Lennox G20 forced air gas fired system about 10 yrs old. After the burner shuts down the fan will continue to run forever. Only by cutting power at the furnace will it stop. Fan should shut off about 90 seconds after the burner shuts down, but is adjustable for a longer interval by a jumper. Does this indicate that what Lennox calls the blower control center (BCC) a circuit board, is faulty or is there sensor or bi-metallic control that might not be opening? Problem seems to be getting more frequent.