[GpsFrontier]

you should be less sceptic with me

I am sorry I didn't mean it to seem that way, I know you had built it and it worked. I just have a hard time understanding unless I can see the whole picture in my mind. I see what you are talking about now and how much simpler it is than I was imagining.

I should have made the complete drawing in the first place. But it has been actually one more hour of work.

I am sorry to have put you to that much work for me, but I do want to thank you for it. I do understand now how it is all set up and runs. If it's any consolation I am sure that this thread will be able to help many more people than just me.

anticipating mistakes and weak points in a design. Nothing wrong with anticipating

Yes for good or bad, that is a part of me trying to see it in my head. I do try to anticipate things that might go wrong before I build because I want to get it right the first time.

Important: as we deal with a closed circuit, the pump sucks water out of the "secondary reservoir" (actually tank) and in the same time it pushes it through the circuit into the heater - and back in the secondary reservoir! Hence the secondary reservoir avoids build up of pressure (but allows a closed circuit) in the most simplest and effective way.

I don't know why I was not seeing it that way but I understand what you are saying now. It's much simpler that way. I want to thank you for taking the time to help me understand.

But I've found this from Australia, which actually is exactly the same: 5 Liter caravan heater

I don't have time right now but want to thank you for this link so that I can see what you are using for the heater, and compare it with the tankless ones that are what I am familiar with (even though I have never used either).

(putting a secondary tank in-between to avoid building up of pressure through expansion of hot water is strongly recommended)

I hadn't really thought about that fact, but makes plenty of sense.

Thanks for your help!!!