4" to 3" Question

I have a 2 family home with all 4" cast iron dwv. There are 2 bathrooms on the second floor, back to back. One 4" cast iron horizontal run serves both. 2 toilets, tubs and vanitys. The horizontal run is about 6 feet to the main vertical stack. I need to replace the entire horizontal section and can also remove most of the vertical run down to the basement and up to the roof vent. The horizontal run has improper pitch and that is the main reason to do this. The pitch is actually going the wrong way and water is not draining from the toilet furthest from the vertical stack! This has worked with zero problems for the nearly 40 years I've owned the place. I have gutted the kitchen below and found this condition. My question is can I use 3" and transition to the 4" C.I. in the basement? The difference in pipe diameter would give me considerably more room to do this. I am a little concerned that 3" would not work as well as the 4" has for all these years.....Thanks....

pipe

3" will work as good as 4", but you can only increase from 3" to 4", you cannot go from 4" down to 3".

Smaller sized branch drains feed into larger sized main runs so why can a horizontal run not be smaller than the larger vertical stack it connects into? The only section that would remain 4" would be in the basement right before it leaves the house. It has a sanitary tee on it's back where the vertical meets that horizontal. It also has a 1-1/2"washing machine drain and cleanout jusst back from the pipe exiting the basement. Very poorly done....a straight 1-1/2" tee right into the 4" serving two washing machine locations......the plumber that did this work must have been a "shoemaker" as my father used to say.....

You misunderstood...you must always go from smaller to bigger drain pipes, you can't go from bigger to smaller. What you want to do should work if it is done properly - 3" to a 4".

Thankyou Jim....I did misunderstand. I thought HJ was saying I could not do what I descirbed. All I was intending to do was rennovate the bathroom and kitchen and once I removed the toilet and opened up the wall below.......well things got a lot more complicted. I have been dancing around this issue since last Sept. and waiting untill spring to do this. It would be very easy if I was not living in one side and trying to keep my existing bathroom working.

hj said:

3" will work as good as 4", but you can only increase from 3" to 4", you cannot go from 4" down to 3".

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Why can't you?

Two reasons. First, it's a code violation. While that reason alone should be sufficient, the rationale behind the code requirement is that reducing a drain size creates the very real potential for repeated clogs. Basic rule. Go from small to large, never large to small. Think of a 4 lane freeway reducing to 2 lanes. We've all seen what happens, when the traffic is heavy on the 4 lane it creates a bottle neck at the two lane.

hj

I assume you are trying to misunderstand. IF you remove ALL the 4" piping, then you can replace it with 3", but you cannot insert a portion of 3" between sections of 4". Once you insert a section of 3", ALL piping upstream, with a single exception, MUST be 3" or smaller.

HJ It really doesn't make any sense that I would be "trying" to misunderstand. If I could make a drawing it would have been easier to understand what I was asking. If I did make a drawing of the entire plumbing system in this house...the answers would be.....man thats ugly and totally wrong, get a plumber....but I have to ask what the heck kind of plumber did this in the first place when the house was built in the '50's. Obviously the guy could do the cast iron joints ok but thats the only thing that is good.....improper pitch, no individual vens for fixtures, with just one vent and that is the main vertical 4"stack. I can't afford to rip the house apart and start over. The plumbing has worked with virtually zero issues since 1971 when I bought it and I'm just trying to make it better while some walls are open and the plumbing is exposed......

From what I read you will be fine as long as you are not going from 4" to 3" then back to 4"

The only fitting that I know that is legal to go from 4" to 3" is a 4x3 closet bend.