Very interesting. I’ve been thinking about this issue of “heat down the drain” a lot lately. I’ve started leaving the tub full of hot water after we bathe the kids under the assumption that the heat in the water will dissipate into the room. I have a feeling I’m not accounting for something in the thermodynamic equation though. I do know the additional humidity I’m creating in the house necessitates more energy to maintain a constant temperature, so maybe it’s a zero sum game I’m playing.

From the minimal information on the referenced web site, it appears that the waste water is being used by circulating it through a water tank thus transferring heat to the water that is inside the tank. It appears that this will take the load off of the conventional heat source to heat domestic hot water.

I did the same years ago, but using solar instead of recovered heat from waster water. It worked very well.