We have an older model (1999) Quadra Fire Classic Bay. It runs great! The problem we recently started having is with the exhaust/combustion fan not shutting off. It is sporadic also. When the unit calls for heat all goes well, shuts off when room temp is reached except that the exhaust will sometimes continue running for hours and sometimes it shuts off when it is supposed to. Sometimes it comes on all by itself when the unit is not even calling for heat, then for whatever it will just shut off. It is very strange?? Anyone have any idea what might be going on?