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DUCT DESIGN
3.4 Sizing
An efficient HVAC system must include properly sized ducts, regardless of duct type, shape, or configuration. Ducts should be sized according to the predetermined energy loads in order to provide necessary heating or cooling throughout the space. Oversizing or under sizing ductwork can lead to unbalanced air distribution and added strain for the HVAC unit resulting in energy loss. The fan in an HVAC system generates positive pressure in the system which results in the flow of air through the ductwork as it seeks equilibrium. As the air flows through the ducts the positive pressure decreases due to friction from the duct surface characteristics, changes in direction from bends, wyes, etc. This decrease is referred to as pressure drop, or friction loss, and it can be measured in inches water column (IWC). Each system component, direction change, etc. produce a resistance to airflow. This resistance creates a pressure drop measured in inches water column (IWC) which is physically equivalent to the pressure drop produced by a straight section of duct. For example—a 90 degree bend in flexible duct is considered to be 20 equivalent feet of duct because it exhibits the same pressure drop as 20 feet of straight duct (see Figures 4, 5, & 6). In determining the correct size, sum the total equivalent length of the duct run, including straight duct, bends, fittings, mixing boxes, and entrance and exit losses (See Figure 3 and example below). Size the duct using this total pressure drop value.
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Figure 3
The worksheet for determining the total pressure drop of the duct run depicted in Figure 3 will look like this:
Entrance fitting = Total duct length =
35 ft. 12 ft. 20 ft. 20 ft. 35 ft.
2 x 45° bends (2 x 10’) = 1 x 90° bend (1 x 20’) =
Exit fitting =
Total Equivalent Length =
122 ft.
Although the distance from plenum to terminal end in this example is approximately 12 feet, the total equivalent length used to determine the correct duct diameter would be 122 feet.
The equivalent length values for bends & fittings represented above are default values from ACCA Manual D and based on 900 fpm at 0.08 IWC/100 ’ for supply ducts and 700 fpm at 0.08 IWC/100 ’ for return ducts. The flexible duct system should be designed per the requirements of ACCA Manual D Residential Duct Design and Manual Q Commercial Duct Design.
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