Specialty Item Air Vents

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In a drain, waste and vent (DWV) system, air must be allowed to enter the drain pipe to prevent a vacuum. Water draining in a pipe acts like a plunger in a syringe drawing in medicine as it is drawn back.  Plumbing vents consist of pipes leading from fixtures to the outdoors, usually through the roof. (Roof penetrations are equipped with a flashing to prevent water entering the buildings.) As water drains through drainpipe, air is drawn in through the vent pipe from above the roof, allowing the drain to flow unimpeded.

Under many older building codes, a vent stack (the pipe leading to the main roof vent), is required within a five foot radius of the draining fixture (sink, toilet, shower stall, etc.). To allow only one vent stack, and thus one roof penetration, sub-vents may be tied together and exit a common vent stack.  The vent system also serves the purpose of venting any sewer gas that forms in the drain pipe through the vent pipe above the roof.

A second way of venting a drainage system uses air admittance valves (AAVs), which allow air from within the building to enter the drain pipe. Since AAVs admit building air, no vent pipes are required to penetrate the roof, thus eliminating a potential leak source and reducing building cavity space and routing for vent pipes.

Air admittance valves (AAVs) are pressure-activated, one-way mechanical vents that function similar to a check valve.  A vacuum caused by the discharge of wastewater causes the AAV to open, which releases the vacuum by allowing air to enter plumbing vent pipe for proper drainage. The valve remains closed when draining does not occur, preventing the escape of sewer gas and maintaining the trap seal. Using AAVs can significantly reduce the amount of venting materials needed in a plumbing system, increase plumbing labor efficiency, allow greater flexibility in the layout of plumbing fixtures, and reduce long-term roof maintenance problems associated with conventional vent stack roofing penetrations.

While some state and local building departments prohibit AAVs, the International Residential and International Plumbing Codes allow it to be used in place of a vent-through-the-roof. AAV's are certified to reliably open and close a minimum of 500,000 times, (approximately 30 years of use) with no emanation of sewer gas.  Some manufacturers claim their units are tested for up to 1.5 million cycles, or at least 80 years of use. Air Admittance Valves have been effectively used in Europe for more than two decades. U.S. manufacturers offer warranties that range from 20 years to lifetime.

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