Building Codes Assistance Project

Duct leakage is a significant issue for forced-air heating or air-conditioning systems. When ducts are run outside of conditioned space, leakage may decrease heating and cooling efficiencies by up to 40%. Over time, this will add up to a great deal of lost energy and money.

The distribution system is an integral part of many mechanical ventilation systems. It can have a significant effect on the ventilation rate and efficiency of a building. Duct leakage is a source of energy loss, ventilation rate loss, and a source of indoor pollution (in return ducts). When ducts are run through unconditioned spaces like attics, crawlspaces, garages, basements and other locations outside the heated or cooled parts of the house, they not only leak air to and from the outside, but any heat lost through the walls of the duct (by heat conduction) is also lost to the unconditioned space instead of heating and cooling the house.

A 2005 LBNL review cites studies showing an energy loss of 30 to 40% when ductwork is installed in unconditioned spaces. Others demonstrate through modeling and field testing that leakage through the average duct system was 37% greater than infiltration through the building envelope. Houses with leaky ductwork and air handlers located in unconditioned spaces are vulnerable to increased infiltration rates -- especially in hot, humid climates. Duct leakage can also prevent effective distribution of the supply air, substantially impacting the actual ventilation rate found in the average house.

There are several new and innovative products and techniques regarding air ducts - like mastic sealing (instead of conventional duct tape), enhanced HVAC ducts and casings, and duct testing -- that are not specified by the model codes that can save energy, money, and enhance thermal comfort. For more information, visit www.BuildingGreen.com. The following section provides examples of these types of code improvement language or methodologies in these areas. 

Policy Options

• Require Sealing and Duct Testing on Replacement or Renovated HVAC Systems

  • California Title 24: 2008 Building Energy Efficiency Standards
    Subchapter 9, Section 152 (b) 1D & 1E (page 157-59)
    D. When more than 40 feet of new or replacement space-conditioning ducts are installed in unconditioned space, the new ducts shall meet the requirements of Section 150(m) and the duct insulation requirements of Package D Section 151(f)10. If ducts are installed in climate zones 2, 9, 10, 11, 12, 13, 14, 15, or 16, the duct system shall be sealed, as confirmed through field verification and diagnostic testing in accordance with procedures for duct sealing of existing duct systems as specified in the Reference Residential Appendix RA3, to meet one of the following requirements:
    i. If the new ducts form an entirely new duct system directly connected to the air handler, the measured duct leakage shall be less than 6 percent of fan flow and meet the airflow requirements of Reference Residential Appendix RA3; or
    ii. If the new ducts are an extension of an existing duct system, the combined new and existing duct system shall meet one of the following requirements:
    a. The measured duct leakage shall be less than 15 percent of system fan flow; or
    b. The measured duct leakage to outside shall be less than 10 percent of system fan flow; or
    c. The duct leakage shall be reduced by more than 60 percent relative to the leakage prior to the installation of the new ducts and a visual inspection, including a smoke test, shall demonstrate that all accessible leaks have been sealed; or
    d. If it is not possible to meet the duct sealing requirements of subsection a, b, or c, all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified HERS rater.
    EXCEPTION to Section 152(b)1Dii: Existing duct systems that are extended, which are constructed, insulated or sealed with asbestos.

    E. In climate zones 2, 9, 10, 11, 12, 13, 14, 15, and 16, when a space-conditioning system is altered by the installation or replacement of space-conditioning equipment (including replacement of the air handler, outdoor condensing unit of a split system air conditioner or heat pump, cooling or heating coil, or the furnace heat exchanger) the duct system that is connected to the new or replacement space-conditioning equipment shall be sealed, as confirmed through field verification and diagnostic testing in accordance with procedures for duct sealing of existing duct systems as specified in the Reference Residential Appendix RA3, to one of the following requirements.
    i. The measured duct leakage shall be less than 15 percent of system fan flow; or
    ii. The measured duct leakage to outside shall be less than 10 percent of system fan flow; or
    iii. The measured duct leakage shall be reduced by more than 60 percent relative to the measured leakage prior to the installation or replacement of the space conditioning equipment and a visual inspection, including a smoke test, shall demonstrate that all accessible leaks have been sealed; or
    iv. If it is not possible to meet the duct requirements of i, ii, or iii, all accessible leaks shall be sealed and verified through a visual inspection and a smoke test by a certified HERS rater.
    EXCEPTION 1 to Section 152(b)1E: Duct systems that are documented to have been previously sealed as confirmed through field verification and diagnostic testing in accordance with procedures in the Reference Residential Appendix RA3.
    EXCEPTION 2 to Section 152(b)1E: Duct systems with less than 40 linear feet in unconditioned spaces.

• Require Testing of Duct System Leakage

  • City of Austin (Texas) Energy Code amending 2006 IECC (applicable to all climates):
    403.2.2.1 Testing of Duct Systems for Leakage. Leakage of supply ducts and return plenum/ducts shall not exceed 10% of total design airflow. The testing procedure shall be based on ASTM E1554, Standard Test Methods for Determining External Air Leakage of Air Distribution Systems by Fan Pressurization, ASHRAE 152-2004, Method of Test for Determining the Design and Seasonal Efficiencies of Residential Thermal Distribution Systems, or a generally accepted equivalent method.

    Testing shall be performed by an independent third-party technician approved by the building official. Documentation verifying duct leakage of less than 10% shall be submitted with the final mechanical code compliance package on the jobsite.

    Batch testing shall be allowed per attachment 1.

    Documentation shall include the following:

    a. Address of residence
    b. Date of final test
    c. Name and company of technician performing duct testing
    d. Type of test performed (duct pressurization method or blower door subtraction method).
    e. Test results in percentage of airflow CFM.

    Exception: Existing construction with no modification of or addition to the existing ductwork.

• Require System Static Pressure Testing

  • City of Austin (Texas) Energy Code amending 2006 IECC (applicable to all climates):

    403.8.1 System static pressure. Total system static pressure (with filters in place) shall not exceed .8" water column on gas furnaces and .6" water column on electric air handlers. Static pressure testing using a digital manometer or magnehelic shall be performed by an independent third party technician approved by the building official. Documentation verifying static pressure testing results within the allowed ranges shall be submitted with the final mechanical code compliance package on the jobsite.

    Batch testing shall be allowed per attachment 1.

    Documentation shall include the following:

    h. Address of building
    i. Name and company of technician performing the testing
    j. Date of final test
    k. Procedure used for the test
    l. Results of the test listing static pressure for applications tested.

    Exception: Existing construction with no modification of or addition to the existing ductwork

**Images courtesy of Building Energy Codes Resource Center