I have a residential renovation that has expanded the floor area from 1000 gross to 1263 gross Sq. Ft. and needs to have the duct work expanded.
The original house had 182.6 Sq. In. of supply and 152 Sq. In. of returning air duct. Of that 128 Sq. In. of supply goes to the bedrooms and bath and is not changing. Rooms excluding bath and kitchen have individual returns.
The longest existing supply duct run is 14 + feet. The longest new supply duct run is 18 1/2 ft.
New returns are already in place and provide 202 Sq. In area.
I am estimating the new supply duct dimensions, in relation to the old area with a desire to have the new duct going to the living, dining and kitchen provide more air than previously. Kitchen gets a 3.5x10 and the living and dining rooms got a trunk line of 10x13 (165 Sq. In.).
With a furnace that provides 1210 CFM what will be the impact of having a greater supply area in relation to return than previously (I get a proportion of 1.2 old versus 1.45 new)? To my best understanding that creates lower air velocity in the supply.
I also have some idea that the supply duct shouldn't be larger than the furnace opening anyway, is that true?
How does the duct size impact the systems efficiency?
Has this system been built into a problem?
Here's my thoughts, I do not have experience with furnace systems - it's hot down here!
There's alot to consider in your system. With the new addition was there added glass and type, added space that may no longer be under (or over) conditioned space, orientation (if the addition now projects further than the original structure). If any of these conditions are altered enough it could create a different heat load on the structure. If there's alot of glass added like a curtain wall you may want a professional heat load analysis done so you can make the best decision.
When we size A/C systems down here as a business practice we never max the unit out, usually shoot for about 80-85% capacity for the given load. We use a ductalator to size the ducting for about .08-.1 (drag coefficient I think as my ductalator is in the office) through the duct. We shoot for this number to minimize noise and maximize air flow. As much as possible we'll take a main trunk out full size as far as possible, then size the branches to the spaces looking to minimize the branch run.
Don't know if this will help you but it might give you food for thought. Hopefully one or two of the furnace experienced guys will give their input.
Semi-professional plumbing designer
Enjoying life in SW Florida