I’ve clipped this from another site:
4 in. soil pipe offers a more efficient way to dispose of solid material, but the introduction of low-volume toilets and other water-conserving measures, significantly reduces the level (height) of water in the pipe, resulting in significant clogging within the soil pipe. In new construction, the 4-in. pipe is now too wide for long, horizontal runs (especially between the home’s main vertical drain, or stack, and the street).
3-in. soil pipes raise the flow level in horizontal runs and, as a result, suffer fewer clogs.
The plumber and I reviewed the plans, which require an array of lines, one main and two branchoffs, to pick up the walkout-basement plumbing as well as the stacks coming from above. I thought the mains would be 3″, and that is what would run out to the septic tank, but he said it oughta be 4″. Nothing to do with code, just his druthers, I believe.
I said OK, but with a little regret. I did not know about the hangup potential, because I had not researched it.
We’ve not done the work yet. What would you do? 3″ or 4″.
Replies
3" or 4" work fine, just make sure you have smooth runs and its all down hill.
Your biggest problem is the toilet. They are mostly "S" trap and narrow pathway. Just a little to much and it clogs! My folks live in the UK and in the 46 years they have lived in their house, have never needed a plunger! I have worked in new USA houses and Had to take a plunger to the toilet! Now thats poor design!
From my vertical vent stack to my septic tank is about 60 feet. Both toilets (low volume flush) are the full 60 feet from the septic. I used 4-inch waste pipe and as far as I know everything is working fine.
The slope is 1/4 inch per foot, and there are four 45-degree bends along the way. I provided myself with a cleanout at each bend, just in case.
I've read that too great a slope can cause the liquids to flow past the solids, and not carry the solids along with them. But I've also read that theory is hogwash. I suppose that argument is similar to the 3-inch versus 4-inch debate.
BTW, when I had a plumber put in some PEX lines for me, he said he would have used 3-inch waste pipes.
Allen
I'm not plumber - wait for PlumBill or someone else who does it for a living - I just hire these guys, but the way it was explained to me is that with 4" pipes you don't get enough velocity of the effluent moving through the pipe. The sewer pipe does need to be 4" downstream once four toilet 3" toilet pipes have been joined into the sewer line. So, in the case of sewer pipes, bigger is not always better.
A bit of tangent is that a 4" pipe can handle a surprisingly large number of fixtures. How many I'm not sure but on one project (designed by architects and engineers) we had 8 bathrooms, 4 kitchens and 4 laundry rooms feeding into a 4" DWV.
Edit - so, as I understand it, a toilet always gets a 3" pipe, but the rule of thumb for the pipe that exits the building is: 3 toilets = 3" pipe. 4 toilets = 4" pipe.
Edited 5/21/2007 7:24 pm ET by Matt
you do need the flow and I believe you will have fewer problems with 3" vs the 4"
my loft project with 20 units and over 80... maybe 100 fixtures is on a 4"... all engineered.... i couldn't believe it at the time but did the research... and yes fast forceful flow beats slow flow everytime...
an old building i looked at.... guy had owned it over 50 yrs... all the toilets were on 2" cast iron pipe.... i asked if it ever casued a problem... was an old factory & had over 40 people working there at one time... guy told me they'd never had a clogged pipe as long as he'd been there... and most of the workers were women... and who knows what they flush...
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I'd be very surprised if anyone would notice the difference in new construction. With the boom in huge houses there are many bathroom that are never used so the long runs of 4" line are essentially only serving one or two bathrooms. It's never an issue.
Your plumber is right, but the trade off is so small it doesn't really make a difference. Good installation practices will make more of a difference.
Beer was created so carpenters wouldn't rule the world.
If you run 3" you limit yourself on future remodels that will limit you to 3 toilets.
1.6 gallon flush verses 3" or 4" is not a big deal, bigger deal is the slope---- too steep the water doesn't carry the crap.
This is all posted by others I'm just confirming what they said.
I fear no man & only one GOD. Me
What pitch would you consider too steep?
I generally don't grade anything more than a 1/4" per foot, ecept for offsets.I fear no man & only one GOD. Me
ain't it strange the things a plumber has to know!
I just learned l;ast month from mine that on a lond run, if it is too steep adding velocity is apt to have the water run off and leave the solids straggling behind like the fat kid on a hike
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
that was on mypth busters couple months ago
Oh Yeah?So is it true or is it busted?
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
they was trying to make the water outrun the solid. could not be done.
Wasn't that on right after that episode of Flip This House where they made $230k on a piece of property they owned 11 days ? ;-)
OK, I measured.
180' to the tank, 4"/foot drop. 13 yrs, no problems. The health dept guy made me install 2 cleanouts due to the length, but was laughing hard about the impossibility of anything pausing midstream.
To use minimal slope I'd need an incredible tower for my septic tank.
The faster water moves, the more work it'll do. At least that's true for driveways and rivers. I expect the inside of my 4" line is similar.
Seems like I remember seeing log sluices kinda like what my kids ride in. Anybody up to calculating velocity?PAHS Designer/Builder- Bury it!
ain't it strange the things a plumber has to know!
I just learned last month from mine that on a long run, if it is too steep adding velocity is apt to have the water run off and leave the solids straggling behind like the fat kid on a hike.
So, metaphorically speaking, one answer might be to...tighten up your extrusions thereby turning your canoes into kayaks and running the rapids swiftly while the spillway is open. Or you could put skates on the fat kids.