Now *this* is the kind of technology we can use. A lot of heat goes down residential and commercial drains, and under the right circumstances it can be reused.
Of course the economics have to be right. For example, if a device took more energy or cost more money to fabricate than it could ever recover over its life, it would make no sense.
Anyway, although the article doesn't mention it, this appears to be an application of heat pipes. A heat pipe is a device which can transfer large amounts of heat at small temperature differentials when properly applied - lots of technical background here.
Why is this a big deal? The rate of heat transfer between two objects ordinarily varies increases with increasing differences in temperature between the objects that are exchanging heat, and it decreases as the distance the heat must travel increases. And heat energy is the "leakiest" stuff around - the farther it must go the more of it you will lose even under ideal conditions. So it's terrific if you can exchange a lot of heat at low difference of temperature, because then it "leaks" the least for a given amount of useful transfer.
The bottom line is that you don't want to have to transfer heat very far, and that the less of a temperature difference you have the less useful the heat is. And those two factors account for the fact that you don't see a lot of household waste heat recovery systems.