@kapellosaur @quixoticgeek whereas i have never lived in a house that has a hot water tank like that. all the houses i've lived in - at least, that i can remember - had either an immersion heater and tank or a combi boiler feeding the hot water taps. immersion tanks are somewhat smaller than 500l (mine here is 180l), are also not considered drinking water (and indeed, the water that comes out of my immersion tank smells distinctly metallic), and these days tend to be fed from water pressure rather than gravity, which means that if the water pressure falters, the only way to get the contents out of the tank is to figure out how to drain it. and combi boilers have no water storage at all.
as an aside: the most frightening heating / plumbing system i've ever seen was in a flat i looked around in Hawick at one point. it was very cheap, and i soon discoered why; aside from the large fungus that had eaten its way right into the lounge, one room - and not a small room - was entirely dedicated to a central heating system that looked like it had lain undisturbed since its inventor had patented the whole idea of circulating hot water around radiators. probably because every new contractor tasked with modifying it had come in, looked in that room, and henceforth resolved to retrain in a less terrifying career - like repairing ultra high tension pylons