I was fortunate to grow up in a family where both of my parents lived through the Great Depression of the 1930s. Surviving during that period of American history established strong traits in both my mother and father. Neither was very keen on throwing away anything but absolute garbage.
There’s an old saying: “The devil is in the details.” This could certainly be applied to almost any HVAC system, including those using hydronics technology.
All factors being equal, warm and sunny days produce the greatest “Btu harvest” from any solar-thermal collectors supplying a domestic water heating system. The more intense the sunlight, the greater the heat input. The warmer the ambient air, the lower the heat losses from the collector enclosure.