Sufficient Cavity Space for Flex Ducts

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Description

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.

 

Flex duct consists of a plastic inner liner attached to a metal coil, covered by a layer of fiberglass blanket insulation, which is covered by foil or plastic vapor barrier.
Figure 1 - Flex duct consists of a plastic inner liner attached to a metal coil, covered by a layer of fiberglass blanket insulation, which is covered by foil or plastic vapor barrier.  Reference

For proper performance, flex ducts in unconditioned space should not be installed in cavities smaller than the outer diameter of the duct, including the insulation. Compression of the insulation around the duct should be avoided, especially in humid climates where compressed insulation could encourage condensation to form. Even when ducts are installed in conditioned space, they should not be installed in cavities smaller than the inner duct diameter. 

During the design phase, duct routing should be identified. Open-web trusses between floors allow for ducts to freely pass through the floor system without compromising the structure. Manual J and D calculations should be performed to identify the size of ducting needed and thus how much space is required for the ductwork. Remember to take into account the thickness of the insulation (Figure 2). For example, if R-6 flex duct is used, then the exterior diameter of the duct will increase by four inches. Plan air handler locations, making sure there is clear access for the flex duct (or other specified ducting) to run from the mechanical space (or room) to the duct zone (Figure 3). 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, Support at Intervals for Flex Ducts, No Excessive Coiled or Loped Flex Ducts and Sealed and Insulated Flex Ducts.

How to Install Flex Ducts within Appropriate Cavity Space

  1.  Design ducts in compliance with Manual D. Once the appropriate duct diameter is determined, establish the duct layout in coordination with the framing plan. Ensure that adequate space is allowed for all trunk and branch duct runs.

R-6 flexible duct has 2 inches of insulation around the inner liner so a 12-inch duct requires a 16x16-inch chase

Figure 2 - R-6 flex duct has 2 inches of insulation around the inner liner. If the required duct run is a 12-inch round, the exterior radius will be about 16 inches. At a minimum, a 16x16-inch duct chase or opening would be required along the full length of the duct.  Reference

Ensure that the air handler location does not conflict with duct routing

Figure 3 - Ensure that the air handler location does not conflict with duct routing.  Reference

If a dropped soffit is used to house a duct, the soffit space must equal the duct diameter plus the insulation thickness

Figure 4 - The height and width of a dropped soffit used to house ductwork should be the duct diameter plus the thickness of the insulation to prevent compression of the duct.  Reference

  2.  If room is tight and there is fear of compressing the flex duct when making a 90-degree turn, use a metal elbow. 

Consider using a metal duct elbow instead of flex duct at boot connections to prevent compressions

Figure 5 - Use a metal duct elbow instead of flex duct at boot connections to prevent compressions.  Reference

  3.  Coordinate with the framer, plumber, and electrician for effective duct installation with adequate space.

Ensuring Success

Consider duct layout in the initial framing design stage to plan for short, straight duct runs where the duct is adequately supported and has adequate space to avoid constriction of flow. Inspect before drywall is installed to ensure that ductwork is not constricted. Use a duct blaster test to detect leakage and to confirm proper air flow at each duct supply outlet.

Scope

Flexible ducts in unconditioned space not installed in cavities smaller than outer duct diameter; in conditioned space not installed in cavities smaller than inner duct diameter

Duct Distribution Quality Installation

Flexible ducts in unconditioned space shall not be installed in cavities smaller than outer duct diameter; in conditioned space not installed in cavities smaller than inner duct diameter:

  1. Install ducts to prevent compression of duct or duct insulation.

Duct Air Flow Basics

  • Each turn, kink, or compression of ductwork reduces air flow.
  • If the recommended amount of air flow is not delivered to the room, it could lead to homeowner comfort complaints.

Duct Air Flow Tips

  • Design ducts in compliance with Manual D.
  • Use balancing dampers in flex ducts to control flow. For metal ducts, butterfly dampers may be used to control air flow.
  • To prevent kinks at the duct and boot connection, ENERGY STAR recommends using metal duct elbow instead of flex duct.
  • Webbed trusses between floors allow for ducts to freely pass through the floor system without compromising the structure.

Training

Right and Wrong Images

Presentations

None Available

Videos

None Available

CAD Images

None Available

Compliance

ENERGY STAR Version 3, (Rev. 07)

HVAC System Quality Checklist, Duct Quality Installation. Flexible ducts in unconditioned space are not installed in cavities smaller than outer duct diameter; in conditioned space not installed in cavities smaller than inner duct diameter.

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

2009 IECC

Flex duct installation is not specifically addressed in the 2009 IECC.

2012 IECC

Flex duct installation is not specifically addressed in the 2012 IECC.

More Info.

Case Studies

None Available

References and Resources*

  1. Author(s): Burdick
    Organization(s): IBACOS, NREL
    Publication Date: December 2011

    Document providing guideance and considerations for duct design in an energy efficient house.

  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  3. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  4. Author(s): Air Diffusion Council
    Organization(s): Air Diffusion Council
    Publication Date: January 2010

    Standard providing a comprehensive approach to evaluating, selecting, specifying and installing flexible duct in HVAC systems.

  5. Author(s): Air Conditioning Contractors of America
    Organization(s): Air Conditioning Contractors of America
    Publication Date: December 2013

    Standard outlining industry procedure for sizing residential duct systems.

  6. Author(s): ACTO Rubber Products Inc., Air Diffusion Council
    Organization(s): Air Diffusion Council
    Publication Date: March 2011

    Presentation about proper installation of flexible ducts.

Last Updated: 08/15/2013

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