Our furnace runs longer than it should. If it’s an air filter, where is it? — Joy H.
The furnace operation is controlled by the thermostat, which monitors the temperature and creates a closed condition that supplies the furnace control board with 12-volt power. If you set the temperature at 70 degrees, once the inside temp drops below that temperature +/- 5 degrees, it sends 12-volt power to the module board and the motor/fan starts until it raises the sail switch and then the gas valve opens and it lights the burner assembly. When the temperature gets to 70 +/- 2 degrees it shuts off.
I would suggest getting a thermometer and verifying what the temperature is at the thermostat. This will tell you if the furnace is actually performing as designed or really is running too long.