Willie, I think what Mattie was talking about, was putting one of these on my proposed *PVC-SIPHON* system (used with your excellent "Folded Riser" design)...
Because with the siphon system, the drainback would take place into the COLD tank, by breaking the siphon (= lifting the brick out of the tank...).
(although, thinking about it, there is no risk in any case of HW flowing back up to the collector, because the end of the HOT delivery pipe is in AIR (it delivers into a colander, fited in the top of the tank, to maintain stratification...).
(guess This is my fault, for mixing two designs...).
Regarding TANKS, Yeah, I like the sound of multidrum storage also ! the reason is, getting *any* storage into my crawlspace is off to be a tough one ! The access door is only about 80 cms tall by 70 cms wide...
So I am intending using THESE... 240-liter Yellow Wheelie Bin
Initially, I would use one YWB to receive the incoming hot water, and it would overflow/feed into 3 x 80-liter black tubs, representing the return side of the system.
So my next design element, is defining the STANDPIPE systems required to transfer the water inter-tanks...
Cannot tell you how much I am struggling here, to keep my feet on the pedals, I feel like the project is starting to take off ! (thanks to all latest feedback !)
In addition, I have to fight with Analysis Paralysis
, since I must admit to STILL having images of Curly Red Pex, laid in serpentines inside Black-painted Steel Trays...
what is keeping me focussed is, we have had a 10-day DARK SPELL - cold and rainy - I even had to resort to putting my infloor electric slabs ON...
so this is encouraging me to pursue this Slate Facade Development !
AND (following from recent post of just today...) I reckon I am abandoning the idea of pulling outside air in thru slates... Instead, I will add EXTRA HYDRAULICS into those 3 air slots -- far more calories to be had !
coffee time now...
cheers to all, thanks again for all the great inputs and feedback (Willie, however on earth did you come up with that Folded Riser idea, it is pure genius !)