Measure Guideline: Optimizing the Configuration of Flexible Duct Junction Boxes

Research output: NRELSubcontract Report


This measure guideline offers additional recommendations to heating, ventilation, and air conditioning (HVAC) system designers for optimizing flexible duct, constant-volume HVAC systems using junction boxes within Air Conditioning Contractors of America (ACCA) Manual D guidance (Rutkowski, H. Manual D -- Residential Duct Systems, 3rd edition, Version 1.00. Arlington, VA: Air ConditioningContractors of America, 2009.). IBACOS used computational fluid dynamics software to explore and develop guidance to better control the airflow effects of factors that may impact pressure losses within junction boxes among various design configurations (Beach, R., Prahl, D., and Lange, R. CFD Analysis of Flexible Duct Junction Box Design. Golden, CO: National Renewable Energy Laboratory,submitted for publication 2013). These recommendations can help to ensure that a system aligns more closely with the design and the occupants' comfort expectations. Specifically, the recommendations described herein show how to configure a rectangular box with four outlets, a triangular box with three outlets, metal wyes with two outlets, and multiple configurations for more than four outlets.Designers of HVAC systems, contractors who are fabricating junction boxes on site, and anyone using the ACCA Manual D process for sizing duct runs will find this measure guideline invaluable for more accurately minimizing pressure losses when using junction boxes with flexible ducts.
Original languageAmerican English
Number of pages29
StatePublished - 2014

Bibliographical note

Work performed by IBACOS, Inc., Pittsburgh, Pennsylvania

NREL Publication Number

  • NREL/SR-5500-61438

Other Report Number

  • DOE/GO-102014-4396


  • available pressure
  • balancing dampers in HVAC systems
  • Building America
  • duct sizing
  • ducts
  • equivalent length
  • flexible ducts
  • friction rate
  • junction boxes
  • pressure losses in HVAC systems
  • residential
  • residential buildings
  • residential HVAC systems
  • static pressure


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