An air admittance valve (AAV) is a plumbing device used to reduce the number of external vents fitted to plumbing fixtures. A supplier got in touch, concerned that the use of these devices in a Passive House building would have implications for airtightness. I was able to reassure him no—and not other problems either—so long as the following provisos are followed.
Let’s start with the messy basics: plumbing fixtures need vents because pipes in your house carry poop away and are necessarily connected to sewers (or a septic tank or dispersal chamber, if you’re not connected to the mains). Poop breaks down and creates gases, potent enough to explode pipes if there was no way for those gases to escape. Those gases also stink, so you need them vented outside of your house. Poop and smells goes down when you flush and air is sucked in through the top of the vent and that all serves to equalise the pressure in the pipe. No smells and no gurgling pipes.
The traditional way of dealing with this involves lots of roof penetrations: it looks like the left hand illustration below. Now imagine how it looks in a luxe house or apartment with an ensuite on every one of its many bedrooms.
Graphic by Mechanical Hub (see Air Admittance Valves: Everything you should know)
AAVs are basically a check value and they are very handy in cases where connecting to an existing venting system is difficult or impractical. Think renovations, or apartment bathrooms that don’t have external walls. They simplify the plumbing system and that’s a good thing; they also save a reasonable amount of energy. You still need at least one vent stack!
Removing the need for an additional poorly insulated vent stack in a small home in New Zealand’s colder southern regions for example, could save 1-2 kWh/m2/year. For a Code-minimum house … well you wouldn’t notice that amount of heat loss. But that amount in a Passive House is huge. It might be 10% of the total heating demand! In apartments and multi-storey buildings the impact can even be larger. Plus every stack requires a hole cut in the roof. I don’t like cutting holes in roofs for any reason I can avoid.
Typical air admittance valve installation © BRANZ 2009
Some architects refuse to specify external vents like this because the pipes running up the side of the wall offend their aesthetic. So it’s quite common for the pipe to enter the house from the street and run up an internal wall and then out through the roof. So now as Passive House designers, we have to start paying attention. That pipe must be insulated and heating losses have to be calculated.
Now I’m an engineer, not an artist. So my advice is, run the vent stack up the exterior of the wall. If you can’t or won’t do that, use AAVs to enable the minimum number of internal vent stacks (hopefully just the one) and insulate it very well. It will mean specifying at least a 140mm plumbing wall, as the amount of insulation you need won’t fit inside a 90mm wall.
My colleague that started this discussion was concerned about reports of AAVs getting smelly. They won’t smell or impact IAQ so long as they were properly installed. That requires dead level installation so they reseal properly every time. Often they are gravity based rather than using a spring, which is why level matters. Any competent plumber can manage this.
There’s an upside to sewer smells: homeowners know something is wrong and they will call a plumber. This contrasts with cooking on a gas hob that is producing odourless PM2.5 particles that cause significant health issues—but the homeowner cannot know this without special sensors. (It’s crazy to me that gas appliances are still being specified and fitted in homes without sufficient ventilation.)
That aside. We know AAVs are being used in New Zealand Passive House projects without incident. I suggest specifying their installation in places that can be accessed by an occupant without needing a stepladder or special tools. This enables them to follow their nose and sniff deeply right at the AAV to know if that’s the source of the issue—as opposed to, for instance, someone spilling milk on the carpet and not telling their parents.
Level has more detail on AAVs.