1. VS pumps are FAR less reliable than 2 speed pumps. Repair costs on just the electronics is a large percentage (>70%) of the price of a new pump. They are a bit more reliable in California than Florida, since they are very susceptible to voltage surges caused by lightning.
2. VS pumps are only slightly more efficient than a *properly selected* 2 speed pump. But they are a 1-size fits all solution, and can be dynamically tuned to a given pool. This is a big advantage, since most pool dealers and builders cannot calculate flow dynamics on your pool.
3. In-floor vacuum systems, and roof-top solar systems typically are huge energy hogs. There's no fix that I know of for the in-floor systems. But, you will probably save money over 5 years by attaching a dedicated 1/2 HP pump to your solar system, that is switched by temperature. Then, you can 'turn down' your VS or 2-speed pump since you no longer have to provide the 20+ feet of head needed to push the water up to, and through, your panels. It's possible to do this with a VS pump, full automatic controller, and several electrical valves. But it's a lot easier to simply put an intermittently operated 1/2 HP pump in place.
4. Whether you can operate efficiently with a single VS pump, controllers and valving while servicing your roof top panels would require looking at your controller, valves, piping . . . and the pump curves of your selected pump. Most VS pumps don't produce the head at their lower speeds, that you'd likely need.
. . . I'll see if Mark wants to comment. He thinks more highly of VS pumps than I do. . . .
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