I knew what I was talking about when I said that the design and the mechanics are unbelievably simple - I forgot to mention that it although has the potential to get a theoretical physicist in trouble
Anyway I should have made the complete drawing in the first place. But it has been actually one more hour of work. But you should be less sceptic with me, I've build and have tested this design 25 years ago. No offense Dear, but you are anticipating too many if-then possibilities, which are 100% home made by your imagination. Nothing wrong though, with being sceptic and thus being able to anticipating mistakes and weak points in a design. Nothing wrong with anticipating - as long as one is right!
Back to the design and the facts:
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.
The design works perfectly well with a heater that has a small tank, 5 Liter as I said. I am not sure if those are available IN US or AUS, - In Europe they're still popular an often in use. But I've found this from Australia, which actually is exactly the same:
5 Liter caravan heater
I am quite sure that a normal tankless heater (the smallest available) will do the deal as well. Because this kind of heater has the same electronics, except that it is "flow triggered" as well. Btw: I've 3 Bathrooms and all have those kind of heaters installed.
They are switched on permanently, but the so called grilon gets only triggered when: A. the water flows through and B. if heating of flowing water is required (thermostatically), - which is perfect for the purpose. PS: In action with my 1000 Liter reservoir, the 5 Liter Heater was switching on and off alternatively for a few minutes, to keep the 1000 Liter stable at 34.5°C. Consumption was acceptable too.
Testing a tankless heater for the purpose shouldn't be rocket science. Just fix fittings to 2 PVC tubes, a pair of knees and fit a piece of hose to them, (putting a secondary tank in-between to avoid building up of pressure through expansion of hot water is strongly recommended) form some sort of coil with the hose and drop it in a bucket of cool water. Watch and learn...