Comment by Aachen

Isn't heat free energy in a place like London? I know very little about metro systems so please correct me if this is insane: wouldn't the people living above the tubes be happy to get a heat exchanger (passive) or heat pump (active, but takes more of the heat) that prewarms their hot water supply? People still take warm showers and boil tea and rice/pasta in summer, and in winter the purpose should be obvious. If the water comes in at 30 instead of 10 degrees C, you need to add only a few degrees for showers and floor heating