Drain heat recovery is quite common here in Canada and mandatory in a few jurisdictions for new builds. https://www.homedepot.ca/en/home/categories/building-ma

Drain heat recovery is quite common here in Canada and mandatory in a few jurisdictions for new builds. https://www.homedepot.ca/en/home/categories/building-materials/plumbing/water-heaters/drain-water-heat-recovery.html https://www.homedepot.ca/en/home/categories/building-materia... As for the hot humid bathroom air, that also exists in the form of ERV (energy recovery ventilators). For well sealed houses you route all the exhaust and intake for the home to one place where there is a heat exchanger to minimize the losses from getting fresh air into the house.