Heat loss calculations for swimming pools

Heat loss calculations are important in determining the appropriate size of a pool heater, as well as in estimating the energy costs of operating the heater. The amount of heat lost by a pool will depend on various factors such as the pool size, the water temperature, the outdoor air temperature, the wind speed, and the pool cover. Here are some common formulas for calculating heat loss in a swimming pool:

1. Heat Loss Due to Evaporation: Water evaporation is the largest source of heat loss in a swimming pool. The heat loss due to evaporation can be calculated using the following formula:

Heat Loss (in BTU per hour) = 1.1 x Surface Area (in square feet) x Temperature Difference (in °F) x Wind Speed Factor

The wind speed factor depends on the wind speed and is calculated using the following table:

For example, if you have a rectangular pool that is 20 feet long, 10 feet wide, and 5 feet deep, and the water temperature is 80°F while the outdoor air temperature is 60°F, the temperature difference would be 20°F. Assuming a wind speed of 10 mph, the heat loss due to evaporation would be:

Surface Area = 2 x (20 x 5 + 10 x 5 + 20 x 10) = 700 square feet Wind Speed Factor = 1.6 (from the table) Heat Loss = 1.1 x 700 x 20 x 1.6 = 24,640 BTU per hour

2. Heat Loss Due to Conduction: Heat loss due to conduction occurs when heat is transferred from the pool water to the surrounding ground. The heat loss due to conduction can be calculated using the following formula:

Heat Loss (in BTU per hour) = U-Value (in BTU per hour per square foot per degree Fahrenheit) x Surface Area (in square feet) x Temperature Difference (in °F)

The U-value represents the thermal conductivity of the pool shell and insulation materials. For a typical outdoor pool with an uninsulated concrete shell, the U-value is around 1.5 BTU per hour per square foot per degree Fahrenheit. For a pool with insulation, the U-value can be lower.

For example, if you have the same rectangular pool as in the previous example, and the outdoor air temperature is 60°F while the water temperature is 80°F, the temperature difference would be 20°F. Assuming a U-value of 1.5 and no insulation, the heat loss due to conduction would be:

Heat Loss = 1.5 x 700 x 20 = 21,000 BTU per hour

3. Heat Loss Due to Radiation: Heat loss due to radiation occurs when heat is radiated from the pool water to the surrounding environment. The heat loss due to radiation can be calculated using the following formula:

Heat Loss (in BTU per hour) = Surface Area (in square feet) x Temperature Difference (in °F) x Radiation Factor

The radiation factor depends on the outdoor air temperature and is calculated using the following table: