plumbingwater-heater
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.
You are fine. Copper is pretty pliable and one soft metal. I am assuming that this branch is already working and capped. So if you do have any issues you should know right away. Also I would tell the plumber what you did so he isn't pulling out his hair if something goes wrong.
There is no benefit to "downsizing" the supply size; however, with tub fillers it usually is not a deal breaker because it just takes a bit longer to fill up the tub. If you look at the size of the tub filler spout, it may be no more of a restriction than you already have.
Both the stop valves and the threaded adapters that are screwed into your cast brass tub valve arms are available for 1/2" O.D. tubing (1/2" compression). Stop valves for 1/2" compression may be difficult (not impossible) to find in a "straight" configuration (as opposed to an angled 90° configuration), I would suggest that is the reason the plumber used 3/8" tubing and fittings.
How will an aesthetically pleasing shower mixer arrangement be created for this? I recommend that you consider changing the filler valve to a faucet set that has a shower diverter incorporated, there are many available styles:
Best Answer
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.