Rooftop water tower
As building height increases, the vertical height of its plumbing also increases. This produces a large water column and the weight of this water produces very high pressure at the bottom of the column. Normally, this would require very thick (heavy schedule) plumbing to survive the pressure. Fittings at the bottom of the column would need pressure reducing valves to operate normally, and municipal water pressure would need to be very high to supply pressure to the top of the column.
Instead, the plumbing at various levels of the building are often sequestered, with pressure supplied from a rooftop water tower instead of the municipal supply. The tower itself is fed by a pump and a relatively high pressure line that carries water to the top of the building from the pipes below. Regular cleaning of tanks in use is necessary, in order to prevent possible water contamination.
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