We learned a long time ago that plumbing must be vented to allow smooth flow through the drain pipes. The way architecture is in our world today; people are putting water fixtures and sinks everywhere! The problem was the venting. Conventional wisdom had them running up the walls and through the roof. But some houses have island sinks or bathroom sinks that cannot be plumbed this way.
Along came the AAV or Air Admittance Valve. They
have been around since the mid 90's or so. They are designed to allow air to enter the plumbing drainage system and equalize pressure when water drains out of the system while maintaining the integrity of the water trap or seal that prevents sewer gasses from entering the home through fixture drains. When water runs into the drain, the negative pressure created by flow causes the AAV to open, allowing air to enter. When the flow stops, a spring closes the valve preventing sewer gasses from escaping. It's as simple as that. They should be installed at least 4" from the top of the branch drain they are being used on. I like to see them just a little bit higher. The manufacturers state that the distance should not exceed 6". Hmmm, ok... gimme 6! You still must have at least one vent pipe extending upwards through the roof though, but these are handy for those special installations.
I have been seeing the ones that have been installed for a number of years failing now. They are supposed to last around 30 years, per the manufacturers, but things fail when they fail! Mostly at the kitchen sink. When they fail in that location, the dishwasher discharging will force water out of the vent and into the room, either under the sink in the cabinet or all over the counter tops and onto the fllor. Had one not that long ago that was bad, the Real Estate Agent had his laptop on the counter, when the dishwasher discharged water, the water ran all over the counter top and under his laptop... panic in the streets!
The fix was evident, the owner had the AAV vent replaced and all was good.
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