Sorry in advance for the lengthy post.
Currently have Nat Gas 90% AFU furnace that is 12+ years old and was way oversized for the house (120k BTUh vs 75k BTUH with 20% oversizing) so I had to derate the furnace to keep if from tripping on hi heat all the time.
Based on age (I know not that old) and look of the heat exchanger I am planning on replacing with new variable modulating variable speed furnace in the spring with 2 zones, one for the first floor and one for the second floor
Did manual J and duct calculations and determined proper size of furnace and A/C. All branch lines are properly sized but the existing trunk lines, including return are wrong.
This is what I have determined. First floor combined CFM (heating and cooling) is 800. Second Floor combined CFM is 520.
Based on the above and about 800 FPM design on the trunk lines I would need a 8x20 for the first floor and a 8x14 for the second floor at 0.08 static
The return at 700 FPM design on the trunk line would be 10x25 or 12x25
My question is being a multizone unit, I believe I would need to increase each trunk line by 1/3 (less the return) to compensate for only a single zone calling for heat and install a static pressure by-pass damper?
Can you pls provide some guidance on the trunk line sizing with multizone units with variable speed blowers?