@Doug_Bostrom
Interesting! My first system 35 years ago was drain-back. The pump required to lift water ~10' was so noisy you could hear it 100' away outdoors. Tried drain-down, the valves failed every year or so due to hard water (no exchanger required in US). So I added an exchanger and glycol beyond it. But the domestic water was in the big part of the exchanger tank, and eventually corroded it into leaking. Now I have a dual exchanger - glycol coil, domestic coil, and ~100 gallons of "dead" rainwater for heat storage. The domestic water is heated as it is used, passing through five parallel coils.

On any sunny day all year, it provides all my hot water, and sometimes floor or hot tub heat. If no sun, there is a gas heater and an outdoor wood boiler in the glycol system. (I'm totally #OffGrid )

Somehow it doesn't gain much without direct sunlight, though. I'm 39° N, and when a tree shades the panels it stops.

#solarDHW #SolarHotWater