This house was built with a kitchen sink, bath sink, and toilet/tub on a 4" CI, with a 3" (might be 2... cant remember) CI roof vent.

About 40 years ago it was remodeled.

On the original drain line, they left the tub and kitchen sink, and added a washer.

On the new drain line they put 2 toilets, 2 bath sinks, and a tub.




I can find no evidence that that second drain system has a vent. It for sure isn't going through the roof, and I'm fairly sure it's not going through the walls to the original vent. I tested this by filling tub 2 up half way, and putting my ear near the old toilet connection on the original vent (they left the stub on the vent open when they moved it). I heard zero air gurgling in that vent when tub 2 is drained, and I 'DO' hear air gurgling when I drain tub 1 and the washer.

This leads me to believe that the 2nd drain system isn't vented. Good assumption?

If I could move the kitchen sink and washer to that 2nd drain system, it would help with another project. Is that possible?

"If" the 2nd drain system isn't vented, it would be great if I could move it over and have everything work like it has been working.

If venting issues show up, could I solve those issues by putting a tee on the washer/sink drain, and run a pipe over to the original CI vent?

 

You're probably right, based on current codes, that 'new' branch likely requires its own vent. It's hard to say whether it was up to code 40 years ago or not... probably not, but maybe.

Is your drawing plan view (looking from the top down)? And where does your whole system drain out to the sewer/septic? Out to the right? And your plan is to cut off the old CI where the 'Existing' arrow is?
Is everything in red new? Or was new in 1980?

Your best bet is to probably add a 2" vent from the sinks. But it's hard to tell at this point.

 

Yes, it's top down.

Everything in red is new, except for Tub 1. (The 4" PVC line that all the reds connect to is new also.)

Both drain lines leave the house right there at the wall. Both drains used to go to a septic tank, now they run out to a city sewer line. Yes, I want to cut near the existing arrow, and run a new PVC line to the new drain.

I guess another way of asking this is:

I'm assuming that the new 4" drain pipe is sufficient to handle everything except tub 1.

If so, can I just put a tee on the washer/kitchen sink line (or any other part of that new drain system) and connect to the original vent, which is about 10 feet away?

 

Have you been up in the attic to see if the vent lines are tied together to share a common roof penetration?

 

Yes, and I put my ear up to the existing vent and heard zero air gurgling/noise when I drained tub 2.

It boils down to these two questions:

If that second drain pipe is vented somehow that I'm not aware of, can I drain into it with the kitchen/washer line?

If that second drain pipe isn't vented, can I run a line over to the existing vent?

 

I think I made my initial post too complicated. Here is a simplified version of what I want to do.

Everything is vertical except for the two drain pipes, and the line I want to cut. Also, there is a vent at washer/sink 1 that I forgot about.

 

Any ideas on this? Now that I realized that washer/sink1 have a vent, it's almost the same situation on each drain system.

The only difference is that the drain 2 system doesn't currently have a vent AFAIK.

Will introducing the vent from the washer line cause issues with the drain 2 system?

Where should I introduce that line into the drain 2 system? On the end where the toilet is with something like a combo & 1/8?