When a cooling coil is to be added to an existing furnace and the ducts are made of tin, the first _____ feet of the tin duct must be removed and fabricated according of the specifications of the UMC.

When a cooling coil is to be added to an existing furnace and the ducts are made of tin, the first _____ feet of the tin duct must be removed and fabricated according of the specifications of the UMC.