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Definitive Design: ACCA’s Manual D Revised

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An experienced HVAC contractor knows there is no shortage of ways a system design could go wrong. There could be errors in the load calculation; incorrect sizing of the equipment for the application, or the air distribution system may not deliver the right amount of air to the various rooms of the house. Of all of these, equipment sizing generally gets the worst rap when a home is not comfortable, however, many times deficient duct systems can be directly attributed to poor system performance. The ANSI/ACCA Manual D (Residential Duct Systems) provides design guidance to ensure that conditioned air gets to where it’s needed, and it has just undergone a revision so that it continues to be the best resource possible.

Standard Procedure
Duct system design can be broken down to five main steps:

  1. Determine the external static pressure (ESP) for the design Cfm from the manufacturer’s blower data
  2. Sum up the component pressure losses (CPL) for things like the coil, air filters, and grilles
  3. Subtract the total component pressure loss from the external static pressure to find the available static pressure
  4. Identify the longest supply and return runs within the home to calculate the total effective length
  5. Use the available static pressure and total effective length to calculate the system’s friction rate

You want the friction rate to fall within ‘the wedge’ (a friction rate between 0.06 and 0.18 IWC) in order to avoid inadequate fan performance. Final duct sizing depends on whether the velocity of the air within the ducts is adequate for noise control.

New Motors – Updated Guidance
One issue that has arisen since the 2009 revision of Manual D is the growing prevalence of ECM motors. These motors are different than ‘normal’ motors because they change their speed in order to maintain a constant Cfm output. That means that the ESP for these motors can vary. That begs the question: what ESP should the designer use for step 1?

The 2009 Manual D advises the designer to avoid operating in the top 1/3 of the blower’s pressure range, which means that the duct system would need to be designed in order to produce no more than 0.67 IWC if the blower’s max pressure was 1 IWC. However, for ECM motors, Manual D now simply instructs the designer to use 0.70 IWC maximum for their design ESP.

Zoning Standard
In 2012, ACCA officially published Manual Zr for residential zoning. This new standard covers everything a designer needs to know about system zoning for enhanced occupant comfort, from load calculations and equipment issues, to bypass ducts. Therefore, the 2014 revision of Manual D referenced the updated zoned system design guidance in Manual Zr. Duct sizing for air-zoned systems remains a part of Manual D in sections 9 – 13.

Low Resistance Return Path
The revised Manual D now includes component pressure loss recommendations for transfer ducts and transfer grilles to be used for the low resistance return path requirement from Section 4. You can find these CPLs in the new Group 14 of Appendix 3.

New Column On Duct Sizing Worksheet
In the past, there has been some confusion about the Cfm used to size the duct airways. Manual D instructs the designer to use either the cooling or heating design Cfm for sizing, depending on which is larger. Some practitioners would then sum up the design Cfms used for sizing, only to find that the total did not match the blower Cfm.

Given that the practitioner must use the larger of the two values (cooling or heating design Cfm derived from the load calculation), it is almost guaranteed that some room Cfm requirements will be for heating while other room Cfm requirements will be for cooling Cfms. However, these two values will not be present simultaneously, and so summing up the design Cfms proves to be unnecessary since it won’t describe the system’s actual performance.

The duct sizing worksheet in Manual D now has a new column for a normalized Cfm. The normalized Cfms are based on the room’s average Cfm (heating and cooling), and can be summed up to equal the blower Cfm. This column will be useful for air balancing purposes when air-zoning is not used.

Great Contractors Invest In Themselves
This is a brief overview of the substantive changes to the procedures included in the 2014 revision of Manual D. Experienced practitioners will be able to incorporate these changes in their design work. Nevertheless if any of the points discussed here didn’t make sense to you, we urge you to take the time to learn (or reacquaint yourself with) the procedures contained in this industry standard.

Luis Escobar

Posted In: ACCA Now, Technical Tips

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