Fan Runs Continuous
FAN RUNS CONTINUOUS – The thermostat maintains the correct temperature, but the indoor fan never quits blowing.
POSSIBLE PROBLEM & DESCRIPTION
Thermostat "Fan" Switch
The most common cause of this problem is the “Fan” switch on the thermostat. It normally has two options – “Auto” and “On”. In the “Auto” position, the fan should cycle on only when there is a call for heating or cooling. In the “On” position, the fan will run continuously. Watch Video.
Some new thermostats have an additional fan setting of “Circ”, which is short for “Circulate”. In this mode the fan will cycle on and off with heating & cooling, but it will also kick-on automatically if there is an extended period without a demand for fan – to provide additional air circulation.
If your system has an added humidifier, this could cause the system to run more than normal.
Some humidification system energize the blower to operate even when the heating system is not running to maintain the humidity setpoint in the space.
Stuck Blower Relay - Qualified Technician Recommended
Many gas furnaces have either a fan relay or a control board to cycle the fan on and off as needed. Sometimes the relay or control can stick which will cause the fan to run continuous.
Stuck Fan Switch - Qualified Technician Recommended
Some older gas furnaces use a heat-activated fan switch to start the blower after the burners have fired and the heat chamber has warmed-up.
The fan should stop once the burners go off and the heat chamber cools.
A stuck fan switch can cause the fan to run continuous.
Some "Fan&Limit" switches have a push-button, manual-override that, when depressed, causes the fan to run continuous.