Background: Late in the summer last year I finally commissioned a DIY solar hot water pre-heat project. Very simple, drain back, open (vented) tank, PEX coil for a heat exchanger, PEX plumbing throughout, panel is about 10x12' with re-purposed aluminium "staple up" radiant floor transfer plates. Anyway, long story short, the performance has been disappointing, bottom line is that the storage tank temperature has never really exceeded 25°C. On the other hand, despite being in an unheated crawlspace in Halifax, over the winter it never was below 15°C, either, so its better than nothing (better then the cost of the pump, I haven't calculated). I suspected the solar controller, or rather, its analogue probes, of being a problem, and finally got around to installing a bunch of digital probes and flow meter (raspberry pi).
After a day of monitoring, its clear the flow is right at 15 l/min. I just now repaired some more wiring, and will update tomorrow with some sample temperature of the panel, tank, and return water.