Is 3" pvc piping too big for a drain which will tie in an existing 2" pvc drain (bathtub, sink, kitchen sink/no dishwasher 1st floor apt) with an new 2" drain in cellar for washing machine and utilty sink. 3" pipe would replace 11' declining (1" per ft)run along cellar wall of exisiting 2" pvc pipe which bends and drops 5" to enter the sewer drain at floor level(yes, a back check valve is installed as well as vent pipe and cleanout) 2" pipe seems too small for the additional volume of water but 2.5" is probably only available at supply houses. The difference of only 1/2" in the size of the pipe seems negligible.

pvc drain for tub and sinks only was rerouted from main stack in order to add a necessary structural element (2x 8" joist) 1st floor bathroom floor below inside wall. Which was never installed. Toilet drain remained unchanged and continues to drain into the main stack pipe.

why would you want to choke down to 2" from 3"? Should be the other way around. Why can't you cut out the 2" and run 3" all the way?
You have a drainage pip[e that slopes 11" in 11'? Way,way too much fall. Should be 1/4" to the foot giving you a 2 3/4" drop.
There's a problem with too much slope. Too little and won't get a good flow. Too much and the liquid runs out so fast to leaves any solids behind.
Bottom line. I would run 3" all the way. Good luck, Tom

Tom,

I think Egret wants to connect one 2" drain carrying bathtub, sink, kitchen sink/no dishwasher (1st floor apt.) with another 2" drain in the cellar for washing machine and utilty sink and increase to 3" pipe to replace older 2"pipe, right?

If so, it doesn't change much as the advice you gave is still the same. Increase to 3" and improve the slope of piping to 1/4"pitch per foot with an offset to drop the piping into the sewer drain at floor level... ;) As you noted Egret, 2.5" size is not common and is not used in most cases residentially or commercially.

My only other thought is to install full-size test tee cleanouts at each 2" branch line just before combining into the 3" pipe.

Thanks...

Mark

PS: By the way, if I haven't said it, it's real nice to see you up and posting again! I swear you answered every question on the plumbing page the other day...all speedball, speedball, speedball, speedball...:) Nice to see that again!!

Thanks Mark! It's good to be back in the saddle again.