Plumbing: Is a larger stack size quieter?

[awetynb.pi]

I am adding a bathroom in my attic. It will be nowhere near the existing stack, so I am adding a new stack.

I am piping the stack in cast iron to keep it quiet. A three inch stack is all that is required, but I can fit a four inch stack in the walls in the two floors below. Would the larger stack be quieter?

Is there any other reason to go for the larger stack or not? I have read where larger drain pipes can be more problematic with the new low flush toilets due to lower water velocities on the horizontal portions of the run, and there will be a 10' horizontal run at the bottom of the new stack back to the house drain, and a 6' horizontal run in the attic. Yes I know the horizontal runs need 1/4" per foot pitch. The three inch would be easier to fit in the floor in the attic, but my primary concern is not hearing anything from the attic toilet as the pipe is going to be in the wall between the living room and dining room on the first floor.

Total height of the new stack will be about 20'.

[Speedy Jim]

stack, so I am adding a new stack.

all that is required, but I can fit a four inch stack in the walls in the two floors below. Would the larger stack be quieter?

where larger drain pipes can be more problematic with the new low flush toilets due to lower water velocities on the horizontal portions of the run, and there will be a 10' horizontal run at the bottom of the new stack back to the house drain, and a 6' horizontal run in the attic. Yes I know the horizontal runs need 1/4" per foot pitch. The three inch would be easier to fit in the floor in the attic, but my primary concern is not hearing anything from the attic toilet as the pipe is going to be in the wall between the living room and dining room on the first floor.

The cast iron will be quite good at sound deadening in either size. There may be some fractional benefit to the 4" size noisewise, but I wouldn't bet the farm.

My choice would be the 3". I would install it in the deep stud space you have so that the pipe has no contact at all with the framing or wallboard. Where clamps/straps are needed, use cushioned ones. Wrap the pipe its entire vertical length with some kind of sound absorbent. I don't think you need space-age material here, just some material to add a bit more deadening.

Pay attention to isolation in the horizontal run upstairs too, as it can make annoying noise in the ceiling below.

Jim

[Joseph Meehan]

Follow Jim's answer. I think he hit all the factors and did so very clearly.