In order for your forced air furnace to work properly, you have to have the right sizes of duct vents in each room expelling air. The duct vent size will depend on the size of the furnace and the size of your home, so you know how many vents you need in each room. The furnace's British thermal units (BTUs) will help calculate the size it needs to be.
Look at the plate on your furnace to figure out how many BTUs it produces. Divide that number by 10,000 BTUs. For example, if your furnace is producing 100,000 BTUs, divide that by 10,000 BTUs to get 100.
Multiply that number by the amount of cubic feet of airflow per minute (CFM) your type of furnace produces. A natural draft furnace produces 100 CMF, an induced furnace produces 130 CFM and a condensing furnace produces 150 CMF. For example, for a natural draft furnace, multiply 100 CMF by 100 to get 10,000.
Divide your number by 10. For example, 10,000 divided by 10 is 1,000 CMF. Your home needs ducts that support this much airflow.
Calculate the size of your ducts vents by multiplying the length by the width. For example, if the vents are 5-by-10 inches, the total CMF for the vents is 50 CMF per vent.
Divide the CMF per vent by the total amount of CMF for your furnace. For example, divide 1,000 by 50 and get 20 vents in the house.
Find the square footage in your home. Multiply each room's length by its width. For example, a room that is 8-by-10 feet would be 80 square feet.
Divide the number of vents by the amount of square footage in each room in your home. You will need at least one vent in every room that is 100 square feet or smaller and two to three vents in every room that is larger than 100 square feet.