Getting lots of useful information from this board. Thanks.
I have some confusion about pool pumps. From what I am reading, the variable speed pumps are much more efficient than the two-speed pumps, which are more efficient than the single-speed pumps, but the typical explanation – that the pump can be operated at lower speed during low demand – is not at all satisfying.
If I used a bank of 3 single speed pumps (for about the same up-front cost as a variable speed) operating in parallel, then I could, in principle, duplicate the multiple 'speeds'. I could run the smallest only for low-level circulation, bump it up during heavy use, maybe run all three when cleaning, etc. But the tables I have seen show that none of the single speed pumps come close to the efficiencies of the variable speed pumps, so I'm obviously missing something.
I can believe that for a given head, a larger pump will pump slightly more gall/kWh than a smaller pump, due to internal efficiencies in the windings. And, for a given internal efficiency and plumbing configuration, a slower pump will be more efficient because the lower flow speed produces less head loss.
Thus, it seems that a given pump could improve its gall/kWh efficiency by running slower, but there must be a limit before residual power losses start to overwhelm the gain from reduced head loss. I find it difficult to believe that a 3HP pool pump stepped down to act like a 10 gallon aquarium pump is going to consume less power than the basic aquarium pump would do. So there must be some flow rate that optimizes efficiency, and I recognize that, if a pump is designed for maximum flow rate, the optimal flow rate may well be significantly below the nominal rate (reading previous posts seems to confirm this)
So do the variable speed pumps achieve better efficiencies simply by operating closer to this optimal efficiency than do single-speed pumps which are designed to maximize flow? If so, are there any single-speed pumps out there that target optimal efficiency rather than maximum flow? It seems that if I only needed x HP of power to my pump for most of the time, then I would be better running a x HP pump optimized for that condition than a much larger and stepped-down pump.
Or, is there something fundamentally different about the construction of the variable speed pumps that is not available as a single-speed?
If anyone has any recommendations on pumps, I would appreciate your thoughts. We are adding a partial in-ground (mostly in-ground, pad will be only slightly below water level) pool, about 17k gallons, with a single-story solar heater. Plumbing will be 2” for most of the run, splitting off to 1.5” lines at the pool.