Honestly? It's simply not worth the effort. Yes, we can put a grey water reservoir who receive water from washing machines, dishwasher, showers, sinks and so on

Honestly? It's simply not worth the effort. Yes, we can put a grey water reservoir who receive water from washing machines, dishwasher, showers, sinks and so on, with a simple three way thermostatic valve who divert just hot water to the reservoir and than a heat pump suck heat for it to push in a room, but the quantity of heat we can recover is simply too little except maybe for some industrial activity that produce hot water as a result of some process (like an industrial laundry). It's simply too costly creating such system than the energy we can recover.