On Sep 11, 12:56*pm, DerbyDad03 wrote:

I've considered venting numerous times, just because "it's supposed to
be vented" but I've never had any reason to go through the trouble.

Up until yesterday I never saw any indication that I needed to alter
anything related to the plumbing. At this point, it's a bit hard to
imagine that after using this bathroom nearly every day since I
remodeled it 20+ years ago I've suddenly got a venting problem. I
guess it could happen, but something else must have changed also.

So here's the next question:

If it were indeed a venting issue, please explain what happened
yesterday. I used the toilet and shower in the morning so I know they
were OK. I was the last one to leave the house and the first one home.
In other words, no water was used from the time I left to the time I
noticed the low level in the toilet.

What could have happened during the day that - vent related - would
have caused the water in the toilet and trap to be sucked out?

Plumbing fixtures that are not vented at the connection
to the drain at each fixture with a T going up to the vent
and down to the drain where the fixture pipe enters can
have the water in the drain traps siphoned out by pressure
changes in the piping or by suction created by a large
volume of water draining past that fixture...

It sounds like your house uses the main stack as a
wet vent, rather than having individual vent pipes
which are connected to a vent stack which can then
be connected to the main stack above the level of the
highest fixture in the building...

It could have had something to do with the town doing
that sewer line cleaning, it could have been a washing
machine emptying at just the right time when a pressure
differential existed or could have been the wind...