Need Help Calculating Total Dynamic Head

[Drew Wathey]

Hello everyone,

I'm looking to install a submersible well pump in my cistern and I'm trying to figure out the total dynamic head to properly spec a pump. The issue I'm running into is I don't know how to calculate it for a PEX system that uses manifolds.

I'm aiming for an overall flow rate of 22 GPM and a constant pressure of 40-50 psi, which I'll be using a CSV to accomplish.

Here are is the piping setup:
Elevation difference: 7 feet

Main Run
32 mm X 30 foot horizontal run from pump to CSV
32 mm X 6 foot from CSV to cold and hot main distribution manifold
20 mm X 12 foot to washing machine (1 tee + 1 right angle)
20 mm X 19 foot to sink (1 right angle)

Run 1 Cold
25 mm X 190 foot to manifold in bathroom (2 right angle fittings)
20 mm X 1 foot to sink
20 mm X 5 foot to toilet (1 right angle fittings)
20 mm X 12 foot to shower (2 right angle fittings)
20 mm X 12 foot to outside spigot

Run 2 Cold
25 mm X 125 foot to manifold in bathroom 2 (1 right angle)
20 mm X 1 foot to sink (1 right angle)
20 mm X 5.5 foot to toilet (1 right angle)
20 mm X 13 foot to shower (2 right angles)

Run 3 Cold
25 mm X 17 foot to manifold in bathroom 3 (1 right angle)
20 mm X 1 foot sink
20 mm X 4 foot to toilet (1 right angle)
20 mm X 10 foot to shower (2 right angles)

Run 4 Cold
25 mm X 33 foot run to manifold in kitchen
20 mm X 1 foot to sink
20 mm X 4 foot to dishwasher (1 right angle)
20 mm X 16 foot refrigerator (3 right angles)
20 mm X 42 foot to shower (3 right angles)

The above information is only for cold runs. I'm no sure if I need to include the hot runs to determine total dynamic head.

 

[Valveman]

You would have to also know the flow rate for each device to figure the friction loss on that line. Normally you would figure the largest flow rate at the farthest distance and add that friction loss to the depth and pressure the pump needs to produce. Pumping from a cistern means no lift. If you want 50 PSI at the appliances that is the same as 115' of lift to a pump. Then if the friction loss in the longest line to the largest appliances has say 10 PSI friction loss, you shoot for a pump and CSV that can deliver 60 PSI or 138' of lift.

 

[Sponsor]

 

[Drew Wathey]

So if I'm understanding you correctly I only need to calculate the friction loss for each run and disregard pipe length when determining which pump to buy? And I would based the calculation off total flow rate for each run? I don't need to factor in any hot runs for this, correct?

Run 1: Flow rate - 14.6 gpm, friction loss - 18.7 psi
Run 2: 6.6 gpm, loss - 4.56 psi
Run 3: 6.6 gpm, loss - 4.56 psi
Run 4: 14 gpm , loss - 16.45 psi

So if I wanted to maintain the 50 PSI at end of Run 1, I would need a pump capable of 70 psi? Or a pump capable of 60 psi if I wanted to maintain 40 psi?

I was thinking either of choosing one of these pumps:
https://www.rainharvestingsupplies.com/grundfos-22sq-submersible-pumps-22-gpm-17-psi-to-95-psi/
or maybe this? https://www.rainharvestingsupplies.com/grundfos-30sq-submersible-pumps-30-gpm-17-psi-to-56-psi/

Also, which CSV would you recommend for my setup?

 

[Valveman]

Even though you could turn everything on at the same time and use as much as 14 GPM, most houses rarely use more than 5 GPM at any given time. If you add the 18.7 PSI loss to the 40 PSI you want, then a 60 PSI CSV with a 50/70 pressure switch is what you want. But when you are not using 18.7 GPM, and only using say 3 GPM on that line, there will be no friction loss and you will have 60 PSI at the shower instead of 40. Although I love 60 PSI constant in my shower, that will be 20 PSI higher than you are designing for.

So if you want 60 PSI in the shower use a CSV set at 60 PSI and a pump that can deliver 60 PSI at the flow rate required. I usually recommend a 10 GPM pump as that is twice what a house normally uses. But with a CSV you can install as large of a pump as you think you may need, and the CSV will make it work all the way down to 1 GPM as needed.

The SQ pumps you are looking at are OK, but they are little 3" diameter pumps that spin 10,700 RPM and have electronics in the motor. The standard 4" pump that only spins 3450 RPM and doesn't have electronics in the motor will cost less and last longer than the SQ. 60 PSI is the same as 138'. Add in the 7' of lift and your TDH will be 145'. A 10S05-9 will give you 11 GPM at that pressure. A 16S07 will give you 18 GPM.

With either of those pumps the PK1A pressure tank kit has everything you need to control the pump and get strong constant 60 PSI.

 

[Drew Wathey]

I guess I never pressed post reply for this post.

Thank you for your help so far. Our town main water supply varies between 10 and 16 gpm depending on the time of day and the pressure varies between 20 and 30 psi. So currently, when we have 2 water fixtures on at a time neither is getting adequate flow or pressure. Using an outside hose at the same time makes things even worse and we use the outside hoses quite frequently.

We live in a hurricane zone so we have a 22,000 gallon cistern, but what I'm doing is converting the cistern into a water reservoir essentially because when we got hit by Irma the water inside became too salty to use. So I want to use the cistern the main water supply for the house (no rain water will enter the cistern). The cistern will be fed by the water main and regulated by a float valve. The pump in the cistern will be the main supplying force of water, which is the 1st reason I'm trying to spec a pump between 20 and 30 gpm. Based on some calculations I did taking into account the 3 time periods each day where we the most water it seems we use on average 12-14 gpm not including outdoor usage of hoses. Which leads me to the 2nd reason, I don't want to be surprised by a sudden drop in flow if someone decides to turn multiple hoses on at the same time, which for some reason is more common in our household than you'd think.

If you feel the 16S07 will be more than adequate for my needs I will take your advice. I'm assuming I need to buy a motor and controller for the 16S07 as well?

 

[Reach4]

Keep the cistern in the dark to inhibit algae. You may need to chlorinate somehow. You could get some low-range chlorine test strips and use chlorine bleach. Or you could automate that in some way.

 

[Valveman]

With a CSV you can use as large a pump as you want and the CSV will make it work like a small pump when small amounts of water are needed. The same PK1A kit will control up to 25 GPM. You can get a 1HP, 30 GPM Hallmark pump for about 150 bucks. I don't know the quality or how long it will last but you could buy 3-4 of those for the price of some brand name pumps.

 

[Drew Wathey]

Thanks for the replies. While the price does seem low, I'm not sure I want to buy something with unknown reliability especially since the cistern and pump will be the main source of water.

 

[Valveman]

I hear you! But you could buy 3 of those for the price of one. In a cistern the pump is easy to change.