I have a heat pump with a gas furnace AUX Heat/Emergency heat backup. About a week ago, the fan would run continuously with the fan set to AUTO (as it has always been), and the house temperature at or above the heat set point. Normal operation should have the fan cycling off if the inside temperature rises to the temperature set point with the fan set to AUTO. So, if the heat setpoint is 70F and the inside house temp is 72F, the fan still runs, it doesn't cycle off. This occurs whether the heat pump is used or the gas furnace is used. The only way to stop the fan is to set the system to OFF.
I researched online and other than failed electronic thermostat circuitry, the most likely cause seems to be a failed fan limit switch. Does this sound right? Is there only one fan limit switch with my dual fuel heating system? I can't imagine there being two of them (for the heat pump and the gas furnace) as the fan runs continuously in using both heat sources. Sounds like the fan limit switch is in the gas furnace chassis; If so, does it affect the system operation when the heat source is heat pump? It seems it would. Would replacing this be the easy/cheap test?
The system is a carrier, built around 2005.
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