Total Duct Leakage Tests
Test heating and cooling duct distribution systems for air leakage using a testing protocol approved by Residential Energy Services Network (RESNET) (RESNET 2013). This testing is typically done by a home energy rater certified by RESNET.
- Conduct the testing at either rough-in (after the air handler and ducts have been installed and sealed but before drywall or flooring and registers are installed) or at final (after the air handler and ducts, drywall and flooring, and registers have been installed).
- If there is more than one system in the home, assess leakage on a per-system, rather than per-home, basis.
- Either visually inspect or duct blower test ventilation ducts (e.g., ducts used for a separately ducted ERV or HRV system).
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.
Central forced air heating and cooling systems use ducts to distribute the hot and cold air. If these ducts have loose connections where sections of duct connect to each other or to duct boots or trunk lines or the air handler, air can leak through the cracks. Leaky ducts can be a major source of energy loss and comfort problems, and can contribute to moisture problems, especially when the ducts run through unconditioned space such as a vented attic or crawlspace.
ENERGY STAR Rev 3 Version 7 requires that if a home’s HVAC system includes a duct distribution system, these ducts must be tested for air leakage. Duct leakage is measured and documented by a certified home energy rater using a testing protocol approved by the Residential Energy Services Network, Inc. (RESNET). The accepted protocols are found in RESNET's Mortgage Industry National Home Energy Rating Systems Standards, Chapter 8, Section 803.3 (RESNET 2013).
As required by RESNET standards, the test is performed using a duct tester such as the Minneapolis Duct Blaster or the Retrotec Duct Tester. The duct tester consists of three components: a calibrated fan that is used to either pressurize or depressurize the duct, a device called a manometer that is used to measure fan flow and building pressure, and supplies such as cardboard and tape or adhesive plastic sheeting to seal off the supply and return registers during the test (See Figure 1). The fan is used to pressurize or depressurize the duct system to 25 pascals (0.10 inch water column [IN WC]) (RESNET Standards). Once at 25 pascals pressure, the air flow through the duct tester is read in cubic feet of air flow per minute at 25 pascals; this measurement is abbreviated as CFM25.
Duct leakage testing should be performed after all components of the system have been installed, including the air handler, the ductwork, and the register boxes or duct boots.
Two types of tests are performed: total duct leakage and leakage to the outdoors:
The "duct leakage to the outdoors" test measures only duct leakage outside of the home’s air barrier, i.e., leakage to the outdoors, for example, into an unconditioned attic or crawlspace. The “total” duct leakage test measures how much leakage there is for all of the ductwork connected to the HVAC system, including ducts located both outdoors and indoors.
According to ENERGY STAR Version 3, Rev 07, for ducts in unconditioned spaces such as unconditioned vented attics or crawlspaces, both tests should be conducted. When all ducts are located in conditioned spaces, only the "total" duct leakage test needs to be conducted if certain conditions exist. Testing of duct leakage to the outside can be waived if all ducts and air handling equipment are located within the home’s pressure and thermal boundaries AND if envelope leakage has been tested to be less than or equal to half of the Prescriptive Path infiltration limit for the Climate Zone where the home is to be built. Alternatively, testing of duct leakage to the outside can be waived if total duct leakage is <= 4 CFM25 per 100 ft2 of conditioned floor area (or <= 5 CFM25 per 100 ft2 of conditioned floor area for smaller homes that have less than 1,200 ft2 of conditioned floor area). For ENERGY STAR criteria on testing of duct leakage to the outdoors, see Duct Leakage to Outdoors.
ENERGY STAR Version 3 Rev 07 allows total duct leakage to be tested at either rough-in or final. “Rough-in” is when the air handler and ducts have been installed and sealed but before drywall or flooring and registers are installed. (This includes cabinets (e.g., kitchen, bath, multimedia) or ductwork that connect duct boots to toe-kick registers.) “Final” is after the air handler and ducts, drywall and flooring, and registers have been installed. If testing is conducted at final, registers installed over carpets are permitted to be removed and the face of the duct boot temporarily sealed during testing. The Rater should also visually verify that the ductboots have been durably sealed to the subfloor (using duct mastic or caulk) to prevent leakage during normal operation. If testing is done at final, this visual check can be done then. If testing is done during rough-in, the rater must return after the drywall has been installed to visually confirm that the duct boot is sealed to the drywall. If the ductblaster leakage testing is done at final, ducts should be visually inspected at rough-in to look for any obvious gaps or misses in duct mastic so those can be corrected before drywalling. Some raters will also recommend that the ducts be tested at rough-in with a a low-CFM smoke machine connected to one of the ducts and the other registers closed off so that the HVAC contractor can clearly see and fix any leaks in the ducts.
The ENERGY STAR Version 3 Rev 07 air leakage criteria specify that duct air leakage must be <= 4 CFM25 per 100 sq. ft. of conditioned floor area at rough-in or <= 8 CFM25 per 100 sq. ft. of conditioned floor area at final.
There are pros and cons to either method. Some builders prefer to test for duct leakage at rough-in when the ducts are easier to access in case additional air sealing needs to be done. Some builders, especially those who install ducts in the attic, prefer to wait until final to test the ducts because the ducts are likely to get moved about by other trades in the meantime and ducts will still be accessible.
One might question why duct leakage needs to be tested at all if the ducts and air handler are located in conditioned space. High-performance homes are homes built tight using materials that greatly retard heat transfer through conduction, convection, and radiation. As a result, high-performance homes require smaller HVAC systems. For example, 20 years ago, a 2,000-ft2 home required a 4- or 5-ton air conditioner for cooling. Today, a 2,000-ft2 home requires only a two-ton unit. This downsizing results in energy and cost savings. However, air handler fans have a standard air flow rate of 400 CFM per ton. If the system size is cut in half, the air flow is also cut in half. Therefore, it becomes even more important that the conditioned air not get lost in duct leakage.
The measured duct leakage can be compared to rated air handler flow to get a sense of the energy penalty that duct leaks are contributing in Btu/h. (This is not an ENERGY STAR requirement.) Cooling systems move 400 cubic feet of air per minute over the evaporator coil per ton of cooling. Each cubic foot of air moved will carry with it 30 Btu/h. A 2.5-ton cooling system moves 1,000 CFM of air and puts out 30,000 Btu/h. If that system has a measured duct leakage of 10% (100 CFM25), it is losing 3,000 Btu/h (1/4 ton) of cooling to the outdoors.
Leakage limits are assessed on a per-system, rather than per-home, basis. So, for example, if a home has two furnaces, duct leakage must be measured in each system and compared to the square footage that the system conditions. Each system must meet the “total” and “outdoors” leakage requirements to qualify for the ENERGY STAR program.
How to Test "Total" Duct Leakage at Rough-In or Final
- For duct testing at rough-in, perform the test before drywall and finished flooring are installed, and after the air handler, ducts, and duct boots (register boxes) are installed. For duct testing at final, perform the test after the air handler, ducts and duct boots, drywall or finished flooring, and registers are installed. Connect the duct tester by attaching the duct that comes connected to the calibrated fan to a return duct grill with suitable tape (Figure 1). If conducting the test at rough-in, if there are any cabinets that connect duct boots to toe-kick registers, they do not need to be installed. If testing ducts at final, visually inspect the ducts prior to drywalling, to fix any obvious areas of leakage. A smoke test could also be conducted at this time to assist in making corrections before drywalling.
- Temporarily seal shut all of the other supply and return duct registers using cardboard and tape or removable adhesive plastic (Figures 2 and 3).
- Set up the duct tester to either pressurize or depressurize the duct system (follow manufacturer’s instructions). Set up the manometer to measure pressure and air flow according to the manufacturer’s instructions.
- Connect the "input" on the pressure side of the manometer to the return (the black tube in Figure 4).
- Turn on the fan to pressurize (or depressurize) the ductwork to 25 pascals. Leave at least one door or window open between the building and outside to prevent changes in building pressure.
- Once a steady 25 pascals of pressure is reached in the duct system, note the manometer reading for CFM. This reading of the fan air flow needed to maintain 25 pascals of pressure is the amount of air escaping through the leaks in the duct system, indicated in cubic feet per minute.
- Reconnect the input (black tube in Figure 4) to a supply duct in a part of the house that is some distance from the return. Repeat steps 5 and 6. Record the duct leakage.
- Add the two duct leakage measurements together and divide by two. This will give the most accurate duct leakage measurement for total duct leakage. See the examples in Figures 5 and 6.)
- If duct leakage is too high, use a theatrical smoke machine to illustrate duct leakage locations to the HVAC contractor.
- The air handler unit can be sealed with tape to reduce air leakage.
Figure 2 - To prepare for a total duct test at rough-in, cover all of the supply outlets and return inslets.
Figure 3 - To prepare for a total duct test at final, cover all of the supply outlets and return inslets.
Figure 4 - The duct tester is set up to depressurize the duct system.
Figure 5 - Sample results from a total duct leakage test at rough-in.
Figure 6 - Sample results from a total duct leakage test at final.
After ducts are installed and before drywall is installed, the duct system should be visually inspected by a HERS rater to ensure that all duct connections are properly fastened and sealed, preferably with mastic. The rater should also visually inspect at rough-in that ducts are fully insulated (to R-8 for supply ducts and R-6 for returns and other ducts) along the length, including all connections, and that the insulation is not compressed by tight strapping, by framing members, or by excessive bending.
If duct leakage is tested at rough-in, a HERS rater should confirm and document that total duct leakage is <= 4 CFM 25 per 100 ft2 of conditioned floor area using a RESNET-approved testing protocol and should verify with a visual inspection that duct boots are sealed to finish surfaces at final.
If duct leakage is tested at final, the HERS rater or builder may perform an optional, additional duct blaster test prior to drywall installation or make use of a theatrical smoke machine to look for air leaks so they can be sealed before drywalling.
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
6.4 Rater-measured total duct leakage meets one of the following two options. See Footnote 37 for alternative:36, 37, 38
6.4.2 Final: The greater of ≤ 8 CFM25 per 100 sq. ft. of CFA or ≤ 80 CFM, with the air handler & all ducts, building cavities used as ducts, duct boots, & register grilles atop the finished surface (e.g., drywall, floor) installed.40
(36) Items 6.4 and 6.5 only apply to heating, cooling, and balanced ventilation ducts. Duct leakage shall be determined and documented by a Rater using a RESNET-approved testing protocol. Leakage limits shall be assessed on a per-system, rather than per-home, basis. For balanced ventilation ducts that are not connected to space heating or cooling systems, a Rater is permitted to visually verify, in lieu of duct leakage testing, that all seams and connections are sealed with mastic or metal tape and all duct boots are sealed to floor, wall, or ceiling using caulk, foam, or mastic tape.
(37) For a duct system with three or more returns, the total Rater-measured duct leakage is permitted to be the greater of ≤ 6CFM25 per 100 sq. ft. of CFA or ≤ 60 CFM25 at ‘rough-in’ or the greater of ≤ 12 CFM25 per 100 sq. ft. of CFA or ≤ 120 CFM25 at ‘final’.
(38) For a home certified in the State of ID, MT, OR, or WA that is permitted before 01/01/2016, as an alternate to Rater-verified duct leakage, a PTCS® Duct Sealing Certification Form is permitted to be collected by the Home Energy Rater.
(40) Registers atop carpets are permitted to be removed and the face of the duct boot temporarily sealed during testing. In such cases, the Rater shall visually verify that the boot has been durably sealed to the subfloor (e.g., using duct mastic or caulk) to prevent leakage during normal operation.
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.
Section R403.2.2 Sealing (Mandatory). Duct tightness test has been performed and meets one of the following test criteria: Postconstruction total leakage test (including air handler enclosure): Less than or equal to 12 cfm per 100 ft2. Rough-in total leakage test with air handler installed: Less than or equal to 6 cfm per 100 ft2 of conditioned floor area.*
Section N1103.2.2 Sealing. Duct tightness test has been performed and meets one of the following test criteria: Postconstruction total leakage test (including air handler enclosure): Less than or equal to 12 cfm per 100 ft2. Rough-in total leakage test with air handler installed: Less than or equal to 6 cfm per 100 ft2 of conditioned floor area.*
Section R403.2.2 Sealing (Mandatory). Duct tightness test has been performed and meets one of the following test criteria: (1) Postconstruction total leakage test (including air handler enclosure): Less than or equal to 4 cfm per 100 ft2 of conditioned floor area. (2) Rough-in total leakage test with air handler installed: Less than or equal to 4 cfm per 100 ft2 of conditioned floor area.*
Section N1103.2.2 Sealing (Mandatory). Duct tightness test has been performed and meets one of the following test criteria: (1) Postconstruction total leakage test (including air handler enclosure): Less than or equal to 4 cfm per 100 ft2 of conditioned floor area. (2) Rough-in total leakage test with air handler installed: Less than or equal to 4 cfm per 100 ft2 of conditioned floor area.*
*Due to copyright restrictions, exact code text is not provided. For specific code text, refer to the applicable code.
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