two- or three-speed unit -- all the way off by using the switch on the furnace unit itself -- not from a switch on the thermostat.