Every pool needs a pump and a filter, but many pool owners add a heater to complete the system. The heater is installed after so that only clean water reaches it. Some people think of pool heaters as unnecessary luxuries. But in cooler areas of the country, a heater lets you open the pool earlier in the spring and keep it open longer into the fall. In hot regions, a pool heater may allow you to use your pool almost year round.

Swimming Pool Heat Pumps
It is the heat pump in which heat is transferred to that water by taking the warmth out of the air that is created by compressing a gas. Pool and spa water circulates through the unit the same way as the other heaters, but does not pump any more heat than any other design of pool and spa heater.
A compressor in the unit exerts pressure on a gas, usually Freon, and generates heat. The water is circulated through a heat exchanger that is warmed by contact with the hot gas. The gas cools from contact with the water and is recompressed and heated to start the cycle all over again. The Freon used in heat pumps is a nonflammable, noncorrosive gas, which makes it suited to this application. Freon does not contain the chlorine component of the Flourocarbon that makes it environmentally hazardous.
Though expensive, heat pumps are energy efficient and last a long time. They are not effective spa heaters because they take a long time to heat the water in the spa. Because they rely in part on taking warmth from the air, the hotter the surrounding temperature, the better and quicker they work.