Stockholm.

Heat-pumps are no longer a single-family home story. From Stockholm’s district networks to a 9,000-flat retrofit in Hamburg, the technology is quietly displacing gas at city scale.

On the morning of 16 April 2020, the last coal boiler in the city stopped burning. The unit was called KVV6 — combined heat and power block six — and it had stood on the Hjorthagen waterfront in the Värtahamnen d…