Can I tie new drain to old vent system?

[diecastdawg]

Hopefully I can explain this sufficiently because I cannot take picture of this.

We are remodelling a kitchen. New cabinets are now in place before I realized what a Studor vent was. The "old" kitchen had a Studor vent (and we have never had any problems with it in the three years we have been there). The Studor Vent was put in prior to our living there. I would prefer venting outside through the roof since it seems safer although I know AAV's are supposed to be safe too.

The Studor Vent (AAV) was put on top of a "new" 2" PVC drain line that ties into the old cast iron drain line under the house (it basically intersects the cast iron and is tied into the cast iron line on both sides (essentially a "bypass" of what was apparently the clogged portion of the drain line). The upper part of the cast iron drain line is now unused and the lower part continues to drain as it always has. So the both the PVC portion and the cast iron portion of the drain lines are completely enclosed although the part of the cast iron line that was bypassed is unused now.

Since I have already put the cabinets and countertops in, it would be difficult to get into the wall and try to tie into the old vent pipe behind the wall without destroying the back of the cabinets.

The "old" system vented through the roof as most older house did. I would like to tie the "new" 2" PVC drain line into the "old" galvanized 1 1/2" stub that is sticking out of the wall under the sink (it is currently capped off). My desire to tie it in would be for venting purposes only because the water will go down the new PVC as it always has and the water should never enter the portion that I tie into the old stub.

Is there any downside to tying into the old portion of the plumbing for venting purposes only? Currently, there is nothing else tied into this old vent. I don't know why the plumber who put the PVC "bypass" in did not do that, so that's making me wonder if it should not be done for some reason. The old drain stub if approx. four or five inches to the right of where the new 2" PVC drain line goes down into the floor. But I can get to 1 1/2" 90's to turn close enougg to tie into the old stub.

Hopefully this question makes sense. Thanks for your help. This looks like a good forum.

 

[Krow]

I'm not sure why anybody would cap off an existing plumbing vent unless it was in the way of something. If it is indeed connected to the vent portion of the stack, there would be nothing wrong with reconnecting it to the new pvc piping.

What concerns me is this so called "By-pass" that you seem to have in your drainage part . This is unclear and maybe needs to be rectified. I'm a little confused because I can't picture it

The Studor Vent (AAV) was put on top of a "new" 2" PVC drain line that ties into the old cast iron drain line under the house (it basically intersects the cast iron and is tied into the cast iron line on both sides (essentially a "bypass" of what was apparently the clogged portion of the drain line).

 

[Sponsor]

 

[diecastdawg]

Thanks for your response. The word "bypass" might not be accurate, but it is what came to my mind first. Picture the original drain pipe: It is capped where it comes out of the wall under the sink. Then they added a 2" PVC drain pipe next to it and it drops down to underneath the house as expected.

Then underneath the house, the new PVC drain is "T'ed" into the old cast iron pipe. This may not be the proper name for the piece, but it has a 45 degree PVC "T" where the new PVC is tied in on the 45 degree part. Then the other two ends of the PVC "T" are attached to the old cast iron with the rubber hub pieces (not sure of the technical name for those). So the part of the cast iron plumbing from the "T" back to the sink is not open anywhere, but it is not used any more. I assume that they left the unused cast iron in place for support purposes and also just to save the labor in taking it out. The cast iron below the "T" drains out to the main system like it always has.

So when I use the term "bypass", I assume they "T'ed" in the PVC below where the original cast iron was clogged. I did find about a 2 foot piece of cut cast iron under the house that was significantly clogged, so I am assuming this is the piece that they cut out on this drain. But in my opinion, they should have just replaced the rest of the straight run of cast iron with PVC because that's not that difficult to do with easy access underneath the house.

But regardless, the "old" unused section of cast iron would be what goes back up through the wall and ultimately to the vent pipe through the roof.

If it is acceptable to tie into the old vent system, my only concern is that I have to use two pvc 90's under the sink to tie the 2" pipe to the old drain/vent because of how close they are to each other. I wasn't sure if venting is harmed by having a quick 180 degree turn in the pipe, but I assume that since it is just air going through the pipe, it should be o.k.

 

[Krow]

The 2 90's will be fine. You will not have any problems . On the vent side of any system , you can have as many 90's as you need .As you stated, air has very little restrictions

 

[hj]

The connection to the "vent" in the wall would have to go "upward" without any drops to make the connection. The pipe going downward inside the wall would have to be cut and capped somewhere, not just left "plugged up". Whoever did it, if it was because they could not unplug the drain must not have been a very good plumber, since I have not had any drain I could not at least get to drain.