I recently purchased an older home with an 18 year old HVAC in an attic closet that seems to distribute air unevenly. It also doesn't perform very well on moderately hot days. The air is delivered via flexible ducts in the closets to the first and second floors. The attic level has 2 flexible ducts and it delivers a strong flow of air. On the 2nd floor 2 bedrooms also get a strong flow but the 3rd bedroom and den only a trickle. All of the ducts on the main floor only get a trickle of air. The thermostat is on the 2nd floor. I also have a thermostat for the furnace on the first floor. The 2nd floor is always 3 degrees cooler than the first floor. Note the return duct is in the attic so it draws air from the attic forcing me to keep the attic door open so sufficient air can be drawn in. On hot days (85+) the system can barely keep the 2nd floor at 77 degrees.

Wondering what people think might be wrong. Is the system too weak to distribute air evenly throughout the house? Something wrong with the unit? It's old so need a charge? Thanks for all replies.

let me guess the contractor that did the job used a large flex duct as the main trunk in the attic and then cut holes in it to tie into for the smaller flex ducts that run to the rooms?

Also why would you not have the return duct into the ceiling of the 2nd floor? I think you answered your own question. That for sure needs fixing.

As far as age, if it works its not to old. The only thing new systems have over older ones is they are more efficent at cooling so cost of operation is lower. But even if you changed the units to the top of the line stuff, the duct would need correction.

As far as a charge. you first need to correct your ducts and air flow. Once that is done then you check your charge if the system is not performing well. To many contractors sell these system checks and adjust the charge as part of the service calls. If the system was installed properly there should be no need to do this. In fact its my opinion based on over 20 years in this industry it shortens the life of the equipment. Your refigerator is a mini ac system that only cools temps down further. When was the last time you had the gas checked. (never) All you do is vacuum the grill on the bottom or the back once and a while. Same goes for the whole house unit.
The biggest killer of AC systems new or old is incorrect air flow and lack of keeping the system clean.
Get an experianced AC contractor that does steel duct work and have him or her review your system. They will want to sell you new stuff because thats their business. They will tell you your system is no good and all other types of things. But stick to your guns and they will correct the ducts only. If the system does work when its cooler out even though it may be older it does not have to be changed, unless you want more modern equipment.