project solar heating greenhouse of 20m²

[tribalwar]

Hello,
I introduce myself: Eric I live in a rather cold area of ​​Germany. Passionate about gardening and new energy but little talented in electronics, I appeal to you for a little advice.
My project is to install a solar water heater on the north wall of my 20m² greenhouse. The principle is the same as a solar heating floor but vertically.
So I intend to use an old cumulus like hot water storage powered by a homemade solar collector.

The principle is described on this diagram and should therefore work. The only concern is at the engagement of the resistance of the balloon in case the solar water is not enough. As can be seen, the return water probe and the start water sensor each control the circulator. So PROBLEM !! When the water in the PER system is too cold, it will trigger the circulator but if the water in the tank is too cold, the resistor will turn on properly and shut off the circulator until the water balloon is hot enough but at the same time the other probe will trigger the same circulator from where a steady stop .... not good. I hope you can help me solve this problem because as said, me and relays ....

schema-chauffage.gif