I'd add to that, it's very dependent on how the system is actually installed. If all the ducting runs only in heated parts of the building, then the potential for heat loss is little. If the furnace is in an unheated basement or even worse, an attic, some ducts run through outside walls, etc, then it's going to be a losing proposition for sure. Plus, I don't see the need. All the houses I've lived in with furnaces it was comfortable with the fan just running when the furnace was on.