The short answer
An air admittance valve is a one-way vent: a plastic cap a few inches across, on a short pipe standing up from a sink drain, that lets air into the drain when water runs and closes to keep sewer gas in. Studor is the best known brand, so most people call it a Studor vent. In Northern Virginia homes we find them under kitchen islands, in basement bathrooms and on vanities added in a remodel, anywhere a pipe to the roof would have meant opening walls. When one fails, the sink next to it gurgles and drains slowly, or the cabinet smells like sewer.
How it works
Draining water pulls the pressure in the pipe down. Inside the valve, a light rubber seal lifts and lets room air in, which is the job a vent pipe through the roof does. When the water stops, the seal drops shut on its own weight, and any push of air from the sewer side only presses it tighter. Nothing to plug in, nothing to set.
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Where you’ll find one
- Under a kitchen island sink, which has no wall to run a vent up.
- Under a bathroom vanity on an inside wall, often added in a remodel.
- In a basement bathroom, sometimes behind a small access panel or a louvered grille.
- On a laundry standpipe.
- In some newer houses, in the attic, on top of a vent pipe that never went through the roof.
The signs it’s failing
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| How it fails | What you notice |
|---|---|
| Stuck shut | The sink gurgles and drains slowly. A toilet nearby may drop its water after the sink drains, and the dishwasher can gurgle while it pumps out. |
| Stuck open | A sewer smell in the cabinet, worst while water’s running or right after. |
| Cracked, or the seal is worn | A smell that comes and goes, sometimes a faint whistle. |
| Put in too low | Water gets into the valve and it stops working, or leaks. It has to sit higher than the drain it vents. |
The rubber seal is what wears. Dust, grease vapor from a disposal and age make it stick. A good valve lasts for years, but they don’t last forever, and a lot of the ones under Northern Virginia sinks went in with a remodel 15 or 20 years ago.
A quick test you can do
- Most valves screw on by hand. With nothing running, turn it counterclockwise and lift it off.
- Run the sink for a minute with the valve off.
- If it drains faster and the gurgle stops, the valve is stuck shut. Replace it.
- If it drains just as slowly, the valve isn’t the problem. It’s a clog in the sink line.
- Screw the old valve back on right away. An open pipe under a sink lets sewer gas into the kitchen.
For a clog, how to unclog a sink drain and what a P-trap is walk through the trap. If the smell was the problem and the valve looks fine, check that the trap still holds water: a sink that’s rarely used can dry out. Sewer smell in the house has the other causes.
Replacing one
Take the old one to the hardware store to match the size, then screw the new one on hand tight. Most are 1 1/2 or 2 inch. That’s a ten-minute job when the valve is easy to reach. The rules it has to follow under the Virginia plumbing code:
- It has to stay upright and sit at least 4 inches higher than the drain pipe it vents.
- It has to be somewhere air can reach it and a person can get to it. Never sealed in a wall without a grille.
- The house still needs at least one vent through the roof. A valve covers a sink or a bathroom group, not the whole house.
If yours is buried, glued on instead of threaded, or in a spot where water reaches it, that’s a short repipe, and a plumber should set it right.
When it isn’t the valve
If drains all over the house gurgle, or the toilet bubbles when the tub drains, one valve under one sink isn’t the cause. That’s the main line or the main vent. Why your toilet is gurgling sorts out which.
When you’d rather we find it
A plumber can usually tell you in one visit where the gurgle starts. A plumbing diagnosis visit is $49, or $250 after hours. Clearing a toilet, tub or branch line is drain cleaning, $200 to $800. The main line to the street runs $550 to $1,650, and a camera down the line is $89 added to the visit, so you see the cause on the screen. When grease or roots keep coming back, hydro jetting clears the whole pipe wall. A sink clogged at the trap is $99 with our sink clearing offer, up to the P-trap. You get the price before we start.
Already holding a quote to dig up your sewer line? Send it to us for a free second opinion. You don’t owe us anything for looking.
Call (703) 997-9757, day or night, book a plumber online, or see every current deal on our offers page. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What is an air admittance valve?
A one-way vent under a sink, or in a wall or attic, that lets air into the drain while water runs and closes to keep sewer gas out. It does a roof vent’s job without the pipe.
Is a Studor vent the same as an air admittance valve?
Yes. Studor is a brand, and most people use the name for any air admittance valve, the way people say Kleenex.
How do I know if my air admittance valve is bad?
A sink that gurgles and drains slowly, or a sewer smell in the cabinet. Unscrew it and run the sink: if it drains better with the valve off, the valve is stuck shut.
How long does an air admittance valve last?
Many last for years. The rubber seal is what wears, and dust, grease vapor and age make it stick. Replace it when it sticks or lets a smell out, not on a schedule.
Are air admittance valves allowed in Virginia?
Yes, with conditions. It has to be reachable, get air, sit at least 4 inches higher than the drain it vents, and the house still needs at least one vent through the roof.
Can an air admittance valve cause a sewer smell?
Yes, when it sticks open or the seal wears. The smell is strongest in the cabinet while water runs.
