If it's a closed-loop, say for hot water heating or chilled water for air conditioning, it's virtually impossible to cavitate the circulating pump. The biggest problem, by far, is selecting a pump with too much discharge head, especially in applications with a big change in elevation. The intial fill supplies the pressure to put water to the top of the system. The weight of the water coming down is the same as the water going up. It's like a ferris wheel. All the circ pump has to do is move the GPM required in a circle. Relatively low horsepower drive and corresponding ly low head.