When a solar pool heater owner says that their pool feels warmer in the morning, especially this time of year, it’s usually a matter of perception. As you would expect, solar pool heaters only heat pools during the day when the sun is out and the pool pump is running. Naturally, the temperature in the pool will typically rise throughout the day, reaching the highest temperature sometime in the late afternoon. The pool then slowly cools overnight, losing part of the heat generated during the day to the ground and atmosphere. The temperature by which the pool drops is a function of the ambient temperature, ground temperature, wind, precipitation, and other environmental factors. A pool cover greatly helps retain heat overnight.
So why might someone think their pool is warmer in the morning than in the afternoon? It’s often a result of the “toe test.” Dipping a toe into the water in the morning, when the outside temperature is relatively low, gives the perception of a warm pool. Later in the day, when the sun is out and the ambient temperature is warm, the pool, which is likely warmer than it was earlier, may still feel cooler using the toe test.
Pool comfort is all about relative temperatures. When it is 60 degrees outside, a 75 degree pool may feel like a bathtub. When it’s 75 degrees outside, an 80 degree pool might feel chilly at first. And when it’s 95 degrees outside, an 85 degree pool may earn you a membership to the Polar Bear Club! Each person has different temperature thresholds for enjoying a pool, and that threshold often varies with season and time of day. The only real way to determine solar pool heating performance is to measure the actual pool temperature throughout the day, and compare that to an unheated pool (or other body of water that serves as an analog). The Gulf of Mexico surface temperature usually approximates the temperature of an unheated pool in Southwest Florida. A solar heated pool is warmer than an unheated pool every day of the year, the degree by which is a complicated relationship of weather and other factors. The perceived temperature of the pool based on the toe test further complicates the equation.
My advice is to jump in and recall the fact that you are enjoying a warmer pool than your neighbor’s unheated pool!