Pool heaters come in a range of types and capacities and you need to carefully select the right one for your pool. This is not as easy as it seems as although it is easy to calculate the amount of water to be heated there are other factors to take into account which are harder to quantify.
Heater capacity is quoted in BTUs per hour. For example the Raypak Digital (2100) models range from 206 – 406k BTU and the Hayward Universal H Series ranges from 150-400k but the Hayward 250 seems to be most popular. (One BTU is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.) A larger heater will be slightly more efficient but, at the same time, it would be pointless to spend more than is needed.
The factors to take into account are:
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