I need to run an ice maker line around 50 feet from the softener and
RO system. I'm on a slab so that presents a problem running the line
underneath. It gets -20F about once a year here so running the line
in the attic so it won't freeze will be tricky. All the roof beams
run across the direction i need to go so I guess I'd have to drill all
the beams and keep the tubing as near the "warm" ceiling as possible.
Plus the attic is very very low with the 3/12 pitch roof.

On the other hand the cold air return for my furnace makes a darn near
perfect line where I need to go. The cold air return is in the slab
and it is crude at best! The bottom of the return is just plastic
sheet on top of gravel. They somehow made a form on the base gravel
and poured the slab on top of it.

Provided I make a nice entry and exit into the return why shouldn't I
run 1/2" PEX tubing through it?

thx