[mwint]

There must be two pipes, one insulated hot side, and one cold side, right? Otherwise the “used” water would cool the hot water going to the next house.

Such a system would presumably be metered by volume, and it’d incentive the right behavior (extracting as much use from each gallon of water as possible).

[dendrite9]

That's what I was thinking. I wasn't sure about things like hot tap water for example. I'd assume the heating water is a closed/mostly closed system like a scaled up version of a radiator system in old houses.