I have been told that 3-inch drain from a toilet achieves a better "swirling" action than larger 4-inch pipe, therefore helping to carry away solid wastes better. Is this true ? And if so, how far should I run before opening up to the main 4-inch waste line that exists my concrete slab ? :confused:

I have been told that 3-inch drain from a toilet acheives a better "swirling" action than larger 4-inch pipe, therefore helping to carry away solid wastes better. Is this true ? and if so, how far should I run before opening up to the main 4-inch waste line that exists my concrete slab ? :confused:


"I have been told that 3-inch drain from a toilet acheives a better "swirling" action than larger 4-inch pipe"

Whoever told you that? Using that logic reducing to 2" or 1 1/2" oughta give you one hellava flush.

Code will allow you to rough in a toilet on the 1st. Or 2nd. Floor at 3 inches.
However on a slab or basement under the cement you have to rough a toilet in at 4". Good luck, Tom

Thanks Tom, Something tells me I will be using this forum again. I think a lot of the self-help "Harry-Homeowner" books leave out details that make the difference between a job done right vs. a job... well you get the idea!

Equal volumes of water will flow differently through different diameters of pipe. A larger pipe will allow the water to run down the bottom of the pipe, like a gutter or trough, rinsing the bottom of the pipe but not the upper half, where material buildup can occur over time. A smaller diameter will force the fluid higher up the pipe walls where the drag will induce a more turbulent flow, helping to flush the whole of the pipe cross section.

Hey Tjpen,
We thank you for the input and you're correct about how water flows, however, you're responding to a five year old dead thread, Look in the upper left hand corner for the date and welcome to Th Plumbing Page. Stick around and answer questions, Regards, Tom