Question on plumbing vents

[Dan Friedrichs]

For some reason, I'm really struggling to get an intuitive understanding of drain venting.

What if a house had 3 stories, one toilet on each floor, all connected to a vertical 4" main stack (which properly vents through the roof). Does this installation require separate vent connections?

It seems like it should - if the 3rd floor toilet is flushed, doesn't the bolus of water potentially create high-pressure air below it, blowing out the trap of the 1st floor? I'm confused because I've seen diagrams omitting venting in this arrangement.

[Bill Dufour]

According to the local plumbing union when you build a 12 story hotel each toilet and sink needs a totally separate vent clear to the roof with no sharing. The city building inspector allowed them to build it with some kind of shared system, maybe some valves, at a much lower labor and material cost.
Bill D.

[Martin Siebert]

I think a good explanation of why you need vents to begin with might help. The vent is probably one of the most misunderstood "need" in the building industry. Very simply, the vent prevents the flow of water down the drain from pulling the water that is left in the trap down with it. It stops your traps from being "sucked dry". That's it. That's all it does.
Ideally, it is always best if each fixture has it's own vent and that vent clears the roof. This is a requirement in some areas, mine being one of them. Some places allow shared vents and this cuts down on the number of roof penetrations. Some areas allow "dry vents" or so-called pop vents, but they are always subject to failure and can allow sewer gas {the reason for the trap on all the drains} into the dwelling. There's considerable consideration that goes along with traps and vents, i.e., every roof penetration is a potential leak, etc., but the concept is pretty simple.

[Bruce Wrenn]

The lower two require a separate vent, but can share a vent stack. The upper doesn't provided the WC and shower/ tub enter at the same level (4 X 4 X4 tee with a side inlet.) Separate vent stack can connect to the main stack 6" above the flood rim of the highest fixture, usually a lavatory on top floor. Lavatory on top floor doesn't require a separate vent. P-1 plumbing licensee holder in a previous lifetime

[Dan Friedrichs]

Thanks, guys - this helps!

[Mike Cutler]

I think a good explanation of why you need vents to begin with might help. The vent is probably one of the most misunderstood "need" in the building industry. Very simply, the vent prevents the flow of water down the drain from pulling the water that is left in the trap down with it. It stops your traps from being "sucked dry". That's it. That's all it does.
Ideally, it is always best if each fixture has it's own vent and that vent clears the roof. This is a requirement in some areas, mine being one of them. Some places allow shared vents and this cuts down on the number of roof penetrations. Some areas allow "dry vents" or so-called pop vents, but they are always subject to failure and can allow sewer gas {the reason for the trap on all the drains} into the dwelling. There's considerable consideration that goes along with traps and vents, i.e., every roof penetration is a potential leak, etc., but the concept is pretty simple.

Martin
I'm having a little trouble with the sentence I put in bold red.
We have a single 4" vent stack in our house. There are multiple fixture vents that are connected to this main vent in the attic, but there is only one roof penetration.
I have seen more than one 4" vent on a residential dwelling but that was due to the amount of fixtures on a single vent and the resultant penalty. It took two vents for all the fixture vents in the house.
I haven't seen a vent through the roof for each fixture in a residence yet.
My downstairs toilet used to be wet vented, but that was changed.
We are allowed to use AAV''s, Air Admittance valves, or the "pop vents" I think you're referring to. They used to allow only one per residence, and then it changed to total vent area per residence, excluding toilets. Now I understand they're allowed on toilets also. It changes quickly.

Regardless though of any difference based on geographical area, a properly vented DWV system is pretty important.