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Properly Design And Size Ductwork To Maximize Comfort And Improve Building Health


The critical role of ductwork in a heating and cooling systems is routinely ignored or simply not understood. Even when an air-conditioner and furnace are properly sized, a deficient ductwork system will adversely affect heating and cooling performance, owners’ comfort, building and occupant health, and operating costs.

Our Director of Training Explains:

Manual DOften, poor heating and cooling performance is attributed to the air-conditioner and/or furnace, when the actual problem is undersized ductwork, improper layout or poor workmanship.

The only method to correctly determine duct layout and calculate trunk, supply and return sizes is to complete an ACCA (Air-Conditioning Contractors of America) compliant Manual D design. In fact, current building codes, specifically 2018 IRC (International Residential Code) section M1601.1, require duct analysis and design according to the Manual D procedure. The codes further specify installation requirements, specifically, 2018 IECC (International Energy Conservation Code) section R403.3, requires ductwork both to be sealed and insulated which is reiterated in IRC section M1601.4.6 and ACCA Manual D A10-5.

Benefits of a Completing a Manual D or Correcting Duct Deficiencies

Correcting Duct DeficienciesThe chief goal of proper ductwork layout and sizing is to design and carefully install a duct system that works with the heating and cooling equipment, specifically the indoor blower motor. Design coupled with high-level installation craftsmanship provides numerous tangible advantages to homeowners. Below are some of the benefits:

  • Improved temperature control year-round. The delivery of optimized airflow and even air distribution in each room or zone offers equal temperatures throughout the home, particularly during extreme weather events such as the heat of summer and the cold of winter.
  • Enhanced moisture regulation and comfort within the home. The ability of a heating and cooling system to maintain consistent indoor relative humidity within a safe and comfortable zone is determined by numerous factors. A duct system designed to minimize air friction, resistance and pressure drop within the ductwork, through the indoor fan motor, and across the evaporator coil enables the heating and cooling system to properly regulate moisture in the air.
  • Healthier home to live and work. Infectious bacteria and viruses thrive in humidity extremes. Bacteria, mold, and allergenic mites and fungi thrive in high humidity environments, while viruses prefer low indoor air moisture to stay suspended longer. Maintaining indoor humidity between forty and sixty percent reduces many adverse health effects. Good ductwork design and sizing ensure proper airflow, velocity, mixing and delivery to keep relative humidity at the optimum level.
  • Reduced operating costs and monthly utilities. Typical duct systems lose 25 to 40 percent of the heat or cooling produced by the furnace or air-conditioner. These losses manifest in two ways, 1) heat loss or gain through conduction due to a lack of duct insulation and 2) air leakage through unsealed joints and connections. Appropriate duct layout, correct sizing, joint sealing, and insulation can achieve a 50% reduction in energy losses resulting in considerable annual savings.  

Ductwork System Design

Manual D DuctworkA Manual D residential duct design begins where the Manual J and Manual S end. Recall from previous articles, an ACCA Manual J is a heating and cooling load calculation aimed at assessing how much heat a home gains or losses depending on the season. The Manual S procedure uses the load calculation to properly select the correct type, and size or capacity of heating and cooling equipment to meet the homes load. The Manual D follows this same logic.

Air Duct DesignThe duct designer begins with ACCA Manual T air distribution design considerations. The designer determines the layout and location of trunks and branches, determines the number and size for each supply outlet and return inlet, and finally select the type of register or grille to maximize throw, drop and air mixing within each room. At this point, the duct system is complete except for the size of the trunks and each branch duct. Good ductwork design uses the heating and cooling equipment selected through the Manual S procedure, specifically the indoor blower motor information to correctly size the ducting. Motor performance data provides the ductwork designer with the amount, or volume of air measured as cubic feet per minute (CFM), and available pressure to push air through the duct system.

As air moves through any type of duct, the sides and surfaces create friction and resistance to airflow. The layout of the duct system, particularly the total length of the duct more specifically the longest run, type of fittings, connectors, and air supply and return grilles and registers all contribute to, and affect the amount of friction within an air distribution system. Given a proposed duct system, as the volume (CFM) of airflow increases within the trunk and branches, the amount of resistance increases. Therefore, the size and type of heating and cooling equipment selected through the Manual S procedure as determined by the Manual J load calculation must be considered when sizing the ductwork. As the equipment size and therefore the indoor fan motor CFM increases, ductwork must correspondingly increase in size to keep airflow friction within a manageable range, and ensure proper airflow across the air-conditioner evaporator coil.

Designing a new ductwork system or assessing existing requires careful attention to detail, skill and training.

Home Performance Group Heating and Cooling Services in Kansas City

If you are building a new home or finishing a basement or space, make sure your contractor is trained and experienced in performing Manual D ductwork design. Be prepared if they ask you to pay for it: Manual D analysis and design is time-intensive and requires several hours or more to properly complete all necessary steps, calculation and analysis.

At Home Performance Group, we continue to invest in technical training and up-to-date software so that we can correctly layout and accurately size our customer ductwork systems. We have performed numerous designs for our clients and prospective clients. Our in-home consultations are no cost and include a copy of the duct design with a system purchase. Alternatively, if you choose not to buy from us, we can provide you the ductwork layout and design for a nominal fee.

If you are interested in a no cost in-home consultation, schedule with a Solutions Advisor today



Article by Larry L. Motley Jr., 21 June 2021

Larry is a graduate of both Wentworth Military Academy and Missouri Western State University earning a double bachelor’s degree in Economics and Finance. Additionally, he maintains six professional tradesman licenses in two states and advanced credentialing in green technology, project and program management, and process improvement. Larry is a three-time combat veteran having served in Operation Iraqi Freedom, Operation New Dawn, and Operation Inherent Resolve. He continues to serve through a value-based building science company focused on providing clients the best design, highest quality installation, and most honest repair services in the community.  

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