What size of ABS for toilet? 3" or 4"

[Jon Eakes]

There was interesting information on the question of 3 or 4" pipe for toilets, but it didn't seem to be aware of the MAP testing on low flow toilets that determined clearly that 3" drain pipe is better for low flow toilets than 4" toilets because with the lower water volume, waste floated better and flowed further in a 3" pipe. This could be important for any long run before an additional water source enters the pipe. A video of that research is available here (yes with transparent drain pipe showing the action): https://joneakes.com/jons-fixit-database/1167-The-low-flush-toilet-controversy-MaP-toilet-testing and the MAP research website is here: https://www.map-testing.com/.

 

[Tuttles Revenge]

This is interesting. The video appears to have been produced in 2015. The old conventional wisdom that 4" carries more capacity and necessary for when we install 4 toilets on a horizontal drain. Theres been debate as to whether increasing slope drops solids out from a the water Glug... and that appears to bear out in these demonstrations.

The new 2021 UPC code changes the requirement to up to 5 toilets on 3" horizontal. Which is a rare instance of the code catching up with technology and changes in behavior.

 

[Sponsor]

 

[Breplum]

Tuttle, thanks for pointing out that 2021 UPC change, not continuing my education these days in retirement, I wouldn't have known that.

 

[Jeff H Young]

I heard about the change on here before to allow 5 w/c well we knew that it would work all along. whether a 3 inch line would work better than 4 I have heard argument not real sure . I think I prefer 4 on underground not going upstairs and to every bathroom but the main and sewer I feel easier to run a machine down but I'm not that experienced with all drain equipment

 

[John Gayewski]

I saw them (in the video) say a pipe graded too steep can cause liquids to run away from the solids, but they never showed it. They showed everything else, but not that.

 

[Jeff H Young]

I saw them (in the video) say a pipe graded too steep can cause liquids to run away from the solids, but they never showed it. They showed everything else, but not that.

Yea weve all been warned about excessive grade ( by experianced and inexperianced hands)yet my code book only calls out minimum fall. not a word about it should be kept below a certain fall where practicle or fall over a certain amount needs approval by ahj . so the code either fails to require a good system or the experts dont belive its important to recomend limiting slope

 

[John Gayewski]

Yea weve all been warned about excessive grade ( by experianced and inexperianced hands)yet my code book only calls out minimum fall. not a word about it should be kept below a certain fall where practicle or fall over a certain amount needs approval by ahj . so the code either fails to require a good system or the experts dont belive its important to recomend limiting slope

If it's true I think the run would have to be very long at a very steep slope which wouldn't be practical. I've never in my life fixed a pipe that was too sloped. Only fixed dips or backfall.