No Excessive Coiled or Looped Flex Ducts
Install ducts without excessive coiled or looped flexible ductwork.
- Consider duct layout in the initial framing design stage to plan for short, straight duct runs wherever possible.
- Install flex duct in the straightest line possible; pull ducting taut and provide adequate supports per code.
- Ensure that the radius of each bend is no less than the diameter of the flexible duct.
- Coordinate with the plumber and the electrician to avoid crushing ducts when other services are installed.
- Design ducts in compliance with Manual D.
- Use proper sized ducts or balancing dampers rather than loops in flex ducts to control air flow. For metal ducts, butterfly dampers may be used to control air flow.
- To prevent kinks at the duct and boot connections, consider using metal duct elbows instead of flex duct.
See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.
Flexible duct, known as flex duct, is very convenient ducting for attaching supply air outlets to rigid trunk ductwork, and, in many cases, flex duct comprises the entire duct system. Flex duct made for HVAC applications is typically constructed of a plastic inner liner attached to a metal wire helix (or coil) to make round, flex duct. The duct comes with a layer of fiberglass blanket insulation already attached around the duct. The insulation is covered and protected by a polyethylene or foil vapor barrier. Flex duct is typically available in insulation values of R-4, R-6, and R-8. For residential HVAC systems, insulated flex duct typically comes in diameters of 4 inches through 10 inches; above 10 inches, it comes in even sizes of 12, 14, 16, etc., up to 22 inches measured at the radius of the metal helix.
Duct runs should be as short and straight as possible and flex duct should be stretched taut before connecting for maximum efficiency and air flow volume. Sloppy installation with flex ducts that loop, coil, or sag excessively adds to the pressure drop across the entire duct system and wastes materials. Some contractors intentionally loop the flexible duct as a way to decrease airflow to a particular room; however, decreases in airflow should be handled with dampers instead as stated in the ENERGY STAR Version 3.0 HVAC System Quality Installation Contractor Checklist, footnote 23: "Ducts shall not include coiled or looped ductwork except to the extent needed for acoustical control. Balancing dampers or proper duct sizing shall be used instead of loops to limit flow to diffusers. When balancing dampers are used, they shall be located at the trunk to limit noise unless the trunk will not be accessible when the balancing process is conducted. In such cases, opposable blade dampers (OBD) or dampers located in the duct boot are permitted."
Manual D Residential Duct Systems (ACCA 2009) sets standards for flex duct design and installation. See Appendix 17 for a detailed discussion of duct installation. The Flexible Duct Performance Standards (Fifth Edition) by the Air Diffusion Council (ADC) provides guidance to designers, architects, engineers, contractors, and installers.
For more on flex duct installation, see No Kinks or Sharp Bends in Flex Duct Installation, Sufficient Cavity Space for Flex Ducts, Support at Intervals for Flex Ducts and Sealed and Insulated Flex Ducts.
How to Install Flex Duct without Excessive Looping
1. Plan for duct runs in the initial house design, especially the framing plan, so that the duct layout is short and straight with ducts resting on ceiling rafters or in duct chases rather than hung, if possible. Coordinate with the framer, plumber, and electrician to minimize obstacles.
2. Lay out ducts so that all bends are smooth and gradual. The radius of the bend (from the center point of the bend to the center line of the duct) should be equal to or greater than the diameter of the flex duct.
Figure 4 - Lay out ducts so that all bends are smooth and gradual. The radius of the bend (from the center point of the bend to the center line of the duct) should be equal to or greater than the diameter of the flex duct.
3. Cut the duct to the measured length. Duct length should not exceed measured span length by more than 4%.
4. Install the duct fully extended. Compressing the duct or using excessive length will noticeably increase friction losses.
5. Keep the duct’s centerline relatively straight, with no significant sag or snaking. Limit sag to 2.5 inches per 5 feet of span, or less, as shown in Figure 6.
6. Use balancing dampers at trunk lines, if needed, to control airflow.
Consider duct layout in the initial framing design stage to plan for short, straight duct runs wherever possible. Inspect before drywall is installed to ensure that ductwork is installed in the straightest line possible from the air handler to the end of the trunk line and from the trunk line to the supply air registers, that the flexible duct is pulled taut and properly supported, and that the radius of all directional changes is no less than the diameter of the flexible duct. Use a duct blaster test to confirm proper air flow at each duct supply outlet.
No climate specific information applies.
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ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Rater Field Checklist
6. Duct Quality Installation
6.1 Ductwork installed without kinks, sharp bends, compressions, or excessive coiled flexible ductwork33
(33) Kinks are to be avoided and are caused when ducts are bent across sharp corners such as framing members. Sharp bends are to be avoided and occur when the radius of the turn in the duct is less than one duct diameter. Compression is to be avoided and occurs when flexible ducts in unconditioned space are installed in cavities smaller than the outer duct diameter and ducts in conditioned space are installed in cavities smaller than inner duct diameter. Ducts shall not include coils or loops except to the extent needed for acoustical control.
ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.
Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.0, Revision 07.
Flex duct installation is not specifically addressed in the 2009 IECC.
Flex duct installation is not specifically addressed in the 2012 IECC.
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Proper Duct Installation = Professionally-Installed Comfort Delivery Ducts