If I were start again, with the benefit of hindsight I would not have installed any thermal solar panels but used all of my available (restricted by virtue of being in a Conservation Area) roof area for pv solar panels. On my back, south-facing roof I have 5 thermal and 6 pv solar panels. My jaundiced views of thermal panels are based on:
1. To prevent boiling of the glycol-water heat exchanger in cloudless skies during summer I need to cover the panels with a shade, rather resembling a banner. Fortunately I have easy access to the thermal panels for manually and selectively covering panels. In France the glycol mixture is drained into a sump whenever the domestic water temperature exceeds a safe level (ca 85 C) which would otherwise cause the glycol to run away to absurdly high (160 C) temperatures at which the fluid boils and lifts relief valves. An obvious solution would be to have electrically operated shades covering panels to an extent determined by the domestic water temperature. Manufacturers show no interest in such a solution!
2. Some of the savings produced by the thermal solar panels are lost in needing the installation company to drain/recharge the glycol/water mixture: cost in excess of £120 and required to be done about every 4 to 5 years.