New York City's heating system is powered from the "waste heat" of nearby electricity plants. https://en.wikipedia.org/wiki/New_York_City_steam_system
Not that NYC is particularly unique on this matter, plenty of different cities do this. https://en.wikipedia.org/wiki/District_heating
As such, New York City's heating is practically free (repurposed "waste heat" from the nearby power-plants).
Your definition of "100% efficient" is rather... bad. Both heat pumps, and NYC Steam Heating systems are far above 100% efficient by your definition.
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There's plenty of ways to go "above 100% efficient", when it comes to BTC-as-a-heater idea.
