It is most normal to see a switch fuse unit on a furnace. This is for a several reasons. One, as has already been mentioned, some central heating equipment calls specifically for fuse protection. Second, it would be normal to find these fuse units with 6 or 8 amp fuses installed, as per the heating equipment manufacturer’s specifications. Third, in days gone by, with very small fuse panels protecting homes, the fuse panels had a set of “subfeed lugs” marked “heater lugs”. These were lugs basicaly unfused terminals, and intended to go to future central heating equipment that may require an electrical connection, and required their own fuse protection at the central heating unit. As a result, these switch fuse units were in use from “way back”, and it became tradition for furnace installers to continue to use them to modern times, even if not otherwise expressly required. Fuses continue to provide superior protection over circuit breakers. It is people who install the wrong size fuse or bypass the fuse protection who create the hazards blamed on fuses.