3/4 vs 1 inch and 4 vs 6 inch new water/sewer hookup

[reubenjacques]

Hi. I am applying for new water and sewer hookup with my city. I plan to possibly have a duplex built on my property in the future. Should I go ahead and get a 1 inch water supply and a 6 inch sewer line if there's any chance I have a duplex built? (Rather than 3/4 inch water supply and 4 inch sewer?) I'd rather have more capacity than I need than not enough. Thanks so much for any advice.

 

[havasu]

I believe it is a wise decision to plan big. The changeover afterwards would be extremely costly.

 

[Twowaxhack]

Sometimes you have to ask yourself what would Elvis Presley do…….

Go big !

 

[havasu]

Umm, you're dating yourself, Kemosabe!

 

[CT18]

I am not sure about the 6" sewer as you can have over 200 DFU's on a 4" line.

 

[pasadena_commut]

Well, two 4" drains going absolutely full bore can theoretically move water at a rate proportional to the area which is 16 sq in x pi, two of them will be twice that. So if the duplex merges two 4" drains into one something is going to back up in that one specific condition. But about the only way I can think of to completely fill a 4" drain continuously is to drain a pool into it, which would never happen as it would go to the gutter in the street. Also what happens to a rooter with a 4" head when it transitions to a 6" pipe? So 6" seems like it is probably overkill for a duplex, but if you decide to go that way, at least put a cleanout of that diameter at the end of the 6" pipe where the 3" or 4" pipe from the units merge in. If there is any chance that more than two units might go on that lot, like a small apartment building, then the case for 6" looks better.

Besides the installation cost, check what the city/county charge for the different size sewer connections. There is probably a premium on the larger size.

 

[MicEd69]

two 4" drains going absolutely full bore can theoretically move water at a rate proportional to the area which is 16 sq in x pi

So, you are saying the area of a 4" circle is over 50 sq. inches, which if it was a square would be over 7" square!!!!!

pi x radius squared; NOT pi x diameter squared.

As CT18 said, a 4" line will take 200 DFUs. A 4" will be just fine!

 

[Twowaxhack]

Sometimes commercial they want 6”

Better call Saul

 

[Twowaxhack]

But honestly with a 1” water line you would be wasting money with a 6” sewer. 1” is what one single family house usually gets installed, minimum.

Ultimately the local codes dictate these things and it’s not always sized by fixture units.

For instances Sussex county, Delaware based the size sewer you install for commercial multi family property nits based of the number of units you’re connecting to the sewer but doesn’t specify how many family units are in each building.


https://sussexcountyde.gov/sites/de...etin for Building Sewer and Water Service.pdf

 

[pasadena_commut]

So, you are saying the area of a 4" circle is over 50 sq. inches, which if it was a square would be over 7" square!!!!!

pi x radius squared; NOT pi x diameter squared.

Right, brain fart of the mathematical variety. But even though I was off by a factor of 4 the conclusion does not change - under normal circumstances two 4" drains can feed into one 4" drain without exceeding its capacity, and two 4" drains won't saturate a 6" drain. (That is, 2 x pi x 4 is still less than pi x 9.)

The pool draining example would never happen, but I suppose there could be a duplex with two tubs each unit, and if they were all full and all drained at once, maybe then it would saturate a 4" drain pipe? Seems like a really contrived example though.

 

[MicEd69]

Right, brain fart of the mathematical variety. But even though I was off by a factor of 4 the conclusion does not change - under normal circumstances two 4" drains can feed into one 4" drain without exceeding its capacity, and two 4" drains won't saturate a 6" drain. (That is, 2 x pi x 4 is still less than pi x 9.)

The pool draining example would never happen, but I suppose there could be a duplex with two tubs each unit, and if they were all full and all drained at once, maybe then it would saturate a 4" drain pipe? Seems like a really contrived example though.

You do realize that properly designed sewer systems do not run full when in normal operation.