Hot air produced by a forced air furnace is cooler than hot water so
the "hot air pipes" (hot air ducts), as you call them, won't be a
If you can touch something comfortably than PEX will be fine.
The furnace flue is an entirely different story.
One of the benefits of PEX is the material is much cheaper than
copper and much easier to run & make connections.
I would suggest doing some research on PEX installation, consider
doing a "home run" installation.
Unless you're going to finish the basement ceiling, there is no need
drill through the joists.
If you do drill, keep the hole out of the middle 1/3 of the span &
drill in the middle of the joist depth.
Also stay away from the extreme ends of the joists.
A "main & branch" system is certainly ok but a home run system has
lots of advantages.
it should answer all of your questions
Thanks for the information, it is very helpful...
I considered a home run system, even bought some manifolds
from Home Depot, but couldnt find a logical and accessable place to locate
I am going to finish the ceiling, so I would like to drill the joists,
but 2 retired plumbers at Home Depot advised agains running the Pex near
the heating ducts
so I thought I would check the concensus here.