Things You'll Need
Galvanized sheet-metal ductwork
High-temperature silicone caulk
A furnace flue directs the poisonous products of combustion out of your home. It typically runs from the furnace to a chimney. The gases it exhausts are hot and moisture-laden. Since a furnace flue is made from galvanized ductwork, it corrodes over time once the galvanized coating breaks down. When a furnace flue corrodes, poisonous gases invade your home, so it's best to repair a furnace flue by replacing damaged piping before it corrodes completely.
Turn your furnace off. This prevents flue gases from entering your home during the repair.
Remove the damaged flue pipe out of the piping system by pulling each connection apart.
Measure the length of the damaged flue pipe and cut a replacement at that measurement. Crimp one end with your pipe crimper.
Slide the new piece of flue pipe into place and push the connections back together.
Install three to four sheet metal screws at each connection and caulk with high temperature silicone caulk.