# How do I calculate the main water line size from storage tank to house?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by RHinNorCal, May 24, 2018.

1. ### RHinNorCalNew Member

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Apr 30, 2018
Location:
North Bay, CA
2,500 gallon storage tank with TBD but ~35GPM pump to supply a 3BR/3Bath house and a detached guest room with full bath, plus mandatory sprinkler system. House is 105 feet from storage tank, plus 28' of elevation. I am digging the trench and running conduit for house water supply pipe next week; 2 different plumbers have recommended very different size pipes: one said 1 1/4", the other said 2". Can someone point me to the right answer (or, even better, to the formula for determining the correct pipe size)?
Thanks,
RH

2. ### plumbsolveMember

Joined:
Mar 1, 2016
Location:
South Africa
My gut feeling is 2 inch is overkill. As long as the pump is as close to the tank as possible and a 40mm or bigger pipe used to feed the pump to prevent cavitation you should be okay.Unless the house is a commercial use area like a b and b and has high demand fittings like i boxes and massive shower heads.

4. ### RHinNorCalNew Member

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Apr 30, 2018
Location:
North Bay, CA
I agree, plumbsolve - it seems big to me, but I'm (obviously!) not a plumber. I don't know many houses on commercial/community water systems that have a 2" main coming in from the street, that's what led to the question.

This is just a single family residence, no commercial interest. Well pump will be a 10GPM Grundfos unit getting water into the 2,500 gallon storage tank, and the pressure pump will have to deliver at least 35GPM ~40GPM @ enough pressure to supply (2) residential fire sprinkler heads concurrently, plus 5GPM of other residential use - i.e. assuming that the fire sprinklers go into service while the bathtub is being filled. So that's the functional delivery requirement for this main water supply.

Still hoping someone can point me to a calculation that will definitively answer the question. Do I need 2", or will 1-1/4" or 1-1/2" suffice?

5. ### RHinNorCalNew Member

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Apr 30, 2018
Location:
North Bay, CA
Size aside (let's assume it's either 1 -1/2" or 2"), what kind of pipe would be best? I'm thinking PEX in a SDR35 or Schedule 40 conduit... or maybe PVC? Thoughts?

nakopf likes this.
6. ### nakopfNailed it.

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Nov 23, 2017
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Plumber
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Bellingham, WA
I'd go for PEX over PVC, for sure. Conduit is spot on for any future replacement - if you're going full overkill, run a redundant line to serve as an easy future replacement.

7. ### valvemanCary AustinStaff Member

Joined:
Mar 15, 2006
Occupation:
Pump Controls Technician
Location:
Lubbock, Texas
At 40 GPM you will lose 8 PSI on 100' of 1 1/4" pipe. You will only loose 1 PSI in 2" pipe. Either size will work, but the pump will have to be larger to deliver 8 PSI more if you use 1 1/4" pipe.

8. ### RHinNorCalNew Member

Joined:
Apr 30, 2018
Location:
North Bay, CA
Interpolating Cary's pressure loss calculation for the 100' run (thanks for that, btw - that's what I was looking for) the PSI drop from 1-1/2" pipe would be around -4-5psi ... does adding in 27 feet of head decrease the pressure even more?

Side question for Cary: is there any downside to having a slightly larger pressure tank with the Cycle Stop Valve? Would be using your first drawing (but with a submersible in the storage tank, not a jet pump).

9. ### plumbsolveMember

Joined:
Mar 1, 2016
Location:
South Africa
Can you not get more info about the pump you are using. If its pumping uphill it will surely lose pressure. Your pump will also have a curve graph.

10. ### valvemanCary AustinStaff Member

Joined:
Mar 15, 2006
Occupation:
Pump Controls Technician
Location:
Lubbock, Texas
Yes going up hill 27" you will lose another 12 PSI. And yes you will need a little larger pressure tank as well as a larger CSV to work with a 40 GPM pump. Plus the higher the pressure setting the larger the tank needs to be. With a 40/60 switch you can use a 44 gallon size tank. But if you increase the pressure setting to like 60/80 to make up for the elevation and friction loss, I would use an 86 gallon size pressure tank. Like plumbsolve says we need to figure the size of pump you need, which is also needed to size the CSV.

11. ### RHinNorCalNew Member

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Apr 30, 2018
Location:
North Bay, CA
This is the pump I was thinking about using:
https://product-selection.grundfos....il.html?productnumber=11890007&qcid=390158090

Would it a) generate enough pressure to support 2 concurrent fire sprinkler heads @ 13GPM plus 5GPM of additional capacity at b) 20 feet of vertical elevation over 160 feet of 2" PVC? And also give me decent pressure for showers (lest we forget the really important stuff!)

12. ### valvemanCary AustinStaff Member

Joined:
Mar 15, 2006
Occupation:
Pump Controls Technician
Location:
Lubbock, Texas
Those are very good pumps. That one will give you 31 GPM from 68 PSI. See how it draws 2.5HP power at max flow and drops to less than 1HP at 5 GPM? That makes it work beautifully with a CSV12560-3. I would set it up with a 45/65 pressure switch and use a 60 PSI setting for the CSV using an 86 gallon size tank. Without the CSV you would need at least two of those 86 gallon tanks and the pressure would be oscillating from 45 to 65 anytime you are using water. With the CSV you would have a constant 60 PSI anytime you are using water, which makes for much better shower pressure.
https://cyclestopvalves.com/pages/csv125

13. ### RHinNorCalNew Member

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Apr 30, 2018
Location:
North Bay, CA
One more dumb question: would you put the pressure tank in the house mechanical room at the end of the 160' 2" main water line, or put it in the mechanical shed at the well head? Both locations are fully secure and weatherized.

14. ### Reach4Well-Known Member

Joined:
Sep 25, 2013
Location:
IL
The pressure tank and the pressure switch go together. So your question becomes " would you put the pressure tank and pressure switch in the house or in the well house?"

15. ### RHinNorCalNew Member

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Apr 30, 2018
Location:
North Bay, CA
Appreciate the enhancement, Reach4. Which would you pick, and why?

16. ### Reach4Well-Known Member

Joined:
Sep 25, 2013
Location:
IL
I would usually slightly tend toward house mainly because the regulation at the house would be a little better at the house. However with your 2 inch pipe it probably makes little difference.
On the other hand, pump house space may be less valuable.

But an argument for the pump house is that you will probably put a power subpanel in the well house. I am not sure how the pressure switch wiring goes for the pressure switch in that case. That is worth thinking about.