Obviously you must use a pump as Hot Water Rises and there is no way it will come down from the roof to floor level.
I would suggest you put any pump at the end of the run before the water returns to the panels and get a metal body pump seperated from the motor if possible.
The reason is that the plastic pumps I looked at had temp ratings (if any) of around 64C and while I doubt the water would get that hot, but you never know.
I put a small pump on a donkey hot water system because I was loosing flow to the shower but I had no choice other than to put the pump right at the storage tank but for central heating, the pump works better when its at the end of the run because the water is cooler and oddly enough, pumps better according to an Army Manual I have on pumping liquids.
Also the motor doesnt get overheated as they are generaly enclosed to be used around water etc.
Due to a bit of enthusiastic over banking, I have blown steam out of the taps so had I used a plastic body pump, it most likely would have over time warped it beyond use.
you will have to investigate if your going to pressurise the system or have it ventilated as hot water expands and steam destroys, sometimes fataly.
I remember a home made Donkey system over pressurised and blew a concrete wall down and killed one or 2 who where in a shower area next to the system.
For pumps, have a look at marine bildge designed ones as they can be cheap and run on low voltage from what I have read.
Unfortunately my old Donkey split the tank and I havent been able to repair it yet so I went to a small Gas fired Instant unit which works damn well.
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