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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.
Basically none beyond the hassles involved in making it and installing it. While there's a bit of pressure drop from longer piping, it's minimal and can be eliminated, if it would be an issue, by choice of tubing size.
There are commercially-produced implementations of this concept, which (as I recall) are optimized for vertical pipe (the supply tubing is coiled around the drainpipe), and save enough energy cost on heating (at least for dorms/gyms/etc.) to make installing a drain pump to elevate the drain water on bottom floors where there's no "natural" vertical fall available pay off. Insulate the completed assembly for best results.
You'd probably be limited to 1/4" copper tubing since the 1/2" would be hard,impossible, to wrap around your drain pipe. You'd need quite a few wraps since the drain water mostly sits /runs toward the bottom of the drain. You could possibly run a larger pipe back and forth along the drain instead of around it. The hassles around fabrication would probably exceed the benefits of any heat recovery from the system. A lot would also depend on how hot the showers are.
