I would definitely not use PVC for forced hot air.
I can't imagine that it is covered by code.
Even though you are not holding any pressure, PVC pipe is rated @ 74 F, so if it gets warm it's strength will go down so the pipe/duct will be sagging and bent all over the place. You also might pick up some smells and VOC's.
Since this is for ductwork, this is going to have to be pretty large pipe (unless it is some kind of high velocity systems) and there fore pretty expensive. Fittings are expensive too.
PVC is often perceived as "cheap" even though it is actually kind of expensive.
I have customers hook up chillers to PVC, and turn of the refrigeration system but not the pump. Lots of sagging pipe the next day.
Good Luck,
Clyde