Help diagnosing gurgling toilet

[AMandM]

The toilet in our basement bathroom often doesn't flush well, instead filling with water and requiring much plunging.  We were planning to snake it, but then this morning something different happened that made us think the problem might not be a drain blockage. The toilet bowl was full of water after an unsuccessful flush, when my husband turned on the shower.  A minute later, all of a sudden the toilet drained with a loud gurgle. This made us realize that when it does flush properly, there's often a gurgle, and that suggested to us a vent problem rather than a drain problem.

Is there a method for diagnosing this?  Or should we just try snaking as if there were a drain blockage, and if that doesn't solve the problem call a plumber to clear the vent stack?  (Neither of us is prepared to get up on the roof.)

If it makes a difference, this bathroom was part of a DIY basement finishing project by a former owner, who did not have high standards for workmanship and didn't know/care about building codes. So for all I know the bathroom drains aren't properly vented in the first place.  Do I have to tear out the drywall to find out?

[lthenderson]

If it makes a difference, this bathroom was part of a DIY basement finishing project by a former owner, who did not have high standards for workmanship and didn't know/care about building codes. So for all I know the bathroom drains aren't properly vented in the first place.  Do I have to tear out the drywall to find out?

It sounded like a vent problem before I read the above sentences and now I'm sure it is a vent problem. Venting is one of the least understood and most often done incorrectly in DIY projects. Unfortunately there really isn't a good way to diagnose the problem because most venting is hidden in the wall structure. I usually start on top and work down. Start on the roof and visually inspect the pipe with a light looking for blockages. Next I go into the attic and see where the pipe comes up from the various places with plumbing (bathrooms, kitchens, etc.) and see how they connect with the pipe going up through the roof. After all visual pipe is inspected, then I start tearing into drywall.

[paddedhat]

As Ithenderson correctly notes, you have  venting problem, which is a particularly odd issue, since toilets are not really that sensitive to a lack of venting, since they tend to flow waste in the bottom of the 3-4" pipe, while allowing air to travel over the flowing waste in the upper 3/4s of the pipe. Basically according to practical reality, and to some extent, the ICC code, you really don't need a vent on a modern toilet, at least not a dedicated vent line for the toilet itself. Therefore, my guess is that you have a clog, or restriction in the drain line under the toilet. I would call around and get a few prices on having a pro run a camera down the drain line. I think you might be a little disappointed in what you discover, but I'm pretty sure that you will find the problem.

[MightyAl]

Is there a method for diagnosing this?  Or should we just try snaking as if there were a drain blockage, and if that doesn't solve the problem call a plumber to clear the vent stack?  (Neither of us is prepared to get up on the roof.)

If it makes a difference, this bathroom was part of a DIY basement finishing project by a former owner, who did not have high standards for workmanship and didn't know/care about building codes. So for all I know the bathroom drains aren't properly vented in the first place.  Do I have to tear out the drywall to find out?

I would say that the basement bathroom isn't even vented.  You will need to tear out drywall to find out.  If it has a drop ceiling then you can check above the wall.

The reason I don't think it is vented at all is that you are getting a hydro lock at the toilet that is resolved when the shower water starts draining.  It cost me $1600 to remedy an issue like this when I sold one of my houses. 

[paddedhat]

If you have zero venting on a system, the toilet will not function. The flip side of this is that a toilet does not need to be individually vented, for reasons I mentioned previously. Building on what MightAl wrote, there is a way to determine if you have a totally blocked, or non-existent vent system, as your description of events seems to indicate that the shower drain acted as a vent. Get a toilet plunger and violently plunge the shower drain. First look in the drain with a flashlight, and determine if you can see the standing water in the U-bend of the trap. Once you plunge, hopefully you blew all of this water past the trap and created a temporary vent. You might be able to get your nose down there and notice a bit of a sewer gas smell?  Now you have a temporary vent system, since the drainage piping has access to outside air. Flush the toilet. Is the toilet working well? if so, you have a blocked, or non-existent venting system. If not, you may have a blockage in the drain below the toilet. Once this experiment is done, run some water in the shower to reestablish the drain seal. If you have a three piece bath with a sink, but no venting, you might be able to get it to perform adequately with a Studor valve, AKA an Air Admittance Valve, or AAV, hidden under the sink. This may not be 100% code approved, but it could make a huge difference and cost $30-40 in parts, not thousands in re-plumbing. Good luck.

[AMandM]

You all know so much!  Thank you!  Off to do some experiments.

[AMandM]

I performed paddedhat's experiment, and the toilet flushed just fine when the shower trap was empty.  The I discovered I could see behind the drywall by removing the inset medicine cabinet above the sink.  Sure enough, there was no venting, just drain pipes from the toilet and sink and the washing machine. I couldn't see the shower drain, though obviously it must be connected.

Next stop Lowe's for an air admittance valve.

Does it matter where exactly I install the AAV along the shared drain line? I don't need one for each of the fixtures, do I?

[paddedhat]

I performed paddedhat's experiment, and the toilet flushed just fine when the shower trap was empty.  The I discovered I could see behind the drywall by removing the inset medicine cabinet above the sink.  Sure enough, there was no venting, just drain pipes from the toilet and sink and the washing machine. I couldn't see the shower drain, though obviously it must be connected.

Next stop Lowe's for an air admittance valve.

Does it matter where exactly I install the AAV along the shared drain line? I don't need one for each of the fixtures, do I?

Glad that my idea worked. Truth be told, I have never done it before, and just dreamed it up while reviewing the symptoms of your predicament. As for installing an AAV, here is an excellent tutorial for adding one under a sink.

 http://www.bhg.com/home-improvement/plumbing/how-to-install-air-admittance-valve/

The other point that comes to mind is that a washer drain without a vent is also nothing but trouble. Given the velocity and volume of discharge from the machine, it can easily blow a trap dry, if the drain is not configured and vented correctly. If you can see the vertical piping for the washer, and access it, I would figure out a way to add a vent line and AAV to it.  The next issue is the difference between an AAV and a Studor valve. A cheap, generic AAV is a $4 part that will probably work fine. A Studor valve is also an AAV, but a much high quality, more durable product. An 1-1/2" Studor valve runs a bit over $20. One of the inspection agencies I used on new construction would no longer approve the cheapy generic AAVs, since they claimed that they were prone to failing. Since you don't think you have any venting, there is a good possibility that both the washer and the sink ended up with "S" traps.

http://www.diy.com/departments/floplast-waste-s-trap-dia32mm/152679_BQ.prd

These are now illegal, as they tend to use debris (especially long hair) inside the second bend as an odd "wick" which can drain the trap and let sewer gas in.