Most residential building drainage systems are vented directly through the building’s roof. Here’s an example. See the white pipe on the roof at left. The plastic pipe is “flashed” or sealed at the roof penetration to prevent rainwater from leaking into the building.
During maintenance you’ll want to be assured the top of the vent is kept open. A blocked vent can be caused by leaves, dead animals and ice dams (in very cold climates). Another “not-so-common” way the vent can become clogged is when a horizontal section of the venting system itself (inside the attic) is sloped the wrong way. When that happens it can fill with rain water or condensation. Experienced home inspectors should be on the look out for these conditions.
Air Admittance Valves (AAV)
Sometimes we can’t (or don’t want to) vent through the roof line.
In these rare cases plumbers use an Air Admittance Valve. Air Admittance Valves (aka Studor vents) are “negative-pressure-activated” one-way mechanical valves. Used most commonly at an island sink or vanity the vents are also located in the attic to prevent the roof penetrations (as seen above) on the front roof line elevation. Think about it – when have you ever seen a plumbing vent on the front roof line?
How It Operates
If we didn’t have either venting method you would have very noisy sink and toilet drains. A discharge of wastewater down your sink drain causes a valve on the AAV to open. When it opens air is allowed to enter the plumbing system. Watch this video to help understand how a plumbing vent works.
An AAV can significantly reduce the amount of venting materials needed in a plumbing system. That’s money in the pocket for the plumber. They also allow greater flexibility in the layout of plumbing fixtures, and reduce long-term roof maintenance. If we don’t have a hole in the roof, it’s easier to maintain the roof. AAVs have been effectively used in Europe for more than two decades. However, there are a few limitations.
Some state and local building departments prohibit AAVs. Check with your local AHJ – Authority Having Jurisdiction for more info.
AAV’s are certified to reliably. However anything mechanical can and will fail. Some manufacturers claim they’re good for 500,000 uses (approximately 30 years of use).
US manufacturers offer warranties that range from 1 year to “lifetime”. You’ll want to seek out the warranty info. Most plumbers won’t have it.
Note: An AAV should should not be tampered with or spray painted. I have had plumbers tell us that sewer flies and bugs have been seen at failed air admittance valves. So if you detect a sewer odor under and around your sink or in the attic, the AAV could have let you down.
NAHB Research Center video showing the basic principals of the AAV