I figured that the most sensible way to install theses is in the cold water path directly in the shower. That way (part of) the heat is recirculated in the show

I figured that the most sensible way to install theses is in the cold water path directly in the shower. That way (part of) the heat is recirculated in the shower. The savings then come from having to run less hot water and thereby having to spend less on water heating. This works even if you have a central store of hot water (as I think is more common in modern European homes these days).