All gas furnaces must be vented, to allow exhaust gases from the combustion process to escape. Vents must be tall enough so burned gases exit above a roof line and cannot filter back into a house through a window or other opening. Even furnaces vented through walls are routed through vertical pipes that extend above the roof. Most furnaces are vented directly through roofs, with either a single pipe or a two-pipe system. A two-pipe system has a small pipe inside a larger one. Exhaust gases flow through the inner pipe while the outer pipe provides fresh air for combustion.
Locate the spot to install the vent pipe, using a tape measure to determine distance from a wall or other identifiable spot. Outline the pipe opening on the roof with chalk and cut a circular hole to the dimension of the pipe with a reciprocal saw; remove shingles to expose roof sheathing to make the cut. Put the roof opening directly above the exhaust opening on the furnace.
Run the vent pipe up through the roof and install a metal or PVC flashing collar over it. Match the vent pipe to the furnace maker's recommendations, either sheet metal or vinyl. Set the collar with the wide flange at the bottom on the surface of the roof. Extend the vent pipe at least 12 inches above the surface of the roof.
Put a cover on the vent. Use a pointed cap, which fits over the top of the vent pipe, with some styles. Install a T-end fitting, with an elbow at the top of the vent in the shape of a "T" with openings for vent gases on both sides. Use elbows to form an upside down "U" on the top of the pipe, with an open end down to allow gas to escape. Make the vent cover conform to the furnace maker's specifications.
Seal the edges of the flashing with roofing cement and put roofing paper over the flat sides. Replace shingles around the flashing, extending over the bottom flange. Cut shingles with a utility knife as needed to fit around the vent. Let the flashing extend over a shingle on the bottom side, but cover the top and sides with shingles.
Things You Will Need
- Tape measure
- Circular saw
- Vent pipe, sheet metal or PVC
- Vent flashing, metal or PVC
- Vent cap
- Roofing cement
- Roofing paper
- Utility knife