The purported savings are comparable to what you would save by switching to tankless water heaters: http://apps1.eere.energy.gov/consumer/your_home/water_heatin

The purported savings are comparable to what you would save by switching to tankless water heaters: http://apps1.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12820 http://apps1.eere.energy.gov/consumer/your_home/water_heatin... The tankless heater also doesn't have the energy cost of building this grey-water heat exchange device. It does have the energy cost of being built itself, but that's probably comparable to the cost of building a standard heater with a tank. Your best bet energy-wise would probably be to go with a tankless heater as near as possible to the shower, maybe combined with a hypothetical system like the one in the article, except it would warm the water going into the tankless heater. Or you could not drain your bathtub until the water cools, if you need to heat your house anyway.