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Local district heating utility here in Oulu, Finland has 190000 m^3 underground heat storage capable of storing 8 GWh of heat with 40 MW of input/output capacity.

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A friend in Linkoping showed me his utility closet where they tie into the city hot water system. I was surprised, they seemed too far from the center of town. But I guess the system of insulation and buried pipes makes it reasonable. I believe they had two metered taps into the system, one for heating the home and one for their pool. I assume the metering is based on the amount of water pumped into/out of the house. Is that correct or does it work from temperature differences?

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).

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.

At least here billing is based on amount of energy extracted, so both volume and temperature difference are metered.

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