Hello Seb:
My brother-in-law had the sweetest solar set up: Big box of water ( 2 tonnes ) with two pumps. Pump 1 would pump the water to the solar collectors when they were hot, other wise they would drain so no freezing problems. Pump 2 would pump water to the floor pipes when the house thermostat asked for heat. The one thing I would add was an underwater Snorkel stove to heat the water on sunless days. I have a snorkel stove in my hot tub and it works great!
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This is a picture where I was replacing a rotten end.
Carl.