Gas pool heaters are rated by Btu (British thermal unit) output. Outputs range from 75,000 Btu to 450,000 Btu.
Calculating Approximate Size
To calculate an approximate heater size for an outdoor swimming pool, follow these steps:
Determine your desired swimming pool temperature.
Determine the average temperature for the coldest month of pool use.
Subtract the average temperature for the coldest month from the desired pool temperature. This will give you the temperature rise needed.
Calculate the pool surface area in square feet.
Use the following formula to determine the Btu/hour output requirement of the heater:
Pool area multiplied by temperature rise multiplied by 12.