Here is a problem I ran into today. I probably do NOT have enough info.
1400 sq.ft. Ranch (one story) with plan to add 800 sq. ft.
100k btu input variable speed/ecm motor and 3 tons of cooling. Fan speed on Highest setting. Furnace located in garage at end of home. Plenume is 7 feet from top of "N" coil to top of Supply trunk. SUpply trunk runs 45 ft. First 20 ft is 24x8 then reduces to 18x8 for final 25 ft. There are currently 11 supplies off the top of the insulated Supply trunk. All eleven supplies are 8" round flex. Each flex run averages 10 ft in lentgh to a 8" ceiling diffuser. Each ceiling diffuser has a damper with handle. There are only 2 return air grills in the house. One is 8x30". It is in a "smoking" room. The other is a 20x25" located 10 ft from the furnace; in a hallway. The returns are also in the ceiling.
The system operates properly BUT has poor air flow/velocity. The supply trunk's final two ducts run to a "three season room". 3 sides of glass/outside walls. These two supplie's registers are are the inside wall. NO returns in that area of house--- idea to "draft" return back thru house/hall about 65 feet!
The 4" Media filter is new/clean.
The T'stat is located in a hallway with most doors closed.
Checked Fan speed/dip switch settings.
AIR VELOCITY IS POOR.
??? Would it help if the ELEVEN flex supply ducts were 6" instead of 8"???????