The complaint is about differential airflow.
Is there a comfort issue here? Different amounts of air are required in different parts of the house due to load requirements.
Basically, you probably have a duct system that is not balanced with the appropriate dampers installed.
From what I’m gathering in your post, it appears that there are too many takeoffs to have sufficient air “throw”. The more takeoffs you have the lower the volume of air because you have more holes in the duct.
The length of the return should not be an issue if it’s properly sized. The static pressure of a return duct must be lower than on the supply side or adverse conditions will exist.
Properly sizing the supply air duct, using the friction loss method will require the Main supply trunk line to be reduced in size as air volume is removed at takeoffs, maintaining static pressure along the entire length of the supply trunk. This design ensures an even static pressure throughout the supply. They can’t stop there.
Balancing dampers must be installed in the shorter high volume takeoffs to restrict airflow to increase static pressure for the longer/crooked takeoffs.
These calculations are performed using a ductulator. As you remove CFM at takeoffs, you adjust the design friction pressure per 100 feet of duct to the remaining CFM scale. This indicates the new duct dimensions that the line should be reduced to.
[size=2]Remember, the HVAC unit is a sucking machine not a blowing machine! It’s about removing the air from the house, reconditioning the air to the proper temperature/humidity and putting it back in the right proportion at the right location.
It’s not about blowing hot/cold air into a cold/hot room.
Both the supply and return systems must be balanced and properly located.
Also remember that one air duct system cannot be efficient in both heating and cooling modes. It must be designed for one or the other.
The location of your supply and return grills are critical for comfort. If you have the return registers and the supply registers in the ceiling, air stratification will occur and short cycling of heat air will occur at the ceiling resulting in a cold floor (in the heating mode).
What is the actual complaint?