wrote:

...I don't have any sense of how big a deal this oxygen diffusion is.

Radiant floor people only worry about cast iron pumps.

For the DHW HX, it seems to me that the dominating issue is safety, and
copper tubing seems safer than plastic tubing...

....which loses 12% of its pressure rating for each 10 F rise above 73 F.

ENDOT has a lifetime guarantee that includes labor for the first 25 years.

Nick