I read a post that said that there is a rule of thumb for sizing ducts with 20-30 sq inches needed for every 6 inch branch duct. I have a branch trunk that is 8x18 in and this serves 4 [6 inch metal] ducts. The main branch duct then tapers to 8x10 inches and 3 [ two long 24 ft 6 inch combination hard and flex ducts and one short 3 ft long 6 inch hard duct] ducts are served by the 8x10 section. If I understand the rule this duct system is at the low end of acceptable size. 8x18=144 and this gives about 20 square inches per 6 inch duct[ 7 ducts]. I had poor air flow from the last three ducts farthest away from the furnace. I opened up the drywall and found that two of the flex/hard duct runs had problems. One long duct was disconnected from the main trunk and the connection between the hard and flex duct had a loose connection with gaps. The second long duct had almost the same problem and was nearly disconnected from the trunk and had a gap where it connected to the flex duct. The 3rd duct at the end of the branch trunk has a big gap where it meets the supply vent.
I was thinking that reconnecting the ducts, sealing them with mastic and converting the flex portions would be all the fix I would need. The airflow is greatly improved just by connecting the ducts back up.I told this to one of the contractors I am considering and he suggested on the phone that I could upsize two of the long 6 inch runs [ one from the 8x18 and one from the 8x10 section] to 7 inches to get more flow into the farthest room. Based on the rule of thumb I think that upsizing the ducts to 7 inches may not help. On the other hand I have high static, .5 on the return and .35 on the supply with a dry coil and maybe increasing my duct size would help. I would appreciate your comments. Is this where a ductulator helps? The alternative would be to run a new 8 inch pipe off the plenum to supply the two long runs. The problem with that would be that the new 8 inch trunk would have 3 to 4 ninety degree turns to get to the long runs.