I am ripping my entire house apart and will be making almost an entire wall out of windows and doors,(glass). I will be changing all of the floor plans, (walls), so at the moment I am adding HVAC to the upstairs. I want to install a furnace upstairs, along with an electrical sub-panel. I plan on making a separate "mechanical s" room for the furnace and electrical, which will be placed in the center of the existing chimney. I plan on installing exposed spiral ducting down the center of the upstairs vaulted ceiling. Do I need to run cold air return ducting back to this secondary furnace? Can I have a gap on the bottom of all doors, instead of running ducting? Thank You
A Manual D load calculation will give you each room's airflow and return air path requirements. Those return air paths can be provided by dedicated return ducts, jump-ducts, or undercut doors. Undercut doors represent the least desirable approach because the amount of airflow that can pass under a door is about 50 CFM per inch, and a one inch undercut is pretty big but 50 CFM may not come close to the airflow going into the room. A one inch undercut also creates privacy concerns and may look just plain ugly.
By far the superior solution if you don't want to use dedicated return air ducting is to use jump ducts that allow air to pass through common walls and into the central hallway, and then you can have your simple central hallway return. The preferred approach is to do the ducts through the wall, not over the ceiling, which would put them in the unconditioned attic.
I am assuming you did a Manual J calculation to size the equipment correctly and determine whether or not a second furnace needed in the first place, right? And that you did the calculation with the assumption that all air sealing and insulation work were done first so when when you eventually do them later, this equipment won't be hugely oversized and create comfort problems, right? And that the results of this calculation convinced you that a second ducted furnace was actually required and cost-effective vs a single unit with zoning, a mini-ducted mini-split heat pump, or a series of ductless mini-split heat pumps? And that you were able to find a furnace with a small enough output that it won't short-cycle all season?
The reason why I ask is that if the answers to most or all these questions are all "No", then it's time to go back to the drawing board, because you could be setting yourself up for disappointment.