And the runner up is the electric heater
Second in popularity to gasfired heaters, electric heaters use an electric immersion element where water is pumped through the heater housing that contains electric heating elements.
Because they do not require venting to the outside, they are commonly found in indoor public pools where combustion heating is not allowed.
However, electric heaters tend to have a long heat-up time requiring a lot of energy; they work best for residential applications and smaller pools or spas.
The heat produced by an electric heater comes from an electric coil that is immersed in water flowing through the heater.
Electric heaters are considered 100 percent efficient, meaning that all of the heat produced is transferred to the water.
Nonetheless, it is best to choose the largest sized heater that the electric supply can support. Electric heaters over 3 kW require a dedicated supply.
To size an electric heater, determine the temperature rise needed for the coldest months the pool will be used.
Refer to the ‘Comparisons of Gas and Electric Pool Heaters’ chart on page 15, and find the required heater output. For example, to achieve a 20 °F temperature rise with a 300 square foot pool, the heater output is 73,000 BTUs.
Since an electric heater is 100 percent efficient, the required output equals the required input, and the BTUs can be directly converted to kilowatts.
Divide the BTUs by 3413 since one kilowatt equals 3413 BTU.
In this example, 73,000 BTU/3413 = 21.4 kW. Finally, round up to the next available heater size.