Sizing a Gas or Heat Pump Swimming Pool Heater
You should have a trained pool professional perform a proper sizing analysis for your specific swimming pool to determine pool heater size.
Sizing a gas or a heat pump pool heater involves many factors. Basically, a heater is sized according to the surface area of the pool and the difference between the pool and the average air temperatures. Other factors also affect the heating load for outdoor pools, such as wind exposure, humidity levels, and cool night temperatures. Therefore, pools located in areas with higher average wind speeds at the pool surface, lower humidity, and cool nights will require a larger heater.
Gas pool heaters are rated by Btu (British thermal unit) output. Outputs range from 75,000 Btu to 450,000 Btu.
Heat pump pool heaters are rated by Btu output and horsepower (hp). Standard sizes include 3.5 hp/75,000 Btu, 5 hp/100,000 Btu, and 6 hp/125,000 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:
This formula is based on 1º to 1-1/4ºF temperature rise per hour and a 3-1/2 mile per hour average wind at the pool surface. For a 1-1/2ºF rise multiply by 1.5. For a 2ºF rise multiply by 2.0.