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Air admittance valves

Air admittance valves are mechanical devices that take the place of plumbing vents. They're inexpensive, they work, they can reduce the number of plumbing vent penetrations at the roof, they won't get blocked by frost, they're listed and approved by several listing agencies, and they cost way less to install than a plumbing vent.

Oh, and they're not legal here in Minnesota.

Minnesota Plumbing Code requirements

Our old home-grown plumbing code used to specifically prohibit the use of air admittance valves (AAVs), but the new plumbing code that was adopted in 2016 is silent on the matter, which means the same thing. AAVs aren't allowed. If my memory serves me correctly, there was some confusion about this back in 2004 (?), and there was a loophole in the plumbing code which may have made them legal for a month or two, but that didn't last long. If you're reading this and you know the exact details, please leave a comment.

At any rate, they're not legal here in Minnesota.

Air admittance valves replace plumbing vents

As mentioned in last week's blog post, plumbing vents allow air into drains to replace water. When water leaves through a drain, it has to be replaced by air. To prevent air from pulling through a trap and pushing water out, a plumbing vent provides an easier, alternative path for air to enter the system.

An AAV can take the place of a traditional plumbing vent at individual fixtures because it allows air into the drain without allowing sewer gas to enter the building. The diagram below, provided by Oatey, shows how air admittance valves can take the place of vents.

air admittance valves

Note: these devices are frequently called Studor vents, named after the original inventors of these devices, Sture and Doris. Studor vents completely dominated the market for AAVs for a long time, but today there are several competitors to choose from. AAVs should not be confused with check vents, which are cheap devices that aren't approved for use outside of manufactured homes. More on the history of Studor vents here:

To help protect plumbing trap seals, plumbing vents also allow pressurized sewer gas to vent to the outdoors. Because of this, every home still needs to have at least one full-sized plumbing vent installed, whether air admittance valves are installed or not.

Home inspection stance

I occasionally run into these during home inspections, and I don't have an issue with them. If I find an air admittance valve, I let my client know that it's not allowed in Minnesota. This will probably create an additional expense if someone has licensed plumbing work completed in their home, and that's the main reason that I mention it.

The concern over air admittance valves is that they rely on a mechanical seal that may eventually fail. Nevertheless, some manufacturers, such as Sioux Chief, offer a lifetime warranty on their air admittance valves. Studor offers a 10-year warranty and claims a 500,000 cycle lifetime.

Another concern with an air admittance valve is that it might leak if there is a sewage backup. That's a valid concern, so I tried testing a bunch of AAVs to see if I could get them to leak. I set up a sink in my garage and blocked off the drain, and let the water go. No leaks. I even turned the vents upside down, and nothing leaked out.

Air admittance valve testing

While doing this testing, I also did some pressure testing on AAVs, and it didn't all turn out as planned. I have some crazy-stupid bloopers at the end of this video that you won't want to miss:

Author: Reuben SaltzmanStructure Tech Home Inspections

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Plumbing Vents, Why Houses Need Them (forget the water bottle analogy)

Plumbing vents protect plumbing traps. They don't make fixtures drain faster; in fact, they do the opposite. When it comes to first time home buyers, one of the least understood components of a home seems to be plumbing vents. They're those pipes sticking up out of the roof that run through the attic and through the rest of the house. All residential plumbing fixtures need to be protected by a plumbing vent.  Vents are frequently connected together inside the attic, which allows for fewer penetrations in the roof.

Plumbing vents

Plumbing vents prevent traps from being siphoned.

Let me repeat that: plumbing vents prevent traps from being siphoned. They also prevent back-pressure on traps, but today the focus is on siphoning. You may have heard that plumbing fixtures will drain faster when they're vented properly, but it's not true. The common, improper analogy is to talk about dumping a soda bottle upside down. You watch the water glug out while air replaces it, and this makes it drain slowly.  Once you put a hole in the top, water drains out very quickly because air can replace the water as it drains.

This analogy doesn't hold water because the top side of every plumbing fixture is wide open. The top of a toilet is open. The top of a sink is open. The top of a bathtub is open. If you wanted to re-create the soda bottle analogy, you would need to block off the top of the plumbing fixture and then try to drain the water out. I can't think of any instance where this could possibly happen.

As I mentioned in last week's blog post, every plumbing fixture has a trap, which prevents sewer gas from entering the building. When a lot of water drains through a plumbing fixture, it can be enough water to create a siphon effect, which has the potential to pull water right out of the plumbing trap. In my blog about S-traps, I included a quick video clip of an unvented drain having water siphoned out of it, leaving the trap with far less water than it should have had.

To help demonstrate this, I made a video showing the difference between a vented drain and an unvented drain. I used clear tubing for simplicity, but the physics are the same. You'll notice that the unvented fixture actually drained about 2 seconds faster than the vented drain. This is because the water that had left the fixture was helping to pull water out. With a vented fixture, there's no pull. The vent allows water to pull air instead. Check it out:

If you hear a gurgling noise after water has drained out of a fixture, what you're hearing is air getting siphoned through the trap. This happens when there is no vent present, the vent is obstructed, or the vent is improperly installed. In next week's blog post, I'll discuss air admittance valves; devices that are designed to take the place of individual fixture vents without running pipes through the roof.

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Author: Reuben SaltzmanStructure Tech Home Inspections

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