How do you repair the plenum box that is separating from the furnice?

To repair a plenum box that is separating from the furnace, first turn off the power to the furnace for safety. Inspect the seams and areas of separation for any damage or rust; clean these areas thoroughly. Use high-temperature duct sealant or metal foil tape to reattach and seal the plenum box to the furnace securely. If necessary, reinforce the connection with screws or brackets to ensure it remains stable.