Dear Mr. Lowry:
Thank you for writing to us. Most heat pumps will run without freezing up when it is above 50 degrees. Some a little higher. Most of the days May 15 to Sept 15 are above that temperature so you don't really need a heater that will defrost for those dates but I like to make the season last as long as possible and open the pool as early as possible. Jandy heat pumps with the defrost are very good below 50 degrees. We have many videos on the site with them at very low temperatures.