Manual D Duct Design Explained | How to Size HVAC Ductwork Step by Step

Learn how ACCA Manual D residential duct design works and how HVAC professionals use it to properly size residential supply and return duct systems. A complete residential HVAC design follows an important sequence: Manual J → Calculate heating and cooling loads Manual S → Select the HVAC equipment Manual D → Design and size the duct system In this YouTube Short, we walk through the Manual D process from determining required airflow to evaluating blower performance, available static pressure, total effective length, fittings, friction rate, duct sizing, velocity, and final testing and balancing. You’ll also learn why friction rate should be calculated rather than automatically assumed, and why elbows, transitions, takeoffs, and other fittings matter. Manual D uses fitting equivalent lengths when determining total effective length, while available static pressure represents the pressure remaining for the ductwork after applicable non-duct component pressure losses are considered. The basic relationship discussed in the video is: Friction Rate = Available Static Pressure × 100 ÷ Total Effective Length Manual D also addresses important real-world issues such as flexible-duct sag and compression, blower performance, duct leakage, noise, air velocity, supply and return air distribution, testing, and balancing. This video is designed for HVAC contractors, HVAC designers, energy auditors, HERS raters, weatherization professionals, home-performance contractors, and building-science professionals who want to better understand residential HVAC duct sizing. For actual projects, use the applicable ACCA Manual D procedures, approved design software where required, manufacturer data, and applicable codes and program requirements. Learn more from the official ACCA Manual D page⁠. Explore BPMS™ — Building Performance Modeling Systems™ at BPMSCloud.com⁠.

Originally uploaded October 7, 2026