I’ve got to thinking about this, before I redid all my plumbing in the basement had maybe 50’ of 1” pex down there. Have almost twice that in copper now, had to add baseboard heaters down there, was never chilly before with the pex. Course, I moved quite a bit around and the longest loop is a secondary to the furnace HX.
Just thinking out loud here, but wonder if it changes how much heat copper would give off if it was painted black like car radiators?