Most homeowners don’t feel that their ductwork is an issue with the home not being cooled or heated correctly. Their first guess is that the unit is old or inefficient. That might be the case but along with the unit the ductwork needs to be addressed. Technology changes and so do codes. Codes that help the customer from having a newly installed unit fail prematurely. The technology comes in the form of how these newer ac/heating systems are more efficient. In most cases you have 2 stage units now with multistage blower motors that will help save you energy. But in the same time they require more airflow. There are cases to where a unit will get changed out and the customer still feels uncomfortable in the rooms addressed before the unit replacement. That is due to old ducting coming apart, poorly designed, not enough duct runs or just undersized ducting. Ducting in most cases do not need to be replaced or upgraded because of the rigidity now a days. But sometimes they do. The lack of airflow, and amount of airflow in a room will be very uncomfortable. Remember, the unit needs to be sized correctly to the sq/footage of the home. So therefore the ducting needs to comply with the same logic. The bigger the home the bigger the ducting. Possibly by adding another return (filter location), or an upsized return using the same one that you already had would do the trick. When these units are installed sometimes the return was never altered to comply with the new unit and the return duct just plainly collapses because it’s not enough airflow passing through that duct. Like breathing through a straw or breathing through only one side of your nostril. It just doesn’t feel right. So therefore you would need the correct air coming through the return grill to give you more sufficient airflow to the rooms. Dampers are sometimes even added to a newly designed ductwork to help balance the airflow correctly to the designated rooms. Duct sizing per room is also measured for correct amount of air for that particular room. The ducting insulation itself also makes a difference and air leakage if not sealed correctly. There are codes installed now when you pull a permit. It comes with a 3rd party company that performs a duct test to make sure everything was done correctly and there is minimal amount of air leakage. It passes, the inspector signs it off. The protection to the homeowner and the piece of mind that everything was done correctly. There are other reasons why your home would be uncomfortable like the fact that the attic is lacking insulation, the walls are thin and lacking insulation, the sun hitting certain areas of the home, the amount of windows, single pane windows, etc, etc. So many other factors. But this one would be the first thing I’d take a look at if the unit had already been replaced and the home is still not comfortable.