I have a very tight crawl. The original contractor was going to use a 16" round trunk, then he switched to a 12" round duct because of space limitations. But it is a mess. New possible contractor is telling me a 8x20 duct will work fine and it does fit into the required space. It would start under the downflow furnace connected to the base/supply plenum in the crawl. One side would turn up through the subfloor going vertical alongside the air-handler to feed 6 supplies from the ceiling of the second floor. The other side would continue the length of the crawl (near 55') to feed nine supply registers. He would supposedly build a directional baffle to help flow.
Before I go cutting holes and reframing some floor joist, does anyone see a reason why I should question this?