A common question we at O.S.B. shop often receive by customers is: “How much time does a heat pump require to heat my swimming pool/spa?” This is a great question, but not one that is easily answered. In this article, we discuss several factors that affect the heating time of your swimming pool or spa. The required heating time of your swimming pool or spa depends on factors such as air temperature, heat pump size, swimming pool or spa size, current water temperature, desired water temperature, and use of a solar blanket. We look at each of these factors in detail below.   AIR TEMPERATURE: As we explain in our article entitled how-does-an-air-source-swimming-pool-heat-pump-work, air-source heat pumps depend on air temperature because they use heat from the air to warm your swimming pool or spa. Heat pumps operate most efficiently in temperatures exceeding 50°F (10°C). In temperatures below an average of 50°F (10°C), heat pumps cannot efficiently capture heat from the air and therefore require more time to warm your swimming pool or spa.   HEAT PUMP SIZE: Swimming pool and spa heaters are sized according to their British Thermal Units (BTU) per hour. One BTU raises one pound of water by 1°F (0.6°C). One gallon of water is equal to 8.34 pounds of water, so 8.34 BTUs raises one gallon of water by 1°F (0.6°C). Consumers often purchase underpowered heat pumps in order to save money, but underpowered units have higher operating costs and require more time to heat your swimming pool. To properly size your heat pump.   SWIMMING POOL OR SPA SIZE: Other factors held constant, larger swimming pools and spas require longer heating times.   CURRENT AND DESIRED WATER TEMPERATURES: The greater the difference between your current and desired water temperatures, the longer you will need to run your heat pump.   USE OF A SOLAR BLANKET: In addition to reducing swimming pool and spa heating costs, solar blankets also reduce required heating time. 75% of a swimming pools heat loss is due to evaporation. A solar blanket retains a swimming pools or spas heat by minimizing evaporation. It acts as a barrier between the air and your swimming pool or spa. Learn more about. Overall, a heat pump usually requires between 24 and 72 hours to heat a swimming pool by 20°F (11°C) and between 45 and 60 minutes to heat a spa by 20°F (11°C). So now you know some factors that affect your swimming pool or spa required heating time. Keep in mind, however, that the conditions surrounding each swimming pool and spa are unique. Heating times vary greatly.