I am planning on installing a new basement toilet. I am not fond of upflush systems therefore I would like to connect the new toilet to the existing house drain by replacing the cleanout wye with a double wye as shown on the sketch below.
I am however concerned that sewage coming from above may cause some accumulation in the bottom part of the double wye where it connects with the new toilet.
More importantly, does this new layout abide by the plumbing code ?
Any comment will be appreciated.

I'm looking for the pipe that will vent the toilet. You do plan on venting the toilet don't you? As long as your going to hafta vent the potty anyhow why not make it a 1/2 bath and add a lavatory. You could drain the lav into the toilet drain where the lavatory vent will wet vent it. Sound like a plan? Regards. Tom

Thanks for your quick response Tom.
Obviously I am planning on venting all the fixtures (you correctly guessed that I intended to install more than a single toilet), I will probably need to install AAVs since traditional venting will be very impractical in this location. I just decided to omit those details for simplicity reasons.
My original question was mainly directed as to whether the 4" double wye shown on the lower sketch was suitable in this particular application. I have not been able up to now to find any positive answer to this question. Since you did not specifically comment on this in your earlier answer, am I correct in assuming that it is acceptable to install a double wye located and oriented as shown on my lower sketch?
Thanks in advance, your help is very appreciated.
Tony

Hey Tony,
The double wye should be OK. I would connect the lavatory drain into the toilet drain and the shower,( if you plan one,) into the lavatory drain where it will be wet vented by it, as will the toilet. Regards and good luck, Tom

Hi Tom,
It's me again...
After doing a little more excavating, I realized that the 4" waste line running below ground is horizontal rather than 45 degree. I therefore needed to review my concept, the newest of which I am attaching for clarity. As you suggested, I am wet venting the toilet via the lavatory vent (AAV) and wet venting the shower via the sink vent (also AAV). I suppose this takes care of the venting part unless you indicate otherwise.
Now my question: I would use a 4x4x3 wye to tie the 3" waste line from my new bathroom into the existing 4" main... Is there a minimum distance to respect before tying my 3" waste line into the existing 4" ?? (see section view on my sketch).
Because of height restrictions, the 3" connexion on the 4x4x3 cannot be vertical (on top of the 4" line). I would however make sure that it is oriented a little bit upwards as shown on the sketch.
Some of the literature I consulted seemed to indicate a minimum of 20 pipe diameters but I am unsure if it was for the same application. Just wanted to double check...
Thanks in advance,
Tony

I would run rhe drain from the sink into the double 3 X 2" wye and wye off from the sink drain line to pick up and wet vent the shower. I'm not happy with using AAVs for vents since it's possible for them to fail but sometimes that's the only way to go. There's no distance required on the 4 X 3" wye.
"Because of height restrictions, the 3" connexion on the 4x4x3 cannot be vertical (on top of the 4" line).
Since this isn't a vent you don't have to roll the 3" branch up on a vertical or a 45 degree angle. Just give it slope.l
Good luck, Tom