New owner trying to better understand flow rates

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qodfathr

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Jul 12, 2015
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GROTON
I have a new in-ground, 16,000 salt water pool, and, as seems to be common, the installer set up the variable-speed pump (Pentair Sta-Rite Supermax VS) to run 24x7, which is both expensive and seemingly overkill.

So I've been trying to find more reasonable settings while maintaining good turn-over. Eventually, this leads into a computation for total dynamic feet of head, and that's where the trouble begins.

One method I used was to figure the average length of the returns. I came up with a very rough estimate of 46', which seemed to be in line with the blind suggestion of 40'-50' I've seen posted here several times. So that made me feel good about my calculation. [I roughly measured the 4 return lines as 30', 40', 55' and 60'.]

But here's where I get very confused -- the pump performance curve. When I look at this chart, assuming 45' of total dynamic head, it would appear that at 2,600 RPM and less, the flow rate should be ZERO. But if I run the pump at the lowest setting (600 RPM), I still feel water coming out of the returns. (Not much, of course, but certainly more than zero.) I also have an auto-pilot nano, and it requires 15 GPM to operate. When I set the pump to 1,600 RPM, the nano stops complaining about flow rate -- suggesting that 1,600 RPM is producing at least 15 GPM of flow, which, according to the pump performance chart, equates to 15' of total head.

15' of head, in turn, suggests a speed of just 2,600 RPM would be producing 72 GPM of flow -- which seems unlikely, but, hey, what do I know? Maybe it is. The installer set the pump to run at 3,100 RPM for 10 hours, which, for 15' of head, is literally, "off the chart," but would be in the vicinity of 92 GPM -- indicating I'm turning over the entire pool every 3 hours, or more than 3x during the day. The installer then set it to run at lower speeds for an additional 8 and 6 hours. (24x7), so, by my calculations of GPM, my total daily turn-over is extremely high (like 4-5x).

So, my real question is: do I trust my determination of 15' of head? Even that has me confused -- for, even at a mere 15', the pump should have a flow rate of ZERO at 600 RPM (by the performance chart), but I can feel that it is non-zero. Is that normal and should I read zero as "really, really low GPM?" If, instead, I use my estimate of 46' of head, the performance chart says I'd have 0 GPM @ 2,600 RPM, but I have far more than that -- it's not a mere trickle -- suggesting that 15' is closer to the truth than 46'.

I hope that wasn't too confusing...

Thanks!
-Todd
 
For a residential pool, turnovers don't really matter so there really is no point in calculating flow rates. Start with this article: Pool School - Determine Pump Run Time

If you want more evidence that turnover doesn't matter, read the article on pump run time in my signature.

FYI, the length of pipe is related to head loss but is not the actual head loss. Calculation of head loss is a very complex process and rarely worth the effort. Also, head loss is dependent on flow rates which are then dependent on RPM, so again, this is not a trivial process.
 
I run mine on low from 6am to 10pm mainly because our electric rates are low and to make it easy of my wife or kids want to turn the fountains on. I've adjusted my SWG accordingly and pool stays nice and clean.


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Sseaman said:
I run mine on low from 6am to 10pm mainly because our electric rates are low and to make it easy of my wife or kids want to turn the fountains on. I've adjusted my SWG accordingly and pool stays nice and clean.
Click to expand...
But probably still far more run time than you really need.
 
Thanks, Mark! I had made the move down to 2 hrs high RPM, 2 hrs medium RPM and 6 hours low RPM (the minimum RPM which still permits the SWG cell to operate). Based upon your advice and info, I may even go lower -- I'll keep an eye on the water quality and if it looks and tests okay, I'll stop worrying about the need for long run times!

We have a heater, and it (obviously) requires circulation to operate, so that may force us to have more than 4 hours of runtime, depending upon desired temp. But I now envision running split time -- say, 2 hours at 8:00a with moderate RPMs to heat the water before the sun takes over (and move debris into the filter baskets) -- and 2-4 hours at 6pm with lower RPMs when the sun has dropped below the tree line for additional heating for those enjoyable post-dinner swims. Ultimately, it looks like the heater and the SWG will be the larger determining factors in terms of needed runtime.
 
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