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Duct sizing rule and chart
Duct sizing should always be based on manual J load calculation and a manual D duct design. If you are not ready to attempt this, an approximate airflow can be found by measuring the volume of the room, and dividing by 6-8 air changes per hour, length x width x height x 6 air changes/ 60min to give cubic feet per minute. This formula simplifies to room volume/10.
You can then find the corresponding duct size on the chart below, taken from ADI duct calculator accepted by the state of Texas, TDLR. steelduct.org
Dirty Air filters reduce airflow. Using hepa or allergen reducing filters will reduce airflow, effectively reducing the apparent cooling or heating power of your system. Replace filters quarterly or monthly for best results. Chosing a mid level pleated filter gives you the better airflow while still protecting the system. If you must use restrictive allergen filters, try to use 2" or 4" wide filters that have more surface area.
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