Thread: AAV vent in attic

Found this at yesterday's inspection. AAV vent terminates in attic. I have read that it's allowed in some cases. This vent is for a master bath addition, 3" drain/vent. I don't believe that it's connected to any other outside vents.

Originally Posted by Trent Tarter

Found this at yesterday's inspection. AAV vent terminates in attic. I have read that it's allowed in some cases. This vent is for a master bath addition, 3" drain/vent. I don't believe that it's connected to any other outside vents.

Depends on local codes. The IRC allows them on a waste stack that terminates in an attic, as long as the AAV is 6 inches above the insulation. There also must be a least one vent that extends to the outside. Some areas do not permit them even though they are approved per the IRC.

I would tell them to vent it through the roof to prevent sewer gas from making squirrels and rats go crazy and damaging the house.
Also its a source of moisture.

Originally Posted by Mark Reinmiller

There also must be a least one vent that extends to the outside.

That is also critical for the proper operation of the AAV.

Originally Posted by John Kogel

Also its a source of moisture.

How so?

Originally Posted by Jerry Peck

That is also critical for the proper operation of the AAV.

"Also it is a source of moisture"

How so?

Warm moist air rises here in my part of the world. Steam from a shower drain, stuff like that.
Sewer air is warmer than the attic air for some parts of the year. Don't ask me to prove it with a quote from a codebook.

Originally Posted by John Kogel

Warm moist air rises here in my part of the world. Steam from a shower drain, stuff like that.
Sewer air is warmer than the attic air for some parts of the year. Don't ask me to prove it with a quote from a codebook.

Warm air rises in my part of the world too, but ... ... but AAVs allow air to enter the air ADMITTANCE valve ... ... but not exit from the AAV.

If sewer gas or air is exiting the AAV, the AAV needs to be replaced as it is defective or has 'gone bad'.

Methane gas pressure will build up in the vent stack and then it will blow, as well as suck.

Originally Posted by John Kogel

Methane gas pressure will build up in the vent stack and then it will blow, as well as suck.

It is designed NOT to blow (release) gas, air, etc., not unless there is something wrong with the system.

Now, if you mean KaBoom! when you say blow because of a build up of gas, then there is something in the vent system which created a spark ... and that something should not be in the vent system.

AAVs are not intended for 'positive pressure' DWV systems, and a properly designed gravity draining DWV system will not have the problem you are describing.

There is nothing wrong with having an AAV in the attic when the DWV system is a gravity (not forced) drain system and at least one vent is open to outdoor air (that keeps the system at atmospheric pressure and prevent pressure build up. If you manage to blow an AAV off the stack it is on, they you have a positive pressure forced system and should not be using the AAV anyway.

There have been occurrences where utility companies 'blow out' the street mains with pressure and, indeed, that pressure can create quite a few problems with all the gravity drain systems connected to the street main which is being pressurized, and not just systems with AAVs. That practice has become recognized as 'not a good practice' because of the damages and messes it can make in the connected systems which were not designed for pressure. Most utilities have, I believe, stopped that practice.

Originally Posted by John Kogel

Methane gas pressure will build up in the vent stack and then it will blow, as well as suck.

This is one of reasons that the home must have a vent to the atmosphere outside the structures envelope if it has an AAV. If the AAV is allowing sewer gas to enter, it is faulty and needs to be replaced.