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Duct Pressurization Testing

A duct pressurization test is used to evaluate leakage of an air-distribution system, including the ducts and air handler cabinet. During the test, all supply and return registers are taped closed, and the pressurization fan is temporarily mounted to pressurize the taped-off duct system. A multipoint test can be performed to infer a leakage area, but it is more common to pressurize to a reference pressure of 25 Pascal (Pa). The flow rate at this pressure is an indication of the leakage characteristic of the air-distribution system. The supply and return sections of the air-distribution system can be separately tested by inserting a blocking section in the air handler. An inference can be made as to the proportion of leakage to the outside under test conditions by pressurizing the house to the reference pressure using a blower door during the duct pressurization test. Because the ducts and house are then at the same pressure, any leakage from the ducts is assumed to be to the outside rather than to the house. The results of this test provide a useful benchmark for evaluating the general quality of the air-distribution system, but the test results do not directly indicate duct air leakage during normal operation of the system. For full details on duct blaster operating procedures, see Minneapolis Duct Blaster Operation Manual [1] and ASTM E1554 [2].

Duct leakage is typically examined using a duct pressurization test to pressurize or depressurize the duct. That section describes methods to test for total leakage and leakage to outside (i.e. to a buffer space), from which leakage to the conditioned space can be calculated. Duct leakage can also be approximated using an orifice plate and powered flow hoods.

Photo of a man kneeling beside a device connected to the duct system of a house (a duct blaster). The device is attached via flexible piping, and has a fan on one end. Attached to that end are a series of wires, which are hooked up to a meter that provides the pressure reading.

A duct pressurization test in process
NREL/PIX 04869

Quantities that can be measured using a duct pressurization test:

  • What is the total duct leakage at 25 Pa?
  • What is the duct leakage at 25 Pa for the house (supply to inside, supply to outside, return to inside, return to outside)?
  • What are the supply and exhaust ventilation rates of the heat recovery ventilation (HRV)? How balanced are the flows? What is the resulting change in pressure for each unit when the HRVs are operating?

Limitations:

  • Similar to the limitations with the blower door test, the uniform pressurization of the ducts makes it difficult to see actual leakage present during normal operation.
  • A simpler method, known as DeltaQ, has been described and may be suitable for many field tests [4]

 

References:

  1. Minneapolis Duct Blaster Operation Manual. 8th edition. Minneapolis, Minnesota: The Energy Conservatory, April 1996.
  2. American Society for Testing and Materials. ASTM E1554, Standard Test Methods for Determining External Air Leakage of Air Distribution Systems by Fan Pressurization. West Conshohocken, Pennsylvania: American Society for Testing and Materials, 1994.
  3. Hancock, E.; Norton, P. and Hendron, R. Building America System Performance Test Performance Test Practices: Part 2, Air Exchange Measurements. National Renewable Energy Laboratory, August 2002. NREL/TP-550-30270
  4. Walker, I.S., D.J. Dickerhoff, and M.H. Sherman. 2002. The Delta Q Method of Testing the Air Leakage of Ducts. Proceedings of the 2002 ACEEE Summer Study on Energy Efficiency in Buildings. American Council for an Energy-Efficient Economy, Washington, D.C.