What do you consider "the most extreme climates"? Most of the US, and
practically all of Canada, are subject to regular conditions _well_ under
freezing, and we do NOT have plumbing along the ridgelines of our roofs.
To prevent ice-damming, attics are kept as cold as possible, and all
plumbing is kept below the insulation. If your attic is warm enough in
winter to prevent a pipe freezing, you're wasting too much energy.

Well... since I live in south Florida, where we haven't had a freeze
in years, and a really low air temp during the winter is 50 degrees F,
I have to respectfully disagree. :-)

You'd have to refer to the original thread for the complete proposal.
I lived in Vermont for many many years, and I'm aware of the issues
there. I would consider that an extreme climate, since as a kid I
delivered papers in -30 degree F windy weather, and moved out of state
after one winter where the sun never broke through clouds for two
months. Nick's stomping grounds of PA don't qualify as an extreme
climate in my book.

The full idea included a reflective barrier above the 4" pipe to
reduce heat loss, and the capability of drain down for those times
when the system wouldn't act as a preheater. IIRC, Nick was proposing
a solar pond or other involved project to preheat water with limited
results, and I tossed back a simple preheater idea that used minimal
materials, took minimal time to build, and had a faster payback.

A vented attic, in which the only function of the rood is to keep off the
house, nothing else.

I haven't a clue (and don't really care) if Nick's idea is sound, but his
objection to a 4" pipe under the ridgeline of a roof certainly is.

Only in Canada type of climates.

Nick objects to a lot of things. You would have to ask him why.