The ACCA's "Residential Duct Systems Manual D."
The following is taken from the Air Conditioning Contractors of America's Residential Duct Systems Manual D.)

If noise was not a consideration, the size of every duct run could be based on a single friction-rate value (which depends on the available static pressure and the longest total effective length that is associated with the duct system) and the cfm value is associated with the run. (A duct run could be a branch run-out, a secondary trunk, a primary truck or a section of a trunk duct.) However, noise is an important design consideration, so these friction rate sizes are tentative.

The designer must also verify that all of the duct sizes are compatible with the velocity limits that are associated with supply side and the return side of the duct system. If a velocity limit is exceeded, the duct run that is associated with an unacceptably high velocity must be resized. In these cases, the final duct size is based on the maximum allowable velocity and the cfm that flows through the corresponding section of duct.

A set of work sheets and a reference chart has been developed to organize the details of the sizing calculations. The material in this section explains the procedures and calculations that are associated with these work sheets.

Basics for the sizing procedure

The duct-sizing calculations must be based on the blower performance data (including the associated footnotes) and the air-side accessory device pressure drop data (when applicable) that is provided by the hvac equipment manufacturer.

This information is required to correlate the design value for the system cfm with the design value for the available static pressure. It is the designer's responsibility to ensure that the pressure drop that is associated with the longest possible circulation path (longest supply run plus the longest return run) does not exceed the available static pressure. It is also the designer's responsibility to ensure that the velocity that is associated with any section of duct system does not exceed the recommended limit.

Balancing dampers are required

A duct system could be self-balancing if the ducts are sized to ensure that the pressure drop that is associated with independent circulation path is exactly equal to the available static pressure. In this case the following rules would apply:

  • Trunk ducts, which are common to multiple circulation paths, must be sized to accommodate the path that has the longest effective length.

  • Since the trunk sized will be too large, as far as the shorter paths are concerned, the run-out ducts must be sized to compensate for trunk sections that do not provide the required resistance.

    Even though this design strategy will produce a duct system this is self-balancing, it is not practical and the system could be noisy. The following comments apply:

  • The calculations would be complex and time consuming.

  • Nonstandard run-out sizes will be required to obtain the desired pressure drops.

  • The velocities that are associated with some of the shorter circulation paths could be too high.

    Obviously, these problems (complex calculation procedures), nonstandard sizes and noise) cannot be ignored. Therefore, the size of any duct section is a compromise between the standard size that provides the desired amount of resistance and the standard size that is associated with a quiet system.

    This means that a properly designed system will not be self-balancing. Therefore, a balancing damper must be installed in each run-out duct. (In this regard, it does not make any difference if the duct system is designed by the simplified method that is present in this edition of Manual D or the more elaborate method that formed the basis for the previous editions of this manual.)

    Available static pressure

    It is absolutely essential for the designer to verify how much static pressure is available to move the air through the supply and return ducts. Steps one, two and three, which are found in the "Friction Rate Worksheet," can be used to process this information.

    Step one

    Use the manufacturer's blower data to determine how much external static pressure (esp) is produced by the fan when it delivers the design cfm. (The value for the design cfm is discovered during the equivalent selection process. As explained in Manual S, the hvac equipment be selected and sized to satisfy the Manual J loads.) Note that it is prudent (but not absolutely necessary) to base the design calculations on a medium-speed fan. (Designing for medium speed provides the ability to adjust the fan performance after the equipment has been installed.)

    Foe example, refer to the blower data that is summarized by Figure 8-1. If 1,250 cfm is required for the application, this table indicates that, at medium speed, the fan will deliver 1,250 cfm when it operates at a resistance of 0.49 per inch water column (iwc).

    Step two

    Evaluate the device pressure losses (dpl) that are associated with the air-side components that will be installed in the critical circulation paths, and that are not generic to the blower performance data. This is important because the pressure that is dissipated by external air-side devices will not be available to the air through the fittings and straight-duct runs.

    Some of the pressure-dissipating devices that may not be associated with the published blower data include DX coils, electric resistance heating coils, heat exchangers, filters and humidifiers. Refer to the footnotes below the manufacturer's blower table for information about the devices that are not associated with the blower data.

    The exact values for the pressure drops that are associated with air-side devices are normally provided in the manufacturer's engineering information sheets. These pressure losses must be subtracted from the external static pressure that is generated by the blower.

    Step three

    Determine how much pressure will be available to move the air through the fittings and the straight-duct sections. To calculate this available static pressure (asp) value, simply subtract the device pressure loss from the external static pressure value.

    Refer to the data used in steps one and two above. In this case, the pressure losses for the auxiliary heating coil, one supply outlet, one return inlet and a balancing damper must be subtracted from the external static pressure. The following calculation shows that the available static pressure is equal to 0.26 iwc.

    Asp = 0.49 - (014 + 0.03 + 0.03 + 0.03) = 0.26 iwc

    For most designs, the available static pressure value should be equal to about 0.20 iwc. Values below 0.10 iwc are normally not acceptable unless the total effective length of the longest circulation path is relatively short. Values above 0.35 iwc are usually too large unless the total effective length of the longest circulation path is very long.

    (For information on ordering ACCA's Manual D, write ACCA, 2800 Shirlington Road, Suite 300, Arlington, VA 22206; call (888) 290-2220; see on the Internet.)