- Alex Mathers
Do You Dalt? You Should.
Air distribution systems use a significant amount of energy. According to the United States Department of Energy (DOE), air distribution systems in commercial buildings use roughly 1.5 quadrillion BTUs of energy, or roughly 1.5 percent of energy nation-wide. Unfortunately, some of this energy is unnecessarily wasted.
According to ASHRAE, almost all buildings have significant duct leakage.
Duct leakage is when distributed conditioned air leaks through joints, seams, and
penetrations in the ductwork before it reaches the desired rooms/spaces. The Sheet Metal and Air Conditioning Contractors’ National Association (SMACNA) states that large, unsealed duct systems may have or develop [air] leakage well above 30% of the total system airflow.
According to the Center for Energy and Environment (CEE) the leading cause of energy waste in large commercial and institutional buildings is duct leakage. This energy waste comes from lost fan energy and the loss of cooling and heating conditioned air. Even relatively small leaks end up increasing energy consumption by very large amounts. In fact, energy loss for a 100,000-sf commercial building in Southern California with a 10% duct leakage rate estimates a cost of $500,000 over 20 years.
Beyond wasted energy and cost, leaky ducts negatively affect the building itself in several ways. It can severely impact indoor air quality due to the introduction of unfiltered air into the duct system. Leaks can cause mold and mildew problems; it is also more difficult to control the space temperatures.
Codes and standards dealing with duct leakage in commercial buildings have existed for many years. ASHRAE Standard 90.1 requires air leakage testing of 100% of all outside ductwork and 25% of representative sections of all other ductwork designed to operate at a static pressure in excess of 3-inch water gauge.
90.1 includes prescriptive requirements:
· The maximum system leakage rate will now be 5%. This is everything from the ductwork to the air handler and all the associated add-on equipment.
· The maximum acceptable leakage rate for the ducts alone will now be 3%.
· Supply and return ductwork leaking to/from the outdoors will now have a maximum acceptable leakage rate of 2%.
The Uniform Mechanical Code (UMC) also requires ductwork to be leak-tested per a SMACNA test manual. That manual is for duct systems having static pressures of 3” W.C. and higher. The test only involves representative sections totaling not less than 10% of the total installed duct area. However, if the tested 10% percent fails to comply, then 40% of the total must be tested. Failing that will require testing 100% of the total installed duct area.
For California specific projects, the 2019 CMC Title 24 Part 4 Section 603.10.1, (based on the 2018 UMC)
603.10.1 Duct Leakage Tests
Ductwork shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual. Representative sections totaling not less than 10 percent of the total installed duct area shall be tested. Where the tested 10 percent fail to comply with the requirements of this section, then 40 percent of the total installed duct area shall be tested. Where the tested 40 percent fail to comply with the requirements of this section, then 100 percent of the total installed duct area shall be tested. Sections shall be selected by the building owner or designated representative of the building owner. Positive pressure leakage testing shall be permitted for negative pressure ductwork…
The permitted duct leakage is based on the leakage class. Leakage class is determined by the construction methods employed in duct fabrication in accordance with the ANSI/SMACNA HVAC Duct Construction Standards.
This test pressurizes existing or newly installed sheet metal duct to determine if it meets either Sheet Metal Industry Standards or Design Contract Specifications. ASHRAE Standard 193 provides standardized methods to test for equipment leakage. The ASHRAE and SMACNA duct testing method uses a calibrated fan that pressurizes a section of duct and measures the airflow with calibrated pressure gauges to indicate total leakage. All openings are temporarily sealed, and fan pressure is read from the gauges and converted to an equivalent duct leakage rate in cubic feet per minute (cfm). If the leakage rate (air loss) exceeds acceptable limits, sealing will be required to correct the condition.
Is DALT Required?
The mechanical code could be read in two ways. The part of the first sentence “Ductwork shall be leak-tested” seems to be clear that the intent of the code is for all new ductwork to be tested. However, the second part of the sentence, “in accordance with the SMACNA HVAC Air Duct Leakage Test Manual” makes the intent less clear.
SMACNA’s most current 2012 HVAC Air Duct Leakage Test Manual states “No leakage tests are required by the SMACNA HVAC Duct Construction Standards, 3rd ed., 2005 or by this leakage test manual. Further, the SMACNA manual states:
2.5.1 The need to verify leakage control by field testing is not present when adequate methods of assembly and sealing are used. Leakage tests are an added expense in system installation. Costs include the time and materials to prepare and test the section of duct, coordinating the testing with other trades and potential delays to the overall construction schedule. It is not required that duct systems constructed to 3 in. wg class or lower be tested. For duct systems constructed to 4 in. wg class and higher, the designer must determine if any justification for testing exists. If it does, the designer must clearly designate in the contract documents the portions of the system(s) to be tested and must also specify appropriate test methods.
SMACNA assumes 2 in. w.c. between AHUs and VAV terminals as a base and therefore does not include this as a requirement for testing. For lower pressure ductwork, particularly ductwork located downstream of VAV boxes, SMACNA states that it does not provide the same potential gain as ductwork located upstream of a VAV box. Although the downstream ductwork is often constructed to 1 or 2 in. w.g. the reality is that these portions of the system rarely exceed 0.25 in. w.g. of static pressure under operating conditions and are often operating near 0.1 in. w.g.. These low pressures greatly reduce the potential for air leakage.
Personally, I read the Mechanical Code as “Ductwork shall be leak-tested in accordance with the SMACNA HVAC Air Duct Leakage Test Manual. That is, the process of testing is based on the SMACNA manual, not the decision on whether to test or not.
In the end, it is up to the engineer of record to specify duct leakage testing. Specifications that read “test per SMACNA” or similar do not provide sufficient direction of the team. A properly written leakage testing specification contains the following:
· Which portions or systems require testing.
· The test static pressure of the system (not to exceed the construction static pressure class of the ductwork)
· The leakage class
The California Energy Commission’s 2022 Building Energy Efficiency Standards (Energy Code, Title 24, Part 6) will include major updates. One of these is a clarification on the CMC Title 24 Part 4 Section 603.10.1. The updated section 120.4(g) now states that duct leakage testing is mandatory. The 2022 CA codes apply to all projects in the state submitted to plan check after December 31st, 2022.