In the business, we call them "Economizers". Often times our waste streams are needlessly hot and our input streams are too cold, so we'll have the 2 trade some heat to reduce out outlet cooling + inlet heating needs.

People have started doing the same thing with home water lines actually' pre-heat the cold water going to your water heater by taking heat from the shower drain essentially.

Also Town of Holland, MI uses waste heat for heated sidewalks in the winter.

Lots of ways to re-use heat if you look hard enough.
Source: 12 yr Chemical Engineer