How to Size Ducting for Your Home
Proper sized HVAC ducting is crucial for maintaining a home's energy efficiency. Many times, a home will have too large or too small duct work installed. Too large duct work will cause your air flow velocity to dramatically decrease. This allows the air to warm or cool before reaching the registers. Too small duct work causes the static pressure to become too high to properly move the required air. This also puts a strain on the HVAC unit and makes it work harder. With a little time and a few calculations, however, it is fairly easy to correctly size ducting for your home. Does this Spark an idea?
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Instructions
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Measure each room to determine the square footage. Multiply the length by the width to get the square footage.
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Calculate the required CFM for each room. CFM stands for cubic feet per minute and is how airflow is measured. Figure on using one CFM per square foot, unless the room has a large amount of windows or is extremely sunny. Then the correct number to use would be two CFM per square foot.
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Use a duct calculator to figure out the duct size for the each room. Trane makes one called a Ductulator or you can visit online duct calculator resources (see Resources section). For example, if you have a 250 square foot room, one CFM per square foot would equal 250 CFM. Plugging 250 CFM into the duct calculator would yield a duct size of 8-inch round or 10-by-6-inch square duct.
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Continue determining the CFM required for each room. These numbers will determine the size of the duct work branching off from the main trunk of duct work to each room.
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Add the CFM of each room together to determine the size of the main trunk. For example, if you have 950 total CFM, your main trunk duct size would be 14-inch round or 20-by-8-inch square.
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Tips & Warnings
Locate the HVAC unit in a central location. This will allow smaller duct sizes to be run out in two directions versus a large duct running the length of your home.
