Please allow me an opportunity to correct some information. The operating range of a heat pump will depend on the manufacturer of the heat pump. Some systems DO operate when air temperature falls below 50 degrees.
edited:[In order to heat down into the 30's, you would need to look for units that offer a "Heat and Cool" option. The best source for reliable information on heat pumps is,
http://www.ahridirectory.org/ahridirect ... earch.aspx
This allows the frost, or ice build up on the evaporator coil, to defrost, allowing the heat pump to continue into cooler temperatures.] end edit.
Heat Pumps generally operate within the normal Northeastern swim season, Memorial Day to Labor Day. However, we realize that there are some limitations, and that occasionally, a cold front may drop the temps way below acceptable temps, but these would also be days that you may not be using the pool anyway.
Normal considerations for pool heating are: pool volume (weight of the water), desired water temperature, pool location (indoor or outdoor), and covered or uncovered. These parameters allows you calculate the BTU's needed. From there, any heater can be used to determine run times, or cost comparisons.
Since you're able to get more BTUs out of a gas heater, it will heat up water faster than a heat pump. Both will reduce in efficiency when it gets cold out.
As already mentioned, a gas heater will heat up the water quicker, but a heat pump will maintain it much more cost effectively. It's a matter of how you will be using your pool. If you only intend on heating the water up on weekends or on-demand, a gas heater is the way to go. However, if you're planning on wanting to use the pool whenever you're in the mood, maintaining water temperature is the main function of a heat pump.