What are the downsides to a DIY heat exchanger on my shower drain?
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 &quot;natural&quot; vertical fall available pay off. Insulate the completed assembly for best results.