I think all of my collectors are Chinese. I bought them all from the same source, at the same time, but that source was just an importer.
The parallel v series thing seems a compromise whichever way it goes. My pipework is all 22mm copper and the flow rate with the pump running at 100% is about 13.5 litres per minute. This seems OK.
When I had the 4 similar panels plumbed as 2 parallel pairs I tried to get the flow between the 2 runs as even as I could. Judging by the output temperature of the 2 runs I was fairly successful in that; not exact, but close enough.
I changed to a series set-up purely to reduce the amount of exterior pipework. I figured that with the pipe 100C or more above ambient the losses must be significant and I could more easily reduce that by reducing the pipe length than improving the insulation. I hope I made the right choice, but who knows?
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