An air admittance fitting supposedly lets air in the system NOT OUT.
They depend on a piece of crap rubber for integrity.
If an insect or some debris gets into the junk there goes your safety
Certain vapors may destroy the flapper from working.
Remember "sewer gases" contain any of the following
Sewer gas is a complex mixture of toxic and non-toxic gases that can be present at varying levels depending upon the source. It is formed during the decay of house hold and industrial waste. Highly toxic components of sewer gas include hydrogen sulfide and ammonia
Hydrogen sulfide poisoning. Exposure to low levels of hydrogen sulfide causes irritation of the eyes and respiratory tract. Other symptoms include nervousness, dizziness, nausea, headache, and drowsiness. This gas smells like rotten eggs, even at extremely low concentrations. Exposure to high concentrations can interfere with the sense of smell, making this warning signal unreliable. At extremely high levels, hydrogen sulfide can cause immediate loss of consciousness and death.
Explosion and fire. Methane and hydrogen sulfide are flammable and highly explosive.
Remember some codes do allow these devices and many code officials are bought and paid for by local politicians who sold thee common decency for a political contribution IMHO
AAV's were primarily approved to keep costs down not a word about safety was mentioned.
Even the powers who did allow garbage to be placed into their Mickey mouse codes did have some sense to put some restrictions to cover their butts.
I personally would try the following.
Have one 4" VTR to prevent hoarfrost and possible either install a return bend (180 deg fitting) or a vandal proof cap on top to prevent leaves and animals from blocking the VT.
Then connect the other vents to this main vent stack or if need be install 2- 4" VTR
The VT if not located near an arch or window or over hang and is used for weather protection can be as low as 24" ( CHECK LOCAL CODE). DO NOT use plastic that may come into contact with direct sun light.
To have a safe plumbing system you need air circulation Unless your just planning on fixing this home and selling it and run off to the hills and forget about the folks living in this structure with a very inferior design .
Not all plumbing should be geared upon bottom line practices.
Heath issues should not take a back seat.
The hospitals are full of good intension DIY folks
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