Updating Your HVAC Unit

Why Does Your Furnace Blower Keep Running (And When Is It A Problem)?

If you have a standard single-stage furnace, you're probably used to its normal heating cycles. These furnaces operate by blowing warm air for just long enough to reach your thermostat setpoint, at which point they shut down and wait for the thermostat to call for heat again. Despite these on/off cycles, a well-sized furnace should maintain a comfortable and relatively consistent temperature.

However, you may have occasionally noticed that your vents will continue to blow warm air even after your thermostat reaches its target temperature. What causes this behavior, and should you worry that there's a problem with your furnace?

Understanding the Heating Cycle

When your thermostat calls for heat, your furnace will initially perform a long and fairly involved startup sequence. This sequence proves several safety features before igniting your burners and turning on the blower. When the thermostat stops calling for heat, the furnace shuts down the burner and turns off the blower – or does it?

In practice, there's likely still quite a bit of heat remaining in the heat exchanger once your thermostat stops calling for heat. It would be inefficient to waste this heat, so the furnace will continue to operate the blower until the limit switch indicates that the temperature near the heat exchanger is cool enough. As a result, the furnace will usually keep running for a short while after hitting its target temperature.

Recognizing Abnormal Behavior

Because the furnace is attempting to extract as much heat as possible at the end of the cycle, you may notice the air temperature at your vents dropping slightly in these last few seconds. However, the temperature should still be relatively warm, especially compared to the surrounding air. The blower shouldn't run long enough to push cool air into your home.

If the blower continues to run for more than a short time or you notice the air from your vents becoming cold, that's a sign of trouble. First, check your thermostat to ensure you haven't accidentally set the fan mode to "on." This setting will run the fan continuously, producing cold air. Switching the fan mode to "auto" will resolve this problem.

However, if you have already set the fan to "auto," and the blower continues to push cold air after your furnace shuts down, it may indicate a problem with your limit switch. The limit switch may not notify the furnace's control board to shut down, leaving the blower to push cold air. This behavior will waste energy, cause excessive wear on your blower, and produce chilly drafts in your home.

If you suspect a problem with your limit switch, it's best to have a professional check your furnace and diagnose the underlying issue. The limit switch is an important safety feature, so leaving this repair to an expert is usually the way to go.

Call a heating repair contractor for help.