A vapor-compression heat pump generates hot air. But many old buildings transmit heat using hot water. This only works if the water is quite a bit hotter than a

A vapor-compression heat pump generates hot air. But many old buildings transmit heat using hot water. This only works if the water is quite a bit hotter than a heat pump usually gets, because the water heats by blackbody radiation, the radiation power is proportional to T^4, and the effective surface area of the existing radiators is calibrated to the old drive temperatures. Heat pumps are a drop-in replacement if you have forced-air heating, but if you have hot-water radiators, it's not so easy. In theory, you could add another stage, but now you're sacrificing a lot of COP. New buildings should always be built with heat pumps (and heat exchangers) but old buildings take a lot of work to fix.