New Basement Toilet Bubbles Up When Upstairs Toilet Is Flushed

[blue hill]

Hi folks,
New member here.
We recently renovated our basement and added a bathroom. Now I notice that whenever the upstairs toilet is flushed, the basement toilet bubbles up, quite violently if you flush the upstairs toilet several times in a row. The water in the basement shower drain P trap moves, but not significantly. The toilets are not connected to a common vent, the basement toilet stack is vented with an Oatey Sure Vent in the ceiling space.
Both toilets are American Standard Champions but purchased several years apart.
Thanks in advance for your help.

 

[Cacher_Chick]

Without seeing the drainage piping, one can only assume. When you flush the upstairs toilet, a large slug of air and water comes down under the pressure of gravity and is looking for the path of least resistance. The slug of air is blocked by the AAV, and at least some of it is making it's way up through the toilet bowl. If the toilet had a real atmospheric vent, the problem would likely not exist, because a real vent allows flow in both directions, whereas an AAV will only let air in, but not out.

 

[Sponsor]

 

[blue hill]

The upstairs toilet has a r

Without seeing the drainage piping, one can only assume. When you flush the upstairs toilet, a large slug of air and water comes down under the pressure of gravity and is looking for the path of least resistance. The slug of air is blocked by the AAV, and at least some of it is making it's way up through the toilet bowl. If the toilet had a real atmospheric vent, the problem would likely not exist, because a real vent allows flow in both directions, whereas an AAV will only let air in, but not out.

The upstairs toilet has a regular atmospheric vent, but you are saying that it's not sufficient? If that's the case there's not likely an easy fix.

 

[blue hill]

So I temporarily removed the AAV and had the upstairs toilet flushed 3 times in a row with basically no effect on the downstairs toilet, so it looks like you're right cacher_chick. The plumber who did the rough in (several years ago) said that an AAV would do the job. What puzzles me is that there was a toilet in the basement previously, almost directly under the upstairs one and there was never an issue. The new one is perhaps 10 or 12 feet away horizontally from the upstairs one. Can someone please advise me on what the minimum requirements for venting with an atmospheric vent will be? I can get up from the basement to the attic and from there to the roof, by running alongside the chimney. Can I get away with a 2" pipe? Is there a maximum run length?

 

[Kreemoweet]

2" is the usual size for a toilet vent. Maximum lengths are usually specified in your local plumbing code.
An older Uniform Plumbing Code version specifies 120 ft max, with 1/3 of that allowed to be horizontal.
Increasing the vent size (to 2 1/2 inch in this case) allows "unlimited" length.

As you found out, air admittance valves should NEVER be used where back pressure is a possibility.

 

[hj]

quote; Increasing the vent size (to 2 1/2 inch in this case) allows "unlimited" length.

Good luck finding ANY 2 1/2" DWV fittings OR pipe.

 

[blue hill]

quote; Increasing the vent size (to 2 1/2 inch in this case) allows "unlimited" length.

Good luck finding ANY 2 1/2" DWV fittings OR pipe.

Thanks for your help. It looks like I'll be fine with 2".
Any insight as to why I had no problem for the first month and now it seems to be getting worse by the day? There was also a basement toilet for many years, almost directly underneath the upstairs toilet with no issues. The new one is further away horizontally by 10 or 12 feet. ???

 

[Reach4]

I was wondering if you have a partial drain blockage. I picture there being an air path in the big drain lines downstream of the toilets, until you back up the water enough to block that path.

 

[blue hill]

I was wondering if you have a partial drain blockage. I picture there being an air path in the big drain lines downstream of the toilets, until you back up the water enough to block that path.

I wondered that as well and it kind of speaks to why the problem seems to be getting progressively worse, but I had my sewer line camera inspected all the way out to the street not very long ago and it was clear. Not saying that I don't have a problem out there now.

 

[Reach4]

I wondered that as well and it kind of speaks to why the problem seems to be getting progressively worse, but I had my sewer line camera inspected all the way out to the street not very long ago and it was clear. Not saying that I don't have a problem out there now.

Starting where?

A backwater valve/ sewer check valve would be a good place for a clog. Did you inspect upstream of that?

Incidentally, the ones with a swinging flapper are pretty useless.

Maybe put 4 gpm or more with a garden hose down the basement toilet bowl for 20 minutes or so. Does the water in the bowl rise past the first 2 minutes?

 

[blue hill]

Starting where?

A backwater valve/ sewer check valve would be a good place for a clog. Did you inspect upstream of that?

Incidentally, the ones with a swinging flapper are pretty useless.

Maybe put 4 gpm or more with a garden hose down the basement toilet bowl for 20 minutes or so. Does the water in the bowl rise past the first 2 minutes?

I checked the backwater valve and it was clear there. Had the upstairs toilet flushed while I was looking at it and it went fine. I will take your suggestion and push as much water as I can down the drain and see if my trouble starts in the line after it leaves the house.

 

[blue hill]

I checked the backwater valve and it was clear there. Had the upstairs toilet flushed while I was looking at it and it went fine. I will take your suggestion and push as much water as I can down the drain and see if my trouble starts in the line after it leaves the house.

Well I ran a garden hose down the drain for close to an hour without incident. Then I had the upstairs toilet flushed a bunch of times with the basement AAV removed. No incident. Then I replaced the AAV and had the upstairs toilet flushed another half dozen times. No incident. Prior to running the garden hose, it had gotten to the point where I couldn't flush the upstairs toilet even once without all the air bubbles splashing water out of the one downstairs. So now I don't have a problem, but what caused the trouble in the first place? I hesitate to suggest that the garden hose fixed it. The water level never rose from start to finish. I watched the main through the clean out port on my back water valve the entire time.

 

[Reach4]

I watched the main through the clean out port on my back water valve the entire time.

Could that open port have served as an air exit during the test?

Let's hope a workman put a yogurt container or drink can into the drain, and your water flow finally washed that through to the city sewer.

 

[blue hill]

Could that open port have served as an air exit during the test?

Let's hope a workman put a yogurt container or drink can into the drain, and your water flow finally washed that through to the city sewer.

No, I closed it before I started the flushing.
Fingers crossed, maybe my troubles are over for a while. I'll probably run an atmospheric vent line as soon as I get some time