The Furnace Fan Motor Will Not Turn Off. It Constantly Runs.

This is usually one of four issues:

  1. The thermostat is set to fan on. When you turn your heating system on in the winter you do not need to turn the fan switch on thermostat to on. The fan switch is labeled On-Off-Auto. If you leave it on auto the furnace will turn the fan motor on and off as it needs to either as a function of time or temperature.
  2. The thermostat fan switch is set to circulating mode. On higher end systems that have variable speed fan motors we normally turn the motor to “circ” mode so that it runs about 20 minutes out of each hour on a very slow speed. This helps to even out temperatures in the home and help clean the air for high end allergy abatement systems that use high quality air cleaners like the IQAIR 16 or the American Standard Accuclean. This motor being constantly on costs about $5 a month in electricity bills.
  3. There is an interruption in the 24 volt safety controls in the furnace. There are numerous flame roll out switches, high temperature limit switches and fuses in a modern furnace. If any of these, including the transformer, fail or open the fan automatically turns on to cool the furnace down. The furnace circuit board will automatically assume that the furnace is too hot with regard to it’s actual temperature. This helps to ensure that the heat exchanger does not crack or rupture.
  4. On older furnaces there is a summer switch above the burners. It could be turned on.
  5. On older furnaces, non-spark ignition types, if the cord leading to the furnace or the outlet is wired backwards, neutral where the hot side should be, the fan motor can partially burn out completing the electrical circuit and causing the fan motor to run at about ½ speed 24/7.