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How Do I Determine the Correct Duct Size?

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  • Written By: Andrew Kirmayer
  • Edited By: Shereen Skola
  • Last Modified Date: 20 June 2014
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There are two main methods for determining the proper duct size for a heating, ventilation, and air conditioning (HVAC) system. The equal friction method determines the pressure loss based on each segment of the ductwork, while the static regain method is based on how static pressure at one duct branch offsets the fiction loss of the section before it. Other factors such as the cubic feet per minute, based on the size of the space accommodated by the air flow, the total length of the system, and installation of proper connections are important for duct installation and deciding on the correct size. If the duct size is too large, then energy expenses will be higher, while ducts sized too small won’t allow for enough air transfer.

With the equal friction method, the longest branch of the ductwork is factored into the total pressure loss, but components such as elbows and fittings need to be included in the calculations as well. The static regain method is more sequential. It takes into account the effects of the previous section of duct into the velocity of the air in the next segment. No matter which method is used, the size of the space to be heated or cooled by the air conditioning duct has to be known. In the case of multiple rooms, this means that one should combine the space of each room to figure out the proper duct size.

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The total duct size also includes the length of other duct supplies. Boots, elbows, balancing dampers, as well as grilles add to the length, so the correct duct sizing should be calculated using all of the elements that will be included. The final number does not have to be calculated manually. There are many software programs that can be used to calculate duct size, and many online programs are available for free. They provide a comprehensive interface for accurately calculating the size as well as the duct layout.

Deciding on the duct type affects what size is ultimately chosen, because duct trunks, branches, and radial parts affect the layout and airflow. If an HVAC duct system is not sized correctly, airflow might not be adequate to heat or cool a space. Air may leak, and the velocity of the air could create high noise levels as well. Low cost or even free software tools aid in the process of determining the correct duct size prior to installation.

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