To determine the correct size of a pool heater, you need to consider the following factors:
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Pool size: The larger the pool, the larger the heater youll need to effectively maintain the water temperature. Pool heaters are typically represented in BTUs (British Thermal Units), which measure the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
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Desired temperature increase: The temperature increase you want to achieve will also affect the size of the heater you need. As a general rule of thumb, youll need a heater with a capacity of 50 BTUs per square foot of pool surface area.
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Heat loss factors: Varying factors like wind speed, amount of sun exposure, altitude, and the presence of a solar pool cover will all influence the effectiveness of a heater.
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Pool and spa combos: Pool and spa combos may require a more substantial heater, depending on the volume of the hot tub itself and the desired heat-up time.
To calculate the minimum BTU rating recommended for your pool, you can use the following formula: Once you know the surface area, divide that number by 3, then multiply by 1,000 to get the minimum BTU rating recommended for your pool. This minimum will typically accommodate a 25–30ºF increase in temperature.
For example, if you have a 100-square-foot pool, and you want to raise the temperature by 30°F, and the heat loss factor is 8, the required BTUs rating would be: (BTU output) = 100 x 30 x 8 = 24,000 BTUs. So in this example, you would need a pool heater with a BTU output of 24,000 to effectively heat the pool water.
Its important to note that most people opt to purchase pool heaters that are sized at least 50k–100k BTUs higher than the recommended minimum to be on the safe side.
