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Speedy Jim[_2_] Speedy Jim[_2_] is offline
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Default Bad studor vent or bad venting altogether?

the_tool_man wrote:

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

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.


Yes, the slug of water when flushing will create a pressure
in the drain. How much pressure will be influenced by the
amount of back pressure or resistance in the house drain/sewer.
If the house drain is beginning to get blocked (tree roots, etc.),
that may produce more pressure.

The vent for the washing machine drain *should* relieve that
pressure, but there may be more we don't know.

The Studor vents won't relieve any back pressure at all;
they are specifically designed to close on pressure.

Adding a vent to the new bath may not solve this since
the pressure is developed further down the line, ahead of
the slug of water.

It may take someone on site to look over the layout
to be able to suggest the best fix.