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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.

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  • Have you measured the temperature of water-at-the-drain? You might be disappointed. Sep 14 '20 at 15:13
  • @Harper-ReinstateMonica of course not. It was just a Shower Thought :) Sep 14 '20 at 15:15
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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.

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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.

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  • This is just a comment, not an answer, but I doubt you'll ever get cost recovery from your proposed solution unless people are taking very long, very hot showers. The waste water from a shower is probably down to about 75-80 degrees by the time it runs down the drain. Your cold water inlet temp is probably about 50 degrees and time is needed for heat transfer. I really doubt you have a cost effective solution. If you are trying to reduce hot water heating costs, have you thought of hydronic solar panels? Sep 14 '20 at 16:04

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