Duct leakage testing should be performed after all components of the system have been installed (including the air handler, ductwork, register boxes/boots, and all air devices such as diffusers, registers, or grills). Leakage limits should be assessed on a per-system, rather than per-home, basis. Duct leakage is determined and documented by a rater using a Residential Energy Services Network, Inc. (RESNET)-approved testing protocol. The accepted protocols are found in RESNET's Mortgage Industry National Home Energy Rating Systems Standards, Chapter 8, Section 803.3 (RESNET 2006).
Duct leakage testing per RESNET standards 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 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. 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.
Two types of tests are performed: total duct leakage and leakage to the outdoors:
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. The ENERGY STAR Qualified Homes Version 3 HVAC Quality Installation Rater Checklist Note 4.1 requires that "total" duct leakage be < 8 CFM25 per 100 square feet (ft2) of conditioned floor area. For more on total duct leakage testing, see Total Duct Leakage.
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 ENERGY STAR Qualified Homes Version 3 HVAC Quality Installation Rater Checklist Note 4.2 requires that duct leakage to the "outdoors" be < 4 CFM25 per 100 ft2 of conditioned floor area. For smaller homes (those with < 1,200 ft2 of conditioned floor area), measured duct leakage to outdoors shall be < 5 CFM25 per 100 ft2 of conditioned floor area.
For ducts in unconditioned spaces, both tests should be conducted, according to ENERGY STAR Version 3, Rev 6.
When ducts are located in conditioned spaces, only the "total" duct leakage test needs to be conducted if certain conditions apply according to ENERGY STAR Version 3, Rev 6. 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 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).
Figure 1 - A technician conducts a duct blaster test.
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 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.
Figure 2 - The duct tester and blower door are set up to measure leakage to the outdoors. The blower door is set to depressurize the house to -25 pascals with respect to the outdoors. Then the duct tester is set to depressurize the duct system to 0 pascals with reference to the house. The flow reading on the right side of the manometer indicates duct leakage to the outside in CFM. In this case, duct leakage to the outdoors is 100 CFM at 25 pascals. Some raters prefer to do this test with the fans reversed so that they pressurize the house and ducts to 25 pascals rather than depressurizing the house to -25 pascals.
How to Test Duct Leakage to the Outdoors
Install a blower door in an exterior door that opens to a central location in the home. If the duct tester is set up to pressurize the duct system, then set up the blower door to pressurize the home. If the duct tester is set up to depressurize the duct system, set up the blower door to depressurize the home. Regarding pressurizing versus depressurizing, the test will work either way, the decision is up to the rater.
Close all exterior doors and windows between the building and the outside during the test.
Attach the duct that comes connected to the duct tester to the largest return duct grille using tape. Set up the duct tester per manufacturer's instructions to either pressurize or depressurize the duct system (whichever is preferred).
Temporarily seal shut all of the other supply and return duct registers using cardboard and tape or removable adhesive plastic.
Turn on the blower door fan and bring the building pressure to 25 pascals with reference to the outdoors.
Turn on the duct tester fan and increase the air flow until the pressure inside the duct system is 0.0 (±0.1 pascal) with reference to the home. When both the house and the ducts are pressurized to 25 pascals with respect to outdoors, there should be no air flowing through duct leaks into the house. During this time, the blower door fan speed may have to be adjusted to make sure the home stays at 25 pascals with reference to outdoors.
Note on the manometer connected to the duct tester the amount of air flow needed to maintain the duct pressure at 0 pascals with reference to the home. This number, in CFM, is the amount of duct leakage to the outside of the home’s air barrier, i.e., leakage to the outdoors, for example into an unconditioned attic or crawlspace.
Take two measurements: the first measurement with the duct tester pressure probe in the return duct as shown in Figure 2 and the second with the duct tester pressure probe in a supply duct as far as possible from the return that the duct tester is connected to. Add both measurements together and divide by two. This will be the average measured duct leakage to the outdoors. Testing both the supply and return ducts can also show you where dominant sources of leakage are in the duct system, on the return side or on the supply side.