About a year ago my wife and I purchased a house built in 1959 that has rectangular metal ductwork (uninsulated), the main portion of which runs mainly lengthwise through the middle of the house. This ductwork is mostly exposed in the partially finished basement. It measures approx. 9" high x 20" wide.
We're planning on eventually doing some remodeling and I'd like to replace the ducts with lower-profile ducts and returns, and build soffits around them where they will hang below the ceilings downstairs. I may also want to adjust the location of two or three upstairs vents, but only by a couple feet or so.
Would I be greatly impacting the efficiency of the furnace? Any chance of harming the furnace by it having to force air through a smaller channel? It appears that the current furnace - a Trane XV90 - was installed in 2003. There is also a Trane AC unit on the outside of the house.
I've seen rectangular and tubular ducting. For interior runs (not attic or crawlspace) does insulation matter? What type of ducting should I be looking at? I'm generally pretty handy, but know next to nothing about HVAC systems. Am I biting off more than I can chew?