If it\'s just a heat exchanger, it\'s almost certainly a feedwater pre-heater. Meaning it captures the heat from the waste shower water and uses it to pre-warm the fresh cold water before it enters the water heater. If they\'re using evacuated heat pipes to transfer the thermal energy and a whopping big exchanger on both ends, it could theoretically re-capture a large percentage of the difference between the gray-water temperature and the cold-water temperature. It would help a great deal with implementation if the water heater were in close proximity to the shower.
Without data, I\'m not willing to say how that adds up, but 15% doesn\'t seem to break any laws of thermodynamics.
Plasma Junkie
You don\'t get how they would use a heat exchanger? Have you seen HVAC heat exchangers? Imagine a set of co-mingled pipes/tubes/grids with the exhaust air or water flowing past the incoming air/water on adjacent tubes. In your house this transfers heat to incoming cold air in the winter time and vice versa in the summer time. 20-30% is entirely believable. They do the same thing with the waste shower water and the cold water that goes into your water heater tank.
Thermodynamically it\'s not a bad idea, but it\'s a plumbing nightmare. At 20% efficiency I don\'t see why anyone would bother except for some eco-nut. They could up the efficiency by using a heat pump and the waste water as the source bath. Again, unlikely to ever be worth it.
\"water heater\"?
If the captured heat is used to preheat the cold water going into the thermostatic shower mixer, no extra plumbing would be necessary.
Edgar Walkowsky
I would like to see them take the ubiquitous home concept further by linking the heat energy produced by the fridge, air con and hot water. For example, In summer, you could use the heat produced by the air con to heat water. Then in winter, you could use the cold air produced by a reverse cycle air-con to cool the fridge.
I think this is gross. I don\'t know about everyone else, but my shower needs cleaning pretty often. I can imagine this $600 device with all its surface area getting pretty nasty pretty quickly. And how are you supposed to even clean it? What about the time and energy it takes to clean it? It probably surpasses the amount of savings you get. I think this is a fine idea in theory, but horrible in reality. No thank you.
Nehemiah Spencer
If you are familiar with countercurrent flow in fish gills, or the blood flow in the paws of wolves tromping through snow, this would be the same concept. The idea is brilliant -- utilize any potentially wasted energy. Example link: http://www.jstor.org/pss/4448790
However, if you ask me, it is very poor design to put this bumpy thing on the shower floor. It seems obvious to me that it should be put under the subflooring i.e. in the joists. And yes, as someone pointed out, it would require the piping coming back from the heat exchange mechanism to be well insulated.
Nehemiah Spencer
@Edgar Walkowsky... very nice idea. You could really drop your energy usage. This would take a lot of engineering and refinement but ultimately could give a significant reduction in power bills for an average homeowner.
Sadly, this is a hard to clean mold factory.
I like the idea, but there are at least 2 products on the market already. One is plumbed into the waste water pipe, and the other one forms an intergral part of the shower tray. The second one is clearly a more elegant design.

Let\'s see, paying $600 for a device that saves maybe $.001/shower while freezing one\'s feet in most areas, such a deal!!
EROI or ROI is about 200 yrs if ever. No thanks.