Pump Selection for a Small but Unusual Pool

[AlanH]

Hi all,

After a somewhat (very?) confusing series of chats with Pentair I would appreciate your thoughts, advice and comments regarding the selection of a pool pump based on the attached diagram and the following specs.

Pool Specs:
-basically two AGPs: one oval (the "shallow end") "joined" with one round (the "deep end").
-finish - commonly available, stock vinyl liners (joined as one would for on-site liner installations).
-walls - ferro-cement. Buried to deck/patio level.
-circulation: 1 rim level drain/filter/sump; 4 wall returns installed 6" above floors.
-specs: 18'x66" round; 10'x16'x18" oval; volume=12,000 gal (approx); GPM=25 (8 hour turnover)
-power: 115 volt; utility cost - $0.33+ per Kwh; ENERGY EFFICIENCY very important. Plan to go off-grid.
-pump location: any elevation between pool floor and surface; about 10-12 feet from pool.
-plumbing: 2" and/or 1.5" depending on TFP input.

First, a bit about my chats with Pentair. Living off the beaten path on the Pacific coast of Nicaragua I use a micro-wave link for internet service. We use Skype for making calls and because or link was cutting out over several days my calls to Pentair for their suggestions would cut off suddenly and, as it turned out, I could never re-connect with the same person. The result, talked with four people and received four answers.

Pentair's suggestions:
1-Dynamo 2 speed, .75HP, but concerned that with 4 returns there wouldn't be sufficient pressure/flow for adequate circulation.
2-Dynamo 2 speed, .75HP, reducing plumbing to 1.5" or 1" would help???
3-Dynamo 1 speed, 1HP, should work
4-Whisperflo 2 speed 1HP (115V), but would be probably be overkill for pool volume and flow (GPM).

Having read a number of the threads on the economic and energy saving advantages of two (and variable) speed pumps they are definitely the route I wish to go (particularly at $0.33+/Kwh and future plans for being totally off-grid). Although the Dynamo (.75HP/2 speed) running on its' low speed (at 115V & 2.6 Amps) seems very energy efficient for the volume that I have (12K gal), would it be suited to the plumbing (friction loss) and number of returns (4) that I would like? Is any model of Dynamo suitable at all? I'm also not stuck on Pentair (was just a starting point). Other suggestions and recommendations welcomed. Thanks.

P>S> Click on diagram for a larger, clearer view.

 

[scooperhsd]

a 2 speed Dynamo (either .75 or 1 HP) would be plenty , even on low speed, to get a turnover at least once per day. You would then use the high speed when cleaning the pool. I don't think your plumbing size is going to matter too much on this either - so use whatever you can get.

For reference - I used single speed Hayward PowerFlo II .75 HP on my 19,000 gallon inground pool for 2 weeks last summer while I was waiting for a replacement to come in. That Powerflo is in the same size class as the Dynamos you're looking at.

 

[Bama Rambler]

AT 33¢ per kwh I'd be seriously considering a variable speed pump. However, if initial cost is an issue, I'd go with the 3/4th hp 2-speed Dynamo.

Going by your graphic, it all looks pretty good except I'd add a second skimmer in the East side of the pool. Just a suggestion for those times when the wind isn't coming from the prevailing side.

 

[AlanH]

Scooper, Dave, thanks. Re-assuring to hear the Dynamo would work.

Cost isn't so much a factor as the fact I would prefer to stick with a pump that operates on 115V, particularly if we go off grid and my solar/wind system is 115V. A step-up transformer is possible but then that has associated energy inefficiencies. From what I know all variable speed pumps run on 230V. Commercially available that is. I could switch out the motor of something like a Whisperflo (or even the Dynamo?) with a third party variable speed drive and inverter-rated motor that runs on 115V. Or, if anybody knows of a 115V variable speed pool pump please let me know.

But back to the Dynamo for just a moment and my own enlightenment (and perhaps other readers of this thread). Assuming the plumbing friction loss is such that 25 GPM for an 8 hour turnover is realistic, and there are 4 returns, does that work out to 6.25 GPM per return and is that sufficient flow through the returns to circulate the water in the pool effectively? And let's say that is on high speed. Running on a lower speed for longer periods would reduce the flow even more. Is there a point (other than 0 GPM) where the flow through the returns becomes ineffective for proper circulation? Am I thinking about this correctly?

 

[Bama Rambler]

The affecting factors are orientation of the returns and flow out of them. You can always install smaller eyeballs to increase the velocity of individual returns so that's not really and issue. You can even plug off returns to get the optimum circulation if need be.

In your scenario, I would install the two returns in the small pool on the North side at different depths and the two returns on the large pool on the South at different depths. That would circulate the small pool clockwise and the large pool counter-clockwise. I believe that would provide the best circulation using the least flow.

 

[JamesW]

The Pentair Dynamo model number DYNII-N2-3/4 HP is a two-speed pump that uses 115 volts.

http://www.pentairpool.com/pool-pro/pro ... mps-46.htm