In alt.home.repair
Jeff Wisnia wrote:

I've never head of a hot water loop

Quite common to see them used where needed in better quality construction.

Maybe more common up north. I'm in Texas and I have never seen them, even
in "better quality construction."

but for it to do any good, each
plumbing run would have to have one and it would require a pump to run
continuously.

Not necessarily. Some of them run by gravity ("thermosyphon"). Cold water is
heavier than hot. They work the same as the old "gravity" hydronic heating
systes.

Interesting but I can't imagine it moving very quickly. And in Texas, the
hot water heater is in the attic so it ain't gonna move uphill.