Problem with very old island loop vent --- should I replace with AAV?

[jharuni]

I have a kitchen island with a loop vent. The sink drains very poorly, and when the sink is fully drained there are odd rhythmic noises coming from it for hours afterwards, like someone is playing bongos down the drain.

The house is 120 years old. I moved in a year ago. I think the plumbing is 1960s or 70s. I replaced the island sink and the trap but connected to the existing drain with its loop. The old sink drained poorly but not as bad as now, and didn't make these noises. All I did was change the sink and started having these problems. Maybe that's just coincidence or maybe I caused it, I don't know.

In the basement I can see where the loop vent pipes go, the drain pipe joins a horizontal 4" run almost immediately, which joins a sewer stack about 4 feet away. The vent pipe goes off to a vent stack as it should. The whole arrangement seems to be just like the diagrams I've seen on this site and elsewhere except that the horizontal runs of the vent and the drain are not vertically offset as most of the diagrams show. They both run just below the joists. There is not much choice because the main sewer line exits the house only 18" below the joists!! The vent pipe also branches off to another pipe that rises up into an interior wall. Only God will ever know where that goes. That wall has been there since 1896, I have no clue how anyone got a pipe up there or where it could possibly lead through an interior wall. There is no bathroom or other plumbing above it.

I've read all the advice and opinion on this board about whether loops or AAVs are better or worse, but in this situation where I don't have much freedom to design what would be best, I'm thinking of just replacing it all with an AAV, which hopefully will work much better than what I have now, even if not perfectly, and will regain me a whole cupboard worth of space under the sink. I'm not going to open walls and I have NO vertical space to play with. The sink is about 18" above the kitchen floor and the sewer line out of the house is about 18" below it. The joists are formidable, they take up about a third of the vertical distance available so everything has to happen in the cupboard or the very limited space in the basement.

I like the design of the Studor Combi-siphon and trap-vent, but they don't seem to be for sale around me. Has anyone ever bought/used one of those in the USA? I'm in New Jersey. Is there any advantage to one of these over the other?

I await your flames and advice.

 

[Cacher_Chick]

Loop vents work fine if they are done properly, and a problem with the vent should not cause slow draining. While we cannot see what you have there based on your description alone, it sounds to me like you have a partially clogged drain line.

 

[Sponsor]

 

[hj]

Your problem is a "drain line" problem, NOT a vent, unless the horizontal portion was improperly installed so it got obstructed. In this case, an AAV would do EXACTLY the same thing.

 

[WJcandee]

As the two pros above have alluded to, the purpose of the vent is primarily to prevent your trap from siphoning when water flows through nearby parts of your DWV system. The suction from that action would/could siphon the trap in the absence of a vent, so that's why there's a vent. (The vent's purpose, of course, is also to provide an outlet for sewer gas.) Contrary to what seems logical to most folks, however, the vent doesn't make it easier for the water to go down -- an unvented system is going to drain just as well. That's why they are recommending that you look elsewhere for the problem, and it's most likely a blockage in what you installed or in your drain line.

 

[jharuni]

Thanks everyone. A little embarrassing, but I have to take the unanimous advice as "stop avoiding the problem and go get your snake". I especially appreciate the pointed note that the vent does not aid draining very much under normal circumstances and is probably not the problem. So I got my snake. To find that there is no cleanout anywhere in sight, and the upper half of the trap is glued in place so I have 180 degrees to go through before I even get started and then another 90 degree bend just below the floor ... and that's as far as I can get. I also discovered a pinhole leak in one of the joints in the loop vent about 4 inches above the trap. It probably doesn't matter much except when there's a sink-ful of water putting pressure on it. So I guess I have to saw out the glued-in trap and the bad joint, snake it out while it's all apart, and rebuild the whole loop with a couple of cleanouts, one in the cupboard and one below the floor in the basement. Waaa! Not what I wanted to do tomorrow.

Can anyone tell my why loop vents are generally designed with a 90 degree elbow at the apex and two 45s so the two vertical pipes are quite far away from one another? Is there a reason not to make it more compact with a pair of 90s butt up against one another at the top? IE just enough distance between the two vertical pipes to make it convenient to work around them?