Venting problem or main line clog

fishheadbarandgrill · Aug 7, 2013

I have a kitchen sink and a bar sink on the same line. This line connects with the main line which then goes to my septic tank. House is 7 years old and I've started to have gurgling between the kitchen and bar sink. Both have Studor mini-vents. I did a test tonight because my plumber thinks I have a venting problem and I think I have a clog in the common line from the kitchen/bar sinks.

If you remove one of the studor vents, the lines drain fine. Both vents are new and work properly. when I place a piece of saran wrap over one of the vent pipes (with the vent removed), fill the sink and drain the water the saran wrap balloons up. After the sink drains and I pull the saran wrap off, then place it over the opening, a very slight vacuum pulls down on the saran wrap.

I think this is telling me that I still have a clog in the line... because I'm getting air pushing up the vent when a large volume of water is draining and then as the water drains past the clog (small volume of water) it creates a small vacuum.

Thoughts appreciated.

BTW, the house has no roof penetrations and has all AAVs. I have no drain/gurgle problems on my other line which serves 2 bathrooms and washing machine.

johnjh2o · Aug 8, 2013

Without seeing how it's piped and from your description it sounds like your plumber is correct. You have a venting problem. Studor vents only allow air into the system but they don't allow positive pressure to escape as an open vent would.

fishheadbarandgrill · Aug 8, 2013

how do you explain the positive pressure generated at the vent? Isn't that generated by back pressure created by water backing up into the line??? As a result of a clog in the line?

johnjh2o · Aug 8, 2013

The back pressure is coming from the air in the line that is being replaced by the water entering drains. With no venting to the atmosphere the air has no where to go. That is where the backpressure is coming from. That is also what is causing your slow drain.