Termosyphon system and closed loop copper heat exchanger "dillema"!

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Romania,Buzau county
Hello everybody!
My name is Gabriel and I have some problems regarding a solar water heater that I just started to build.
I am using as a solar collector a central heating radiator type 11(one water circuit and one convective surface) wich I considered to be the best choice because of the "off the shelf" availability and ease in connecting it to the whole system.
I will not be directly heat the water and will use a heat exchanger wich is basically made out of 10 meters of 15mm copper with a wall thickness of 1 mm.
For the water container I bought a HDPE 200 liter barell but now when I "study" the problem further I am not so sure about this HDPE "thing",because in a hot summer day the water on top of the barell MIGHT reach and pass the 100 degree Celsius "limit" wich is the maximum for this material ,HDPE.
I will use a steel one, but my "problem" consists of the flow in the heat exchanger , considering that I don't use a pump to force the liquid through the heat exchanger, although the difference in height between the bottom of the barell and the top of the solar collector will be about 50 cm.
What do you all those with more experience think about this?
Should I worry or the system described by me will work with no problems?
Right now I am building the box for the solar collector , and after some thought I decided to make it out of steel and a 10-20 cm of glass wool insulation at the bottom and the sides.
Thank you in advance and please keep in mind that this is my first solar water heater , so experience is not my "strong part".
The good thing is that a lot of information is available on the internet.
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agost alicante SPAIN
Hi, maybe you could use a small pv powered pump to circulate the water through the copper coil heat exchanger.

I deal in solar water heaters which have copper coils heat exchangers, I think 10 meters will be a bit short, also it will be important to insulate the storage tank.

Good luck with it

Regards Tony cssolar Spain
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