How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manufactured and modular homes

What is an auto vent, air admittance valve, or check vent?

Thursday, July 26, 2018

This device goes by a number of different names, including Studor® vent, in-line vent and mechanical vent. It is designed to open and admit air into a sink drain line when there is negative pressure (suction) in the pipe, which allows the drain to flow freely without gurgling for air. The one-way vent only allows air into the system for drainage, then closes to keep sewer gas from escaping out into a home. When the location of a plumbing fixture makes it difficult to run a regular (passive) vent pipe through the roof this type of vent is an alternative solution.

    Although all the different names are often used interchangeably, there are actually two different technologies used to open the valve for air when needed and then close it afterwards. The older system is spring-loaded and allowed in mobile/manufactured homes under the jurisdiction of HUD, but not site-built homes. It is shown below and correctly called a mechanical vent, auto vent, in-line vent, or check vent.   The more recent design utilizes a rubber-like membrane that flexes to allow air into the drain system. It is an air admittance valve, sometimes referred to by the acronym AAV. The Studor Corporation is the largest manufacturer of AAVs, so they are often referred to as Studor® vents or Studor® valves. An air admittance valve made by Studor is shown at the top of the page, and an example of an AAV we saw under a kitchen sink at a recent home inspection is shown below. 

    Two requirements for correctly installing an AAV are that it must a minimum of 4” above the fixture drain and readily accessible. Although they are rated for a minimum of 500,000 open-and-close cycles, which is approximately 30 years of usage, they do fail eventually and, occasionally, prematurely; so it’s important to be able to get to the AAV to change it out. They have a screw fitting, so switching out a dead air admittance valve is an easy plumbing chore. 

    Air admittance valves can also be installed in an attic, but must be 6-inches or more above the insulation. Because an AAV requires negative pressure to open, it cannot be installed to vent a sump pump, which generates positive air pressure when operating. Also, a home’s plumbing drainage cannot rely entirely on AAVs; a minimum of one vent through the roof is required by the building code. 

    Also see our blog posts Can an air admittance valve (AAV or Studor) be installed horizontally? and How many plumbing vents through the roof are required by code?

• • • • • • • • • • • • • • • • • • • • • • • • • • • •

Here’s links to a collection of our other blog posts about PLUMBING:

How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

Why is there sand in the bottom of my toilet tank?  

What causes a gurgling sound when a bathtub or sink drains? 

Are drop-in toilet bowl cleaner tablets safe? 

What can I add to my septic tank to help it work better?

What are the code requirements for layout of drain piping under sinks? 

Why is there mold inside my toilet tank?  

What are the pros and cons of a wall-mounted toilet?

Which plumbing fixtures require water shut off valves in a home? 

How can I tell if a house is connected to a septic tank system or sewer?

Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

Why is a backflow preventer required on lawn sprinkler systems? 

How can I locate my septic tank? 

Is a hot water faucet handle required to be on the left? 

Can you live in a house while the plumbing is being replaced? 

Why is the European-style bottle trap not approved by the plumbing codes in the U.S.? 

Why can't PVC be used for water pipe inside a house? 

What are the common problems to look for when the plumbing has been replaced in a house? 

What's that powdery crust on the pipe connections at the water heater? 

How can I tell what type of plumbing pipe I have? 

What causes low water pressure in a house?

      Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.  

Water Heaters

Water Heater Age

"What Are The

Signs Of..."

Septic Tank Systems

Structure and Rooms

Plumbing Pipes

Termites, Wood Rot

& Pests



When It First

Became Code

"Should I Buy A..."

Park Model Homes


Shingle Roofs




Wind Mitigation

Roof and Attic

"Does A Home


Pool and Spa

"What Is The Difference Between..."




Concrete and

Concrete Block

Metal Roofs


Modular Homes

Rain Gutters

Mold, Lead & Other Contaminants


Older and

Historic Houses

Crawl Spaces

Mobile-Manufactured Homes

Building Permits

Life Expectancy

Clay Soil





Exterior Walls

& Structures


Common Problems

HUD-Code for

Mobile Homes

Garages and Carports

Flat (Low Slope) Roofs

Electrical Panels

Sprinkler Systems

Electrical Receptacle Outlets

4-Point Inspections

Hurricane Resistance

Frequently Asked Questions (FAQ)

Home Inspection

Heating and Air Conditioning

Building Codes

Fireplaces and Chimneys

Inspector Licensing

& Standards

Energy Efficiency

Washers and Dryers



Doors and Windows



Electrical Wiring

Click Below  

for Links

to Collections

of Blog Posts

by Subject

Plumbing Drains

and Traps


Smoke & CO Alarms

Aging in Place

Top 5 results given instantly.

Click on magnifying glass

for all search results.






Air Conditioner & Furnace Age/Size


Electrical Switches





Water Intrusion

Electrical - Old

and Obsolete


Foundation Certifications

Tiny Houses

About McGarry and Madsen



Buying a home in North/Central Florida? Check our price for a  team inspection by two FL-licensed contractors and inspectors. Over 8,500 inspections completed in 20+ years. In a hurry? We will get it done for you.

Moisture Problems

Crawl Spaces