In some houses, the hot water is fed by an open header tank in the ceiling. Whatever dust, dead mice or bird poos fall into that, can contaminate the tap water. I went off drinking hot water as a kid after seeing the deep layer of fine dust built up in the bottom of it.
As pointed out in a different comment thread here, that’s a header tank which was used in older, multi-level buildings where city water pressure was not adequate for the rise to upper floors. Builders would set up a tank that slowly filled using the lower pressure and could act as a sort of mini water tower for the building. This did not contain hot water, but there is a chance a hot water tank was using this supply.
Where I lived, the cold water was at much higher pressure than hot. I assumed it was common for cold to be mains pressure and hot to be from a header tank when present. Maybe it was an unusual case of retrofitted hot water (house was likely built before the invention of hot water).
