9 years ago we replaced a leaking 1.5 HP "Hydramax II" pump with a Whisperflo 1HP dual speed. There were issues with the plumbing, including a leak, so much of that was redone as well. However, we had an issue that when we'd turn the pump off, the water level in the pump would immediately drop and it would lose its prime. The only way to prevent it was to quickly close all the valves to prevent the water drop. Should we have installed a check valve before the pump? I guess we had an air leak as well or perhaps it was the sticky skimmer weir, because running the pump long enough, we'd eventually get too much air in there and lose prime again.
We just lived with these issues and now our Whisperflo died (it ran dry too often, I'm sure) and we want to make sure we correct the issue with this maintenance. One guy who came out took one look at the pump and said 1HP wasn't enough. He didn't seem to take into account the model of our pump, so we tended to discount this. However, our equipment is much higher than the pool and even more-so after our remodel 9 years ago, so we're wondering if this means we need a more powerful pump. We were hoping to use low speed most of the time to save power and ensure enough chlorination from our salt water chlorinator.
It looks like pump sizing depends on a lot of factors, some of which I probably don't have the answers to right now! We have a pool with attached spa that holds around 35,000 gallons of water. There are 3 skimmers, a main pool drain, main spa drain and 2 returns (that run into the same pipe). All the pipes from the pool are 1.5" and the new plumbing (above ground to the pump and filter) are 2". I'd have to take a wild guess on feet of head. Many of the lines are buried. The pump is about 2-3' above the water level. I plan to go out there and try to get some actual measurements, but maybe this is a start? The filter is an FNS Plus 48.