What Type of Vent Pipe Do I Need in an Attic?

The DWV System

• The drain-waste-vent system has multiple vent pipes that attach at specific locations to the sewer or septic drain system of the house after framing is complete but before the contractors close in the walls with insulation and sheetrock. These pipes extend up through the walls into the attic and out the roof. Local building codes, the number of fixtures on the system, horizontal branch drains and total system vents drive the size of pipe required. This piping promotes the movement of sewer and gray water through the house and regulates the air pressure in the system. The piping used is a black acrylonitrile butadiene styrene, or ABS, pipe, specifically designated for use with drain-waste-vent systems.

Exhaust Fans

• Venting a home's exhaust fans involves using a flexible duct-type piping that vents to a unit attached to the roof. The distance the covered unit sits above the roof is code-related, but the piping that runs through the attic is typically a flexible pipe approximately 3 inches or larger in diameter, depending on the exhaust unit. Use insulated flex ducting when using this pipe in the attic. Alternatively, vent exhaust fans through gable walls if you don't want to go through the roof.

Hot Water Heater

• The hot water heater requires venting through the roof according to local building codes using a metal piping that exits the roof through a thimble, an insulated and protected unit. The best way to vent the hot water heater is straight up to allow heat expansion of the exhaust. Don't reduce the vent pipe to one smaller than the exhaust pipe location on the hot water heater; keep it the same size or larger for adequate venting. The higher the pipe is able to vent, the better for moving the exhaust away from the hot water heater.

Furnace

• A furnace may vent through the house's roof, depending on where it is placed. If it's located in a basement or in the garage, it vents through the side of a house. Most of the reason for venting a furnace is to allow condensation to escape the system through the vent piping. Venting the furnace through a side wall is an alternative to roof venting and is usually a more inexpensive option. Most furnace piping is similar to that used for DWV systems.

Fireplaces and Stoves

• Fireplaces, wood, pellet or oil stoves must be vented appropriately for installation. While some vent through a side wall through a wall thimble, the majority of these installations vent through the roof. Traditional methods called for using masonry chimneys but other options are available today. Double- and triple-wall insulated stainless steel pipe fitted into a chimney support box allow the venting of stoves and some fireplace inserts directly through the attic without the installation of a masonry chimney. Local building codes determine the height of the metal chimney pipe above the roof based on the roof's pitch and other factors.