This is always a good question. How fast should I run water through my solar pool heating system? Most think that running water fast through solar panels will not let them heat the water adequately. In fact, the opposite is generally true.
Solar pool heating panels work most efficiently when the difference between the panel surface temperature and the pool water is greater. As the pool water temperature approaches the solar panel temperature, the rate of heating decreases, making the panel less efficient. Most solar pool heating panel systems require an absolute minimum of 2 gallons per minute (GPM) per panel. Three to five GPM is a better target, depending on panel size. You won’t get much additional heating performance from higher flow rates.
Do not attempt to make your solar pool heater work better by slowing down water flowing through the panels. On the other hand, it is unnecessary to put more water through the panels than needed, which would just waste energy. The ideal scenario is to use a variable speed pump to “dial in” the optimum flow rate depending on the number of panels in your system. For example, if you have 8 panels, 32 gallons per minute might be ideal depending on the panel size, leaving a little extra flow to account for your filter getting dirty. Flow rates up to about 7 gallons per minute are fine for larger panels in the 4’x12′ range, but you get greatly diminishing returns from higher flow rates.
As you can see in the chart below for a FAFCO brand Sunsaver ™ solar pool heating panel, the panel reaches 80% efficiency with 4 GPM flow rate, and the curve really flattens out for flow rates above that.