[btbuildem]

Something as common as automotive coolant/"antifreeze" could efficiently travel via thin lines alongside regular plumbing.

I think a big part of the challenge would be in having multiple heat exchangers. Perhaps (if plumbing codes allowed) a plain p-trap could be swapped out for an exchanger unit that also serves as a sewer gas stop.

I don't know anything about thermodynamics; how long would it take to transfer a reasonable amount of energy from waste water into the coolant?

Another issue again, running all these coolant lines back to a central water heater w/another exchanger. It does all sound pretty complicated and expensive.

[rainbowzootsuit]

In vertical stacks surface tension makes flowing water cling to the perimeter of the pipe. The supply can then be coaxial like I think you are saying.

This is one product (linked below) like this although I think plastic would be fine if a little less thermal conductivity and less expensive than this much copper.

https://renewability.com/