You can heat your swimming pool in a range of ways:

You could use a gas or oil boiler with a heat exchanger.

Bowman Heat Exchanger
We recommend the Bowman Heat Exchanger as it runs smoothly with the salt water purification system, being made with corrosion resistant cupronickel tubes and bronze end covers. The units are very easy to maintain as both the end covers and tube stack can be removed easily for cleaning, this is unique to the Bowman design.

Alternatively you can use Economy 7 – off peak electricity with a direct electric heater for use with the smaller pools.