Speedy Jim wrote:
Peter Langevin wrote:
I live in a 1 1/2 story home, one bathroom on the first floor. I had
a remodeling contractor out last week to give me an estimate to add a
second bathroom to the upstairs bedroom. His quote came back
significantly ($5000) higher than the other 2 I got, and when I asked
him what the difference was he said it was due to a "wet venting
issue." I mentioned that 2 other contractors hadn't mentioned it,
and he essentially told me that they didn't know what they were
doing. SNIP

The last guy is 100% correct.

Here is why:
When you flush the upstairs toilet, the slug of water rushing down
the stack will create a vacuum as it passes the lower toilet and
will try (might succeed) to siphon the water out of the bowl.

That's only if the water moves like a cylindrical hunk of Jello. I can't
imagine the amount of water it would take to fill the cross-section of a
3-4" pipe under the influence of gravity. I don't think you can do it absent
much higher pressure, like a fire hose. The water is going to run down the
sides of the pipe maybe 1/16" thick.

Now is some kind of SOLID plug (like a diaper or football) is moving down
the drain, maybe you can generate a vacuum. Water alone won't do it.


It's a big job in a residence to add another stack thru existing
walls.

Here's a compromise: Do the remodel without the new stack. If it doesn't
work, retrofit enough parts to make it work.