We recently added on to our house. The addition included a new bathroom. The vent in the bathroom terminates in the attic with an Air Admittance Valve. When we run the washing machine, I get gas bubbling back through the toilet. I think the solution is to just extend the vent through the roof and vent to the atmosphere. Does this sound right and am I missing anything? Thanks in advance.

It is strange that the vent didn't go through the roof in the first place because there will be foul odors/gases coming out of it, and you wouldn't want that building up in your attic where it could potentially find its way back into the house.

However, I think the vent being in the attic is not what is causing the bubbling. The attic is open air which should allow the plumbing to get air. I think there is either a vent blocked, or the new vent isn't adequately sized.

Can an air admittance valve be used to vent a toilet? - Quora. NO. The toilet uses the main stack of the building. If you blocked off the main stack any poisonous sewer gases could force back through the various water traps in the building including the toilet, so my contention is you must have a main stack.Apr 17, 2020

It may be stuck BUT I AGREE with you = replace with through the roof SOP vent!

How many fixtures are on the new vent and what size pipe is the vent ? Also are you on septic or public sewer ?

The vent size is 2 inches. We're on an aerobic system.

What is on the new vent ?

Toilet, shower and 1 sink ?

And do you have a newer style HE washing machine?

JP76 said:

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What is on the new vent ?

Toilet, shower and 1 sink ?

And do you have a newer style HE washing machine?

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The new vent includes a toilet, shower, and two bathroom sinks. Our washing machine is is a newer top load machine with agitator, but it allows us to select our water level. We usually use the full setting. The new addition ties into the main line coming from the original construction about 8 feet upstream of the aerobic system.

If you can reach the AAV in the attic, unscrew the top of it off and run the washing machine and see if it still bubbles in the toilet. If it does not, then figure out how to terminate the vent outside of the attic space. Also it does not have to terminate through a roof if you have an exterior wall that is easier to put it through

According to the Uniform Plumbing Code (UPC), a 2 inch vent can handle up to 24 fixture units (F.U). However, some say that a 2 inch vent can handle up to 40 F.U. For example, Studor's Mini-Vent can vent six F.U on a 2 inch line

In a residence, a lavatory sink= 1.0 F.U., bathtub/shower= 2.0 F.U. and a toilet 1.6 gallon or less= 3.0 F.U, greater than 1.6 gallon= 4.0 F.U.

You are at 7 or 8 fixture units and the max the air admittance valve can accommodate is 6