Pump Efficiency

[chem geek]

Thanks for all of the useful info. The formula you had for calculating output power turns out to be the same as I had (except yours is simpler and more direct). I now see that the main flaw in my solar panel argument was thinking that the head loss for each panel would be cumulative while, because they are hooked up in parallel, the loss should actually be the same as a single panel -- on the order of 2-4 feet of head (1-2 PSI). I have the FAFCO Revolution which has specs at this link where they have a chart for the head loss for each panel. So most of the head loss is probably in the piping to and from the panels and these are indeed long runs since to equalize pressure, the distances going to and from each panel are made approximately equal so that it looks like the flow goes through the full length of the roof twice (since that's what the furthest panel has to do), so that's probably about 80*2 = 160 feet. Then there's the path to and from the roof to the pool house which is another 50*2 = 100 feet. I understand I can ignore the height difference (except for priming) since the pressure loss going up is cancelled by the pressure gain coming back down, but I still need to count the height length of 15*2 = 30 feet. So the loss is 290 feet of 2" (inner diameter) pipe at the 65 GPM should be about 6.8ft/100ft so about 19.7 feet of head (8.6 PSI) so the total expected loss should only be about 10-13 PSI or thereabouts (not counting the 90 degree elbows). When I said I am seeing 15 PSI, that is probably exaggerated and the actual amount may be around 13-14 PSI so it seems to be in the ballpark.

I think it's the size of my property, the layout of where the pump house is relative to the pool and the house, and having a long and narrow (as opposed to square) house has led to long runs and large friction losses. The pool builder should probably have used 2.5" pipe for the long runs -- I'm assuming that the pipe in the ground is the same as the pipes "entering into" the ground at the pool pump (and verified that the pipes going to/from the roof are also 2" internal diameter). If 2.5" pipe were used, then the friction losses would be less than half of what I am currently seeing. Too bad for me -- I wish I had known all of this when the pool was installed (not that I could have convinced a builder otherwise -- I suppose he wouldn't care so long as I paid for the difference in pipe cost). And the solar panel people should have also used the larger pipe as well (at least for the long return runs, if not the panels themselves which can't be changed).

I suppose there is a lesson for others in all of this. If you have long runs of pipe, be it to your pool or for a solar installation or both, then encourage your pool builder to use the largest pipe (especially for the solar) to minimize friction losses. This will likely let you use a smaller pump motor and save on energy costs. Does that sound about right?

As for my situation, a replumbing to the solar (and maybe of part of the solar) may not be out of the question since the pipes don't go under any sort of hardscape -- they run under dirt near the property line. Changing the lines to the pool is out of the question, but that's not where a large pump is needed anyway. I'll see about getting an estimate and see if something can be done that would save enough money to pay for itself in a reasonable period of time. Cutting down to a 3/4-HP unit would probably save me about 30% of pump costs or about $350 per year. I suspect the new pump plus labor is going to make the payback time a bit long, but we'll see.

I'll also look at pump-replacement-only options such as a variable speed pump since that will help in the hotter part of the summer when the solar turns off because the pool is warm enough. That's probably a more economical option (more expensive pump, but maybe a lot less labor).

Thanks again, salinda and Mark (mas985) for your helpful advice.

Richard