My system is now running.
In total I have pipes with a length of 11 meters (36 ft), diameter 2.5 cm (1 inch) and with a surface of about 0.9 square meters (9.7 sq. feet) installed.
They're served with a pump with a maximum capacity of 600 l/h (160 gal./hour).
I can regulate the flow rate and I think I'll put it down to a minumum, because I read that's best for solar absorbers.
My tank has a capacity of 200 l (53 gal.).
On the pics you see all the pipes facing the south side of my greenhouse and the small pump (connected to the green pipes which serve as inflow and output pipes to/from the black absorber pipes).
I'll cover them with black insulating material from the back, so the sunlight will reflect and the additional heat can be absorbed by the pipes.
I'm planing to run the system (with an automatic time switch) only during daytime and heat up the water in the tank. The pump can handle up to 35° C (95 F) warm water, but I don't expect it to go that high.
I hope that the heat radiation of the tank's surface will be enough to get some extra warmth for my plants.
If it won't be enough I can install an extra heating circuit, with an seperate water pump to let the warm water flow through at night. I have a temperature controlled automatic switch, which I could use for the pump to start below a certain temperature and stop again if temperature is high enough. But that's still optional. I'll see how it works for now.
In the last days the water at the top of the big tank reached around 21° C (70 F) in the day, but the water at the bottom was cooler, because there was no circulation. Now there is, because the pump gets cool water from the bottom and the water gets back in the tank at the top.
From tomorrow on, I could post updates and let you know if it works and the water get's warmer. Today it was too late to see some effects.