How to Calculate CFM in HVAC

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In order to have a comfortable living space or room, you need to have a properly designed HVAC system. If you have too little airflow in a space, it may take a really long time to heat or cool to the desired temperature. It could never reach the desired temperature if airflow is really low. Too much airflow will result in a drafty feeling to the room and inefficient heating or cooling. Both of these can put a strain on your energy bill.

Things You'll Need

  • Tape measure
  • Ductulator or computer with Internet access
  • Calculator

Instructions

  • Measure the length and width of a room where you need to calculate the required airflow. Multiply these together to get the square footage of the room, i.e., if the room is 10 ft x 10 ft, the square footage would be 100. sq. ft. Airflow is measured in CFM, or cubic feet per minute. It is a measurement that shows how much air flows into a space per minute. You want 1 CFM per square foot, so the above room's requirement would be 100 CFM. For rooms with a lot of windows or have long periods of direct sunlight, you want 2 CFM per square foot.

  • Continue measuring each room and calculating the CFM requirement for each.

  • Add up each room's required CFM. This total is the amount of CFM your HVAC unit needs to produce in order to properly heat or cool the space.

  • If you don't already have a HVAC unit for the space, purchase one at your local HVAC retailer that matches your CFM requirement. Install the HVAC unit per manufacturer's instructions.

  • Using your Ductulator, match each room's CFM requirement to your planned friction rate. Most HVAC systems use a friction rate of 0.1" water column per 100 feet of duct. As you match each room's CFM to the Ductulator, it will give you the size of duct you need to run to the room.

  • Work back from the farthest room. As you add a room, make sure you add the CFM and increase the duct size accordingly. Install the HVAC duct from each room to the HVAC unit. A professional HVAC installer may need to be contacted for proper duct installation.

Tips & Warnings

  • Round ducts and fittings can usually be purchased at your local home improvement store if they are 12" in diameter or under. Square ducts and fittings may need to be fabricated at your local sheet metal shop.
  • Proper installation of all HVAC equipment is crucial for safe operation. Improper installation can result in a fire, and possible injury and/or death. Consult a qualified HVAC installer if you have any questions.
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