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 yo

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.