So I've read the hydraulics sticky and still have some questions.
I have a small kidney shaped pool - I've heard estimates of size ranging from 12,000 to 15,000 gallons, it's 15X30 ft, 6 ft in the deep end. The bottom drain was closed by a previous owner. There's just 1 skimmer line and 1 return, both on 1.5" piping. The pool was originally designed with the bottom drain feeding into the skimmer, so it seems to have never had more than 1, 1.5" line going back to the pump even when the bottom drain worked. The pool stays perfectly clear and seems to have decent enough circulation (it's screened in so the skimmer doesn't have much to skim).
The current pump is a hayward maxflo .75 hp, I've been told it's 10-15 years old although the model number (SP2805X7) is a current model. We thought about replacing the pump with a variable speed after all the solar installers said it wouldn't get the water up on the 1 story roof. Of course, it in fact does, the solar panels (8) are installed and the water is circulating as usual. I don't know if the flow is optimal for the solar - 4 gpm per panel (8X4=32gpm) but it is circulating and the flow doesn't feel much different than it did before the solar install. There is a suction side air leak, I think the pump may have been run dry at some point since the pool did have a severe leak when we bought the house last year (had to remodel the pool and repair the bond beam). Filter is a cartridge type, also hayward (c900). Pool pad is on opposite end of pool from where the skimmer and return are (probably 40 feet away at least).
I have no idea what the TDH is for this system and I know that no pump is going to push more than about 44 gpm with the plumbing restriction. So bigger is no better. I was thinking that a 1hp whisperflo would be good, guessing the tdh was maybe 70-80 for the system. However, looking at the performance curve for the current maxflo pump, it looks like it is incapable of pumping beyond about 55 TDH so I'm guessing the TDH for the system, even with the added piping for the solar, must be less than 55 TDH or there'd be no flow, right?
So, now I'm looking at the whisperflo specs and it looks like a full rated, energy efficient, .5hp will generate 40gpm at 60ft head. And the 1.5" plumbing won't support any more than that anyway so more pump would just be stressful on either the pump or the plumbing and my wallet, right?
I'm concerned about downsizing the pump and I want to run the system at or above 32gpm for the solar but it sure is looking like the .5hp whisperflo would work, or am I missing something?
Given the pump is so small and I would be running solar (and thus require the higher flow rate) I'm also thinking that a variable speed pump, or even a dual speed pump is not worth the cost. Electricity here in south fl is 11 cents at most anyway.
Can anyone chime in and let me know if downsizing the pump really is a good idea? Am I going to end up with a pump that can't work the solar efficiently or can't generate the pressure to overcome the static head to get the water on the roof in the first place? I feel like I know just enough to get myself into trouble but not enough to be sure at all in my decision.