hoping i could get some advice from the experts on this board. i have a CAC installed back in 1990 in my attic in an 80 year old house. it still works but appears to have several issues. the main air supply goes through a large trunk that has branches to flexible ducts that run through the closets and such providing supply lines throughout the house. out of the 11 supply lines only the 2 lines in the finished attic space and 2 lines in 2 bedrooms on the 2nd floor get sufficient air flow to make a difference. the whole first floor and half the rooms on the second floor get a trickle causing a good 4-5 degree overall imbalance in cooling between the 1st floor and 2nd floor (ironic since i always read about how the 2nd floor is always hotter than the 1st!).

i only have one return line and its in the finished part of the attic right next to the air distribution unit. i think this was done because the return registers would need to be run alongside the supply lines and there's no room in the closets for them.

so i have 2 questions. how do i get a better distribution of cooling throughout the 11 registers? second, is having one return line in the attic causing problems with cooling the house? thanks for your help.

could the problem have been caused by someone driving a nail through the wall (as in hanging artwork and the like) and piercing the supply lines in the affected rooms? Also, there should be balancers at each of the 11 supply lines in the attic which allow for manual flow control - could some of these have been knocked into a closed position by accident? Have you checked all filters? Have you called a reputable HVAC tech to pressure test the line? There are many variables here, also not sure if it's just it's 18 year old age starting to show - time gets us all in one way or another!

To correctly answer your question on the air flow to the rooms and making it work better. We need some more information from you.
1. What is the main trunk in the attic made of. Steel with insulation. Round duct that appears hard when you push on it, or round duct that is the same as the ones that run down to the rooms>
2. When was the last time the filter was changed? and where is is located? Ceiling or inside the blower located in the attic?
3.Where exactly is the return duct opening? In the attic or in the hallway in the 2nd floor area?
Its not uncommon to find diffrent air temps in each room. With load profiles such as the sunny side of the house. Amount of trees covering the house, the amount of windows, and size of each room. and a host of other things that effect balance.
4. Was the system working right when you had it put in many years ago? Or was it always out of balance?
5. What size are the ducts? and to what rooms are they running to.
Reason for this answer is that quite often installers will oversize the ducts to a room so the air flow going to it slows down and become more quite. If you push air through a smaller duct the air flow speed will pick up and become more noisy as it leaves the register at the ceiling. To offset this quite often the installer will install two ducts in lets say a bedroom wiht lower air flow in each but enough combined to cool the room.
6. Are the air blance dampers open at the duct connection points? These tend to loosen up and close.
As far as balanceing of the ducts. you need to be carefull. Closing off air supply to one duct to push more air into another room does not always work. Its working depends on the quality and proper sizing of the ducts that go to the rooms in the first place.

so sorry, simply trying to provide a layman answer for whatever value one can derive from it - did not realize a certification was required to post on this board

Also, be sure that something hasn't collapsed one or more of the flexible branch ducts, or that the duct connections to the trunk duct and the diffusers haven't slipped off or begun to leak air to the attic.