In my 1970 1700sq ft single-level ranch, I have a 1994-ish Rheem 80% single-stage 100K BTU down-draft gas furnace (no AC). The furnace itself is in a small closet in a hallway in the center of the house, about 15 feet away from where I sit to watch TV. The door to this closet is unventilated but hollow-core and weather-stripped - combustion air comes from the ventilated crawlspace and vents out the roof.
The supply ducts in the crawlspace seem well-sized - nine 14x4 registers fed by 6" metal duct mostly directly from the plenum except for one 10" trunk and one 8" trunk feeding 2 6" take-offs each.
The attic return is, I think, undersized: one 14x24 register box in the hallway with an eight-foot 16" spiral duct directly to the furnace closet with one 90 deg elbow to turn the duct down to the furnace. There is a short length of insulated flex duct between the elbow and the furnace return inlet.
The furnace has an integrated filter holder for 2 12x20" filters in a "V" above the blower. It's set on Med-Hi speed which for this furnace is around 1000-1200 CFM depending on static pressures. I've tried Med-low but the temp rise is too great.
The furnace always sounded like a jet engine when the blower kicked on, and I figured it was just a noisy blower until yesterday when I removed the return air grille while it was running and that seemed like half the sound right there.
So I did some research, and it does look like 14x24 is undersized for ~1100 CFM. So I need to add some return ducts. The return register box is more like a plenum - it's a good 26" tall - so I was planning to ask my HVAC guy to add 3 returns to each of the 3 bedroom ceilings. This would be one 8" duct to the master bedroom (which combined with the master bath has two 6" registers) and two 6" return ducts to each of the other two small (11x11) bedrooms.
I've been wanting to do this anyway because the furnace will pressurize the bedrooms.
1) Does my plan for additional returns make sense? What size registers should I be looking at for each of the bedrooms? Should I add even more returns to the living or dining rooms?
2) How do I minimize the sound transmission from the bedrooms to the hallway through the returns (for privacy)? Is 10' of insulated flex duct plus 2 90 deg elbows enough?
3) This furnace is still loud. The noise seems to be coming in equal parts from the burners, the blower, and the induced draft blower. I can feel vibrations in the hardwood floor outside the furnace closet. Is there a good way to vibration-isolate a down-draft furnace? It's currently sitting directly on the Rheem combustible-floor adapter, which sits on the plywood sub-floor as designed.
Thanks in advance,