I have a shower stall built into a bathroom. It will not accept a tub in its current incarnation, so this applies to showering only. Outside of the hassle around fabrication, what are the downsides to thermally bonding the cold water copper pipe to a segment of copper pipe on the shower drain? The drain is sloped, so there is a lot of time that the water is in contact with the piping.
The idea is to preheat the incoming water so less hot water is needed from the thermostatic valve.