Hi all:

We added a bedroom and bathroom onto the house three years ago. The
existing waste plumbing is PVC, properly sloped, etc. All of the
vents for the existing bathrooms connected to a single vent pipe with
a studor vent on top in the attic. The kitchen sink also uses a
studor vent under the kitchen cabinet. The washing machine drain
(last one in the line before exiting the house) is vented through the
roof. When the addition was done, the new bathroom vent was tied into
the vents in the attic, nearby. The waste pipe from the new bathroom
was tied into the existing waste piping under the house such that it
flows past the point where the master bath toilet is connected. For a
while, things were fine, but I have noticed recently that sometimes,
when the new bathroom toilet is flushed, we get a few bubbles of air
forced through the trap in the master bath toilet. This happens
infrequently, but seems to be happening more often with the passage of
time.

It seems as though flushing the new toilet pushes a charge of air down
the main waste pipe and pressurizes it enough to force a small amount
past the master bath toilet trap. However, it doesn't happen every
time, or even very often. No other traps are effected. Also, it
didn't always do this. Is the studor vent the problem? Does the new
bathroom need to be vented separately? I'm not sure where to start.

FWIW, I don't have a problem with getting rid of the main studor vent,
as the new roof penetration would not be any more visible than the
existing one over the laundry room. But, because of the height and
slope of my roof, I would have to pay someone to do the work, so there
is some financial incentive to avoid removing the studor vent.

Any insight would be appreciated.

Regards,
John.