I live in a 30 year old block of flats; two flats per floor, three floors.
Each flat has its own, conventional, vented hot water cylinder.
In the roof void of the building there's a set of header tanks, all interconnected (so effectively one big header tank).
In the service duct through the building is a single 3 inch riser, supplying mains water to each flat and refilling the header tanks in the roof.
From the base of the header tanks there's a single 3 inch downcomer, feeding the base of each of the six hot water cylinders. Another 3 inch downcomer feeds domestic cold to each flat.
There appears to be a separate vent pipe (22mm copper) running from the top of each cylinder up to a separate 'overflow' tank up in the roof (access is difficult, so we're not entirely sure about this). This tank has six copper pipes hooked over the top - and it's empty. There's no ballcock to fill it, and it just has a discharge/drain pipe to a visible point outside the building.
Can I, unilaterally, convert my hot water cylinder into a heat bank? Would Building Regs forbid it?
I've been trying to figure out the risks, in this arrangement, of the contents of any cylinder commingling with the contents of another cylinder in another flat. There are no check valves, so reverse flow into the common downcomer is theoretically possible, but I can't see a mechanism that would cause it.