The terms pressure drop and resistance are the same with respect to HVAC units and the elements, including filters, that comprise them. Airflow is a very important factor in an HVAC system. It helps in determining the efficiency of the unit and an excess amount can even spell problems that could result in expensive repairs or replacement of a system. The more pressure drop or resistance experienced by an HVAC system the less airflow that system has. The way a system is designed and installed as well as the filter used determines how much resistance to the airflow there is. This includes the type of duct work used, the amount and size of the room outlets and return air openings, the length and amount of turns in the system duct work, the filter size and other factors. It is important to try and keep the filter changed or washed as often as needed. As the filter gets dirty the resistance increases, thus increasing the total resistance of the HVAC system and lowering the amount of airflow. There is no easy way for a homeowner to tell how much resistance there is in the HVAC system. Check you filter often and if it is dirty, then it is time to change your filter or wash it.