The good thing about AC is that it generates heat as a by-product but in most cases, this is not used. Imagine if local air-con could provide all of a hospital/school/government building's hot water (and heating since they tend to roast in the UK).
Perhaps there is a market for a company that links the air-con user and local large users of hot water or even factories that need heat for their work.
Yes heat exchangers are well understood. You're mainly talking about the condenser side of an HVAC system. Cooling towers have been around a while. I imagine you'd need a hybrid approach. I doubt you'll find many buildings where the hot water needs are 1:1 with the waste heat from the HVAC system. Now you have two problems. Your hot water heater just got really complex.
However, building codes could mandate that energy for heating water first come from waste heat from the HVAC system. That would drive innovation in the space and who knows maybe it would become common. It's kind of like killing a fly with a hammer though, plus existing structures would get grandfathered in. Energy generation seem to me to be the better bang for the buck.
What would be really awesome is if someone invented a very efficient industrial thermoelectric generator that could be used in the condenser of an HVAC system. Think, regenerative breaking.
Perhaps there is a market for a company that links the air-con user and local large users of hot water or even factories that need heat for their work.