Pump Sizing Math Check

[Deleted member 62417]

Hello everyone:

My old Pentair Challenger 1HP pump is cracked and I am going to replace it this year. Getting good information from the pool store is impossible since they all want to sell me a Hayward EcoStar or TriStar VS pump.
Here is my setup:
18'x36' rectangular pool which holds 29000 Gallons (maybe 26687 depending on the exact geometry)
My turnover rate in 10 hours is 48.6 GPM for 29000 Gallons (maybe 51 GPM for 9 hours of 26687 Gallons)
I have 1-1/2" lines which I have read should be kept at 6-8 ft/s or 38-51 GPM.
I am therefore using 50 GPM for my desired flow rate.

Head Losses:
I have a Hayward SP220T Sand Filter rated at 60 GPM which looks to have a head loss of 12.2 ft according to information on this site (comparable Pentair unit from the Hydraulics sticky)
I measure 52 ft of suction line on my return, and 77 ft of pressure side piping approximately for a total of 129 ft or 130 ft. Using 16 ft of head per 110' of 1-1/2" pipe at 8 ft/s (information from Hydraulics sticky) I get 18.76 ft of head.
I estimate a 1HP pump to have a 9 ft head loss factor.
Total Head Loss = 12.2+18.76+9 = 39.96 = 40 ft.

So it looks like I should be looking at a pump that can deliver 50 GPM at 40 ft of head to turnover the entire pool in 9 - 10 hours per day.
If I look at the chart for a Jandy FloPro FHPM.75 (See http://www.zodiacpoolsystems.ca/~/media/Zodiac/Global/Downloads/SL/SL6206.ashx) it seems to line up with the curve pretty well although I am not really sure what the grey curve represents in the chart? Anyone? A Hayward Super Pump SP2607X10 would also seem to do the trick (See https://www.hayward-pool.com/prd/In-Ground-Pool-Pumps-Super-Pump-_10201_10551_13004_-1_14002__I.htm). However the Jandy is running at 3/4 HP so would therefore draw less electricity?

Question about 2-speed pumps:

For all the 2-speed pumps, the low-speed curves don't seem to cover the 40-ft of head even at really low flow rates? Should I therefore not consider a two speed pump? I wanted to run the pump at 50 GPM for 9-10 hours per day as per the above calculation and then run it at low speed the rest of the day to circulate the water. Is that correct?

Any pump recommendations or math corrections would be appreciated.

Thanks

Derek
derekmjenkins

 

[JasonLion]

Welcome to TFP!

You did fairly well in your reasoning, better than most ever do, though you got a few details wrong. The largest mistake you made is assuming that TDH is constant. TDH depends on the flow rate, which will be different for each pump. To make this kind of calculation you need to compare the pump curve to the plumbing curve to find out what the TDH and flow rate will be for the combination.

However, doing this kind of calculation isn't really needed in this situation (unless there is a spa you didn't mention). The whole two or three turnovers a day rule of thumb does not apply to residential pools. Just about any pump that is sufficient for vacuuming will provide a far higher flow rate than you need for most everything else. A residential pool normally requires only four or five hours of runtime at quite low flow rates, only enough to get the skimmer (and SWG and heater) to work efficiently. Those rates are often around 25 GPM, though some equipment/situations may require a bit more.

Two speed pumps, and variable speed pumps, have the huge advantage that they can be run at higher speeds when vacuuming, or cleaning up algae, and run at lower speed all the rest of the time, resulting in huge savings because of reduced electrical usage at lower flow rates.

To make specific recommendations we need to know what your electrical rates are. Higher electrical rates provide more savings, which can sometimes justify the more expensive permanent magnet variable speed pumps, while lower electrical rates tend to give more savings to two speed pumps with their lower up front costs.

 

[Deleted member 62417]

Thanks for looking over the math!

We have tiered electrical pricing.
7.2cents, 10.9cents,12.9cents.

I expect the rates will go up this summer, so I would assume either 11 or 12 cents per kWh.

Derek

 

[JasonLion]

Those are what I think of as reasonably low electric rates. That favors two speed pumps and perhaps some of the least expensive variable speed pumps if you have a long swim season.

The two speed version of the 1 HP Hayward Super Pump is a good choice, as are several other similar models.