what is up with this drain backup

[happydude45]

I have attached a diagram of the system. The basement sink drains very very slowly, and gurgles loudly when using the upper sink.

Actions taken so far with no success:
- snaked from basement sink to where the pipe goes into basement floor (at the red marker - i cut into it at this point and verified that there was no clog upstream to sink)
- snaked downward 20' to 25' below ground with my 3/8" snake, came up with a bit of hair, but can't be sure if there is a clog down there.

I am hoping the problem is a clog in the vent, otherwise it must be a clog underground??...

thanks for your help!

[Peter Griffin]

It's probably clogged underground and a 3/8 snake is next to useless.

[Terry]

You have run two sinks on the vertical without proper venting.
Anytime you drain the top sink, it will pull water from the lower sink trap.

[happydude45]

Yes I know the plumbing system is not done properly. It wasn't me, just a gift that came with the house!


I should add that when I run lots of water in the upstairs sink, water will actually come out the lower sink drains! In other words, the air is pushing out. Later it starts gurgling "the other way" as it tries to pull air in, as the water slowly drains back down.

does that provide more evidence for a clog underground?
any tips on how to add a vent to improve the system?

[Doherty Plumbing]

Yes I know the plumbing system is not done properly. It wasn't me, just a gift that came with the house!


I should add that when I run lots of water in the upstairs sink, water will actually come out the lower sink drains! In other words, the air is pushing out. Later it starts gurgling "the other way" as it tries to pull air in, as the water slowly drains back down.

does that provide more evidence for a clog underground?
any tips on how to add a vent to improve the system?

As long as the pipe shown coming off the top of the highest sinks connection to the stack is a vent then you're fixtures would be vented properly as far as I know about your code down there.

You would be simply wet venting the bottom sink.

To remedy the situation you should run a snake from the roof down through the venting and down to larger piping (like a 3 or 4" connection). Flush water through the sinks as well.

Be careful because if you DO knock out a bunch of gunk to the lowest point on the vertical soil-or-waste stack you could create a temporary blockage at the bottom and flood out the lowest sink ... until the snake gets there.

[Terry]

When an upstairs sink is above a downstairs sink, on the same line, then yes, it overflows the lower sink.

I run into that all the time with handman jobs.

Wet venting is only allowed with fixtures on the same floor, and they need to be sized for that.