The American Red Cross recommends maintaining a swimming pool temperature of 78 to 82 degrees Fahrenheit. Pool temperature is continuously influenced by wind, shading and elevation. The benefits of keeping a swimming pool at the right temperature stretch beyond simply comfort. Temperature swings affect pool chemistry and also the health of the water. The formula for sizing a gas purifier establishes the heater’s British thermal unit output per hour required to heat the pool into your desired temperature in one day.
Figure the surface area of a normal rectangular residential pool by multiplying the length in feet times the width in feet.
Consult an online weather data service to obtain the average temperature of the coldest month the pool is going to be used (see Resources). These statistics are generally available by zip code. Subtract that temperature in the desired temperature between 78 to 82 degrees to get there at the temperature rise.
Multiply the surface area of the pool by the temperature rise. Multiply that result by 12 to arrive at the required Btu-per-hour output of the heater. This figure represents the heater output necessary to heat the pool into the desired temperature in 24 hours, based on an average wind velocity of 3.5 mph.
Compensate for increased heat loss in windy places by scouring the required Btu/hour by 1.25 for places with an average wind velocity of 5 mph, or by two to get regions with a mean wind velocity of 10 mph, like coastal locations.