I'm skeptical. To get meaningful energy transfer in a heat exchanger, you need time for the two material streams to be in thermal contact with each other. Thi

I'm skeptical. To get meaningful energy transfer in a heat exchanger, you need time for the two material streams to be in thermal contact with each other. This is done by either increasing the surface areas using some porous or tortuitous channel, or just making the channels really long. In either case, it means you have significant pressure drop. Now, a significant pressure drop in the incoming flow is bad enough, as I like a strong shower. But more concerning, is the pressure drop in the drain, which is already only at atmospheric pressure. I predict lots of plugged drains and showers that you're wading ankle deep in water. Or... more likely, they found all this out, reduced their pressure-drop, and ended up with a device that doesn't really save that much energy.