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.
Figure 1 - A technician conducts a duct blaster test.
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.
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 more on duct leakage to outdoors testing, see Duct Leakage to Outdoors.
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).
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
Figure 2 - The duct tester is set up to measure total duct leakage. The manometer shows that this duct system is leaking 100 cubic feet per minute at -25 Pascals of pressure (100 CFM25). (Note, the duct tester is set up to depressurize the duct system.)
- Connect the duct tester by attaching the duct that comes connected to the calibrated fan to a return duct grill with duct tape. Set up the duct tester to either pressurize or depressurize the duct system (follow manufacturer's instructions).
- Temporarily seal shut all of the other supply and return duct registers using cardboard and tape or removable adhesive plastic.
- 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 2).
- 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 2) 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.