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.
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.
- Pulling flex duct taut when installing greatly reduces the amount of friction caused by the ducting.
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.
- Coordinate with other trades including framers, plumbers, and electricians to prevent needless looping of flex duct.
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%.
Figure 5 - Cut the duct to the length needed. Do not loop excess duct.
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.
Figure 6 - There should be less than 2.5 inches per each five foot of duct length.
6. Use balancing dampers at trunk lines, if needed, to control airflow.
Figure 7 - If airflow must be limited to a supply register, use balancing dampers at the trunk line rather than looping duct to control airflow.