Increase main drain line bottlnecked at 2" iron

I'm redoing all the drain lines. Very simple setup.

1 kitchen sink (2 DFU)
1 tub/shower (2 DFU)
1 lav (1 DFU)
1 CW (3 DFU)
1 DW (obviously not going directly to main)
1 toilet (but this is directly tied into the 4" stack)

Currently everything is 2" iron pipe which ties into the 4" stack. I plan to replace it all up to the 2" piece of pipe coming out of the stack. According to my local code I can only have 8 DFU on a 2" horizontal main line so I'll be maxing that out. I was thinking I could replace the 2" main with 3" but is that pointless considering I'll be bottlenecking at the piece of 2" pipe coming out of the stack?

Thanks!
Justin

Basic rule of drains. You can never decrease the size. As you said, it creates a bottleneck which equals clog heaven.

If you upgraded to a 3" line you would have to redo its connection to the 4" main, you CANNOT reduce the size of a drain line in the direction of flow. As far as it being a "bottleneck", how many of those items will you be using at the same time?

I should have thought of that. Makes perfect sense. I can't imagine that I'll be using all of them at the same time. I'll also be installing a clean out at the rear of the 2" main so I guess I'll just move ahead with the 2" main. I definitely don't want to mess with the stack!

All of the fixtures will be coming into the main horizontally. My understanding is that using wyes instead of long sweep 90s promote better flow. Perhaps that will keep the 2" from being taxed.

The house is only 1050 sq.ft. and I don't see anything else being added. If someone where to add another story or something down the road they would just upgrade the stack at that time anyway.

Thank you so much for the help.

Whoa.... you got too much on that 2 inch line. also im not sure when you list your fixtures what is a CW? is that a clothes washer? If so you really got a problem. Washing machine drain line BY ITSELF is required to be 3" at the point where it turns horizontal. Thats without all that other stuff added onto the line. you need to re think this install. that line needs to be 3" throughout its entire length. yes, that means no 2 inch bottleneck. Sorry to be the bearer of bad news, but your only gonna create a bad problem down the road.

Yes, CW is a clothes washer. Interesting. My local code says I'm within legal. I'm maxed, but still legal, unless I'm missing something which is entirely possible.

No you are on the left coast using the UPC. where 2" is legal for a c/w only!

He is on the right coast using a different code book where 3" is required for a clothes washer, !

But notice I said 2" for the clothes washer only you still need to upgrade to 3" for all the fixture units you have on that line

I have done many, many installations with the CW tied into a 2" line connected to other fixtures.

I believe the UPC counts a kitchen sink with dishwasher as 3 DFU, so you are over the limit.

(OOPS: I was consulting a non-current edition of UPC, evidently this has been changed to 2 DFU,
so nevermind, please.)

A domestic kitchen
(with or without food-waste grinder and / or dishwasher, 2.0 DFU
It's 2 for the kitchen sink.

Max units on 2" horizontal = 8
Max units on 2" vertical = 16

Max units on 3" horizontal = 35
Max units on 3" vertical = 48

UPC in Oregon

I tried yesterday to respond to this thread but it wouldnt let me in (my computer's fault). Sorry for being late to the discussion. Heres what I know about this. My code book that we use here NC and SC says 6 dfu on 2" horizontal drain. So youre over. More importantly, the reason we have switched to 3" for washing machine drain is that the newer style washers can pump and dump so much water so quickly that they "may" over run a 2 inch horizontal drain, And thats going to cause you some serious issues. Maybe today you have an old washer, but maybe next month you buy a new one and youre screwed. Also I have to disagree with the "I'm only gonna use one fixture at a time" way of sizing pipe. Thats never been part of the pipe sizing equasion by any standard. Try to remember that codes are "minimum standards" In other words, this is the very least that you must do in order to have a functional system. I know codes are different from region to region, but this is YOUR HOME.. you have to live there... dont save a nickel now only to have it cost you a dollar later.

The DFU numbers were pulled directly from my local code book. I totally agree that upgrading to 3" would have been better. That would have meant replacing the whole cast iron stack since you can't go down a size. I have a 2' crawl space and the drain comes into the stack right were the stack goes into the earth and out. I considered replacing it, but I would have risked having to start digging up earth. So I wasn't so much trying upgrade my drains as I was replacing the old galvanized clogged ones.

I understand, sometimes a project requires things that you just dont want to do, or just cant do. That being said, This is a great example of when you really need to have brought in an expert to help you through the tough stuff. Saving money and doing it yourself are always good things in my book, but sometimes its best to have a pro do certain parts of a job that are over your head. Cutting out old cast iron, adding in a new 3 inch fitting and reconnecting to the cast is a job most plumbers can do in thier sleep. ( A low crawl just makes the job more fun!) Yes, it would have cost you some money now... but if you remember the old fram oil filter commercials.... "you can pay me now.... or you can (REALLY) pay me later"... Seems to be very appropriate in this case.
Im a rookie in the plumbing world compared to some folks, Ive only been in it for 15 years, Thats a minimum of 30,000 hours experience with plumbing. Chances are ive already done a simular repair or modification 100 times before i got to yours. What looks impossible to you, may be a 1 hour job for me, making it totally worth the cost for you to have it done. Imagine if I showed up where you work and tried to do your job... Tasks that you can routinely handle while drinking coffee and chatting with co workers would likely take me all day to figure out (Ive got no experience)... and even then I'd probably get it wrong. The funny part is, I'm sure you could fix my mistakes pretty quickly.... Kinda like I could with your plumbing.. lol. Just something to think about for next time. Good luck!

I totally get what you are saying. It is interesting that you think I work in an office. I'm actually a general contractor and I work around this stuff all the time, though I have my subs doing the actual work. But this one is my house and even my plumber said the 2" will be fine since I'm just replacing anyway. Ultimately you are right though, 3" would be better and if when we get a new pulsing clothes washer things get all backed up then I'll just bite the bullet and get under there again.

On the West Coast, this works.