Plumbing scheme - please review!

[eljefe281]

I was hoping some of you could review the plumbing scheme that I've come up with, after lots of research and self-education (much of it here!).

~12,000 gallon rectangular pool, with (2) 24" sheer descent waterfalls
~850 gallon square spa, raised 6", with six jets

Main circulation pump
I am proposing to use a 2-speed 3/4 hp Pentair WhisperFlo pump for main circulation. I would like to be able to turn my pool over in roughly six hours while operating the pump at low speed. This would require a flowrate of 33 gpm.

I have not yet performed a rigorous calculation of my system head loss. I may, however, open a bottle of scotch one evening and do just that. But I digress...

Using a guessed range of 45-65 feet of head at high speed, I come up with a high speed flowrate ranging from 54-76 gpm (read from the high speed pump curve).

Then, by trial-and-error, I ratio those head-flow combinations down (head loss proportional to flow squared) until I find a point that lies on the low speed pump curve. This gives me a low speed flowrate of 27-37 gpm at 11-16 feet of head.

This suggests to me that my proposed main circulation pump is big enough to do the job, but not oversized.

Waterfall pump
I am proposing to use a Pentair WaterFall pump AFP-75. Based on preliminary data, sheer descents require 10-15 gpm per foot of length. This would require 40-60 gpm for the entire assembly.

The AFP-75 will put out 16-22 feet of head at this range of flow. This seems to me to be more than enough. There will be very, very little friction loss in this circuit - the water isn't going through the filter, heater, etc. In fact, I intend to have a valve on the pump discharge to give it a little backpressure to work against so it doesn't cavitate.

Spa jet pump
Originally, I was planning on having the main circulation pump feed the spa jets while operating at high speed. I am now rethinking this and leaning toward a separate pump to feed the spa jets. There are two reasons - first, it allows each pump to operate most of the time at its most efficient speed, and second, it allows me not to operate the spa jets until the spa reaches the desired temperature. Does this make any sense? Will the spa heat up noticeably faster if I pump a higher flow through the heater during the heat-up cycle? Anyway, the jets my builder is proposing evidently deliver 13 gpm each at 12 psig. I assume there is some pressure drop across the jet nozzle, but I don't know what that is (5 psi, maybe?). Anyway, this setup would require 78 gpm at maybe 45-50 feet of head or so. It seems like a Pentair WhisperFlo 1 hp would probably do the trick, albeit a bit oversized (I may be able to find a better fit). One concern I have is that I may have to add another non-jet return to the spa to accommodate all of the flow of the main circulation pump in spa mode.

Your comments are very, very much appreciated.

[eljefe281]

Nobody?

OK, let's boil it down to a couple of questions.

If I run my main circulating (filter) pump on low speed while heating up the spa, is it going to take a lot longer to heat up? I wouldn't think so - the heat input of the heater is the same.

How many standard 2" pool returns (not jets) will I need to flow 30-40 gpm to the spa? I would think one would probably do it, two at the most.