Plumbing vents and thermal bridging
I’m currently working on the plumbing vents for my new house. I had thought the best plan would be to connect all the vent pipes in my unconditioned attic into one pipe to leave the house so that I would only have one penetration. Now I’m wondering if all that additional pipe it would take to make the connections would potentially allow more cold air into the house than if I had multiple penetration and much less pipe in the unconditioned attic space. Or am I just overthinking this and the cold air hitting the pipes in the attic won’t have any appreciable effect, considering the pipes are bringing in cold air from the outside Everytime a plumbing fixture is used anyway. Weather I have one penetration or multiple I plan on them coming out of the top of the wall as opposed to the roof.
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"A plumbing vent has no connection to the interior air of a home, so there shouldn't be any air leakage associated with a traditional plumbing vent, as long as the penetration is properly sealed. There will be some conduction losses through the PVC or ABS pipe, but these conduction losses aren't very significant."
-- Martin Holladay
I would prioritize two other concerns instead: Whether you want multiple roof penetrations, and which layout minimizes horizontal lengths of vent. To me the best vent layout is one that has straight vertical drops which can be easily cleared of blockages, while if possible minimizing the number the holes you have to punch in your roof.
I see Malcolm posted while I was typing, but here you are as well.
I think that the issue is much smaller than you are imagining. For septic tank and city sewer connections alike, the air on the vented side of all trap connections should be mostly static, even when a trap is passing water. The vent is meant to equalize the air pressure on both sides of the water trap in order to keep the trap water levels above the bend providing the separation between both sides.
It is possible to see a toilet bowl's water level fluctuate in response to the pressure drop on the vented side of the system created by extreme wind conditions. This pressure drop is related to the Bernoulli effect and is also how an old time spraying device worked. My grandmother used one before spray bottles existed. Given enough of a wind across the end of your vent stack, the water in at least one trap could be emptied before the system equalized. I suspect your house might be gone before that happened, but I have never experienced a tornado, so maybe a near miss by one would suffice.
In any case, I believe that the air exchange in the vent stack is limited to convective looping, which itself is a pretty small hit in the grand scheme. The earlier discussion about AAVs and stack exit points may be of some relevance to your situation. For my part, my main goal was to have no roof penetrations and to eliminate the web of piping associated with multiple baths, a laundry and kitchen. Some localities do not approve of AAVs, some people just do not like them, sometimes they are the best answer for and island sink.
Of more importance is making sure that you set your horizontal runs in the attic to drain back to the nearest vertical drop or tee into another well pitched run to the vertical. I have encountered sloppy work by "pros" and amateurs alike that created full blockage or extremely restricted venting thanks to condensate build up in a reverse pitched pipe run.
Thank you for the responses. This confirms I was indeed overthinking this. I think I am still going to go with multiple vent penetrations to allow for vent cleaning and save money and time on connecting pipe in the attic. Now that I have decided to vent through the wall instead of the roof I am less concerned about the additional penetration and am confident in my ability to seal the penetration.
"I am less concerned about the additional penetration and am confident in my ability to seal the penetration."
That's my feeling too. I recently had to clean out a vent pipe at our community hall which had clogged where it transitioned to a sloped horizontal run. Over the years it had slowly accumulated debris from nearby trees - a mixture of acorns and fir needles - which were enough to stop it working.