Feet of head

A_kraker99

Well-known member
Jul 18, 2015
103
Zeeland MI
I feel like I need a physics degree to figure out feet of head in my pool after reading through some tutorials. I just want to know a good esitmate of where I am at for flow rate at certain RPM with my Intelliflo VSF. I see the chart for that corresponding to RPM and feet of head but when I try to calculate feet of head they want to know the flow rate. uhhhh. That is what I am trying to figure out. Mind blown.

Is there a simple calculator somewhere that I can put in my pipe diameter, total length, bends and any accessories and get a somewhat accurate number for feet of head?
 

mas985

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First, why do you need to know flow rate? In most cases, it really isn't that important.

I have few spreadsheet tools that can help in my signature.
 

A_kraker99

Well-known member
Jul 18, 2015
103
Zeeland MI
Just curious for the most part. Also want to make sure I don't exceed the 125 maximum for the heater and that I am above its minimum flow rate for operation.
 

mas985

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That pump on most any plumbing will never reach 125 GPM.

Also, you will know when the flow rate is too low because the heater will alarm.
 
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Jimrahbe

Mod Squad
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Jul 7, 2014
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99,,

I always recommend that when using the VSF pump, just ignore the Flow side of the pump and just use the RPM settings..

99% of pool owners do not need the flow option.. It just causes confusion in most cases.

Thanks,

Jim R.
 

A_kraker99

Well-known member
Jul 18, 2015
103
Zeeland MI
Sorry. I have a VS pump, not VSF. Is there any general guess as to what 1000 2000 and 3000 RPM would give me for flow rate? You can see my pool specs in my signature. I just added a 266k BTU gas heater as well.
 

mas985

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What is your filter pressure at full speed?

What is the height of the filter gauge relative to water level?

How many suction runs pump to pool, distance and pipe size?
 

A_kraker99

Well-known member
Jul 18, 2015
103
Zeeland MI
What is your filter pressure at full speed?

What is the height of the filter gauge relative to water level?

How many suction runs pump to pool, distance and pipe size?
Mas985,
Here is my best estimates and measurements

filter pressure at 3450 RPM is 29 psi
Gauge is about 5.5 feet above water level.

there is a main drain suction run and skimmer run. Main drain is about 50 feet and skimmer is about 45 feet. Both are 1.5” lines and run into a 3 way valve just before the pump.
 

mas985

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Assuming the filter pressure is correct (gauge goes to zero when pump is off), the flow rate estimate would follow this equation:

Flow Rate (GPM) = RPM / 45.4

Plumbing curve:

Head (ft) = 0.01573 * GPM ^ 2

At full speed, flow rate is about 76 GPM @ 91 ft of head.

3000 RPM, 66 GPM @ 69' of head
2000 RPM, 44 GPM @ 31' of head
1000 RPM, 22 GPM @ 8' of head
 

A_kraker99

Well-known member
Jul 18, 2015
103
Zeeland MI
Thanks MAS985. I have always been pretty good at math but this whole thing is not clicking in my head. I am comfortable with trusting your numbers though :)

What confuses me is that I thought feet of head was a static number that was figured out and that the flow rate was determined by feet of head and RPM. How did you get the flow rate = RPM/45.4 ?
 

mas985

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First, head loss is not a constant and depends on the flow rate through the plumbing. However, the flow rate through the plumbing produced by the pump is dependent on head loss. It is a system of two equations and two variables to solve for. The pump has a head curve (equation 1) and the plumbing also has a head curve (equation 2). Where those two cross is the system operating point. The combination of the pump's head curve and pool plumbing head curve follow the pump affinity laws for changing RPM:

  • GPM B = GPM A * (RPM B / RPM A)
  • Head B = Head A * (RPM B / RPM A) ^ 2
  • Hydraulic HP B = Hydraulic HP A * (RPM B / RPM A) ^ 3

I have several tools that I can use to estimate the operating point of a pool plumbing system depending on the amount of information available. See signature. The tool I used for your setup was the PSI2GPM tab in the pump tools workbook. Basically, it uses the filter pressure to estimate the head loss from the filter gauge forward and a description of the plumbing from the filter gauge backwards.

BTW, which Intelliflo do you have? Most will display the flow rate as well as the total pressure loss (aka head loss).
 
Last edited:

A_kraker99

Well-known member
Jul 18, 2015
103
Zeeland MI
You rock. I used your calculation estimates as a part of a excel document I made for myself to figure out turnover rates and energy usage based on some other metrics unique to my pool. I would attach it but it does not allow XLS files on here.
 

mas985

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Again, which Intelliflo do you have? It should display both flow rate and total pressure loss.
 

mas985

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The VS should show flow rate as well. The difference between VSF and VS is that the VSF can set flow rates but both can display them. Unless you have a very old VS-3050.
 

mas985

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What is the model# for that pump?

You may have to look at the pump display.