Bubbling toilet, is it caused by "Hydraulic Jump"?

Help Support Plumbing Forums:

SilentR

New Member
Joined
Sep 18, 2021
Messages
2
Reaction score
0
Location
Arizona
Hello, folks, I have a bubbling toilet problem and a few ideas, but want to run them by you experts before I tackle any modifications. This is a long term thing, it’s been going on for years, the good news is I don’t have any back ups or drainage problems and it’s not getting any worse. What I have is 2 toilets, one on the second floor, and one on the main, both are in line above each other on the same 3” stack. They drain into a 4” line through the basement wall to a septic tank about 8 feet away. I’ve attached a picture showing the piping directly under the main floor toilet and where it goes out the basement wall. Elbow 1 is the wet vent stack from the upper floor toilet, straight up through the roof , about 20’ long. Elbow 2 is the main floor toilet, pipe 3 is from the laundry room, and 4 has a trap on the other side of the wall from the kitchen sink and dishwasher.

Here’s the symptoms: When the upper floor toilet (1) is flushed, the lower toilet (2) burps up a big bubble and the kitchen sink drain (4) gurgles loudly from the trap water being forced towards the sink. If the lower toilet is flushed, there’s no noise from the kitchen sink so the air pressure build up is way less or even non existent. If I take the cap off the cleanout on the laundry drain and flush the upper toilet, there’s a strong gush of air out of the hole and no bubble in the lower toilet. So my original thoughts were either a plugged top on the inlet tee inside the septic tank, preventing access to the air space at the top of the tank. Or a sag in the 4” line from the wall to the septic tank where water would form a seal between the same cushioning air space. But as I looked at this over the last few months, I’m wondering if maybe the high velocity 1.6 gallon slug of water from the upper toilet forms a seal after it hits elbow 1, preventing air from venting up the stack and forcing it out the toilet and sink. I’ve read about something called a “Hydraulic Jump Zone” which seems to perfectly describe this problem, and is apparently part of the plumbing code. One other thing to note, the bubbling only happens when the toilet is flushed, there's no problems when a full bathtub is drained through the same second floor line.

Have any of you pro’s dealt with something similar? If you think there’s merit to the jump zone idea, it’s a reasonably easy fix, other than a bunch of drywall work. I’ll take out everything starting at elbow #1 and extend the wet vent straight down to a new elbow pointing directly at the 4” septic line. Toilet 2 and both the dishwasher and laundry will drain into this short vertical stack extension, so all 3 drains will have a direct connection to the vent before any elbows that could be affected by a hydraulic jump. I appreciate any advice you guys can give me!
 

Attachments

Twowaxhack

Professional
Professional
Joined
Feb 9, 2021
Messages
4,340
Reaction score
1,907
Location
United states
#3 is not vented, #4 probably isn’t either.

Add vents to the open air on drains #3 and #4.

The vent needs to be placed at your sticky note on #3. Can’t tell on #4 because I can’t see the rest of the piping.
 

SilentR

New Member
Joined
Sep 18, 2021
Messages
2
Reaction score
0
Location
Arizona
Thanks for the reply, Twowaxhack. You're right, no vents on 3 or 4, although 4 has an AAV at the sink. 3 isn't giving me any problems, it's sealed by the washing machine drain valve. I can't add a vent to either without tearing out a lot of sheetrock so I'm thinking I'll try my idea of running both of those drains plus the lower toilet into the vertical wet vent stack. (2 Sketches Attached) I'm just hoping the 1.6 gallon slug of water from the upper toilet doesn't fill the vertical part of the vent enough to block the air in the pipe from escaping up the vent, which would put me back to square one.
 

Attachments

Top