I replaced the impeller on my Jandy Zodiac FloPro FHPM 2.0-2 with a 0.75 HP impeller to try to get better energy efficiency. I have 9 Heliocol HC50 solar panels on my roof, which is 25' high.
The solar was working fine with the 2.0-HP impeller in high-speed mode. When I changed to the 0.75 HP impeller, when it's in solar heating mode, with the pump in high speed, I can barely feel a current from the pool return in the spa. I see some spillover into the pool, but not a lot. At least today, I didn't see the pool temperate rise with the solar, and there was plenty of heat. When I would turn off solar (valve automatically shuts to the roof), even in low-speed, I could feel plenty of water return. Note that when solar heater is on, all of the water goes through to the roof with no bypass.
According to Jandy's specs:
there should definitely be over 25' of dynamic head at the flow rates I would expect. Would it be reasonable to just try a 1.0 HP impeller to see if that works, and if not, then a 1.5 HP impeller?