Manual D - Residential Duct Systems is an ANSI-recognized national standard for residential HVAC duct design. The publication provides (i) a single set of ANSI-recognized duct sizing principles and calculations that apply to all duct materials; (ii) system operating point (supply CFM and external static pressure), and airway sizing for single-speed and multi-speed (ECM) blowers; (iii) a method for determining the impact of duct friction and fitting pressure drop on blower performance and air delivery; (iv) comprehensive equivalent length data. This edition of the publication has been significantly revised to include (i) updated and expanded VAV guidance with detailed examples; (ii) impacts of excess length, sag and compression in flexible ducts; (iii) new equivalent length values for flex duct junction boxes. The procedures within this publication can be applied to both constant volume- and zoned variable air volume systems over a full range of duct construction materials. Includes a number of appendices related to air distribution systems including equipment and air-side components, controlling excess air when VAV dampers close, duct loads, duct leakage and duct system efficiency, air quality issues, noise control, minimum air velocity for ducts, codes, standards, best practice issues and commissioning.