The third is under-sizing
Some times contractors are in haste and they make erroneous calculations of load requirement of individual rooms in your house. They also fail to consider the type of system you have,the type of duct material, and also their location to be. As result they design undersized ducts. Undersized ducts restrict airflow. You will often hear whistling noises coming out of your ducts.
The fourth is Long ducts
The location of your HVAC is also plays an important role in allowing a good air flow. If your central unit is located in a corner from where serpentine ducts run to deliver air to your rooms then you should forget about good airflow. The unit should be centrally located to ensure minimal duct run to all rooms. Long ducts decrease the system’s ability to move conditioned air to remote sections of the house. Its velocity and pressure drops on the way.
The fifth is leakages
Leakages in the duct can cause havoc. They lead to loss of heated or cold air into the surrounding.Your system's efficiency can go down by 30 percent or more. So when designing and installing the ducts, care should be taken so that joints are secured well in mastic and foil tapes. There should not be leaks in unconditioned zones such as attic, basement etc.
The last is lack of returns
This is very common flaw in HVAC duct design. You can relate the situation to breathing through a straw when jogging. If your return ducts are too small it will not be able to return enough of room air to the cooling init. According to National Comfort Institute on an average a duct system returns 25% less air than required. The poor airflow will make your system work longer to produce the desired cooling. This will increase your power bills.