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Blower Door Test

A blower door is a device used to pressurize or depressurize a house to determine the leakage characteristics of the building envelope. A variable-speed fan is temporarily mounted in a doorway or other opening to pressurize (or depressurize) the house by specified amounts. The flow rate through a calibrated orifice is measured at the different house pressures. The relationship between flow rate and pressure difference is an indication of shell air-tightness. For full details on blower door operating procedures, see reference [2].

A blower door installed in the
front door of a test house,
from the outside
Ed Hancock

A blower door installed in the
front door from the inside
Dennis Schroeder

The results of the blower door tests can be expressed as one of several figures of merit. One simple expression of the envelope tightness is the blower door flow rate at a pressure difference of 50 Pascals (Pa) or "CFM50." This test is the simplest and easiest to perform because it requires the blower door operator to achieve only one pressure (50 Pa) and measure a single flow rate. The 50-Pa pressure is the highest used in a typical blower door test and is the least sensitive to the influence of wind variation during the test. It, therefore, tends to be more repeatable than other tests that require measurements at lower pressures. However, it does not provide adequate information to use in algorithms for calculating actual air-exchange rates [3]. Another quantity that can be calculated by pressuring the house to 50 Pa is the air changes per hour (ACH50) at 50 Pa (equal to the CFM50 multiplied by 60 minutes per hour and divided by the house volume). ACH50 can be a useful metric for comparing houses of different sizes.

The equivalent leakage area (ELA) is defined as the area of a calibrated orifice that would have the same air flow rate the house does at a pressure of 4 Pa. The ELA, therefore, is an estimate of the aggregate size of all the leaks in the building. An ELA can be calculated from the results of a multipoint blower door test. This is generally done using a laptop computer with TECTITE software [4] to automatically control the blower door for multipoint tests. Typically, 100 data points at each of 8 different pressures between 15 and 50 Pa are used to determine the relationship between pressure and leakage rate for the test home. The ELA is based on the leakage rate at a 4-Pa pressure difference, which is determined by a curve fit to the blower door test data at various pressures. The ELA can then be used in conjunction with weather conditions at the home site to model the natural ACH at particular times and to estimate long-term or annual infiltration rates [2, 3, 5].

If attached units are being tested, guarded tests should be performed with adjacent units pressurized at the same level as the test unit to determine the leakage between units. A guarded test refers to a test with one or more adjacent units pressurized, which should eliminate any leakage between units. The leakage measured in a guarded test will be the leakage to outside and any unguarded adjacent units. Caution must be used when there are buffer spaces connecting adjacent units, such as a common crawlspace or attic. Such spaces can become partially pressurized during the blower door test, making interpretations of the data more difficult. For guarded tests, the unpublished Building America report for Building Science Corporation's Burlingame Community in Aspen, CO [6].

Quantities that can be measured using a blower door test:

  • Air change rate of the house at 50 Pa pressurization/depressurization
  • Effective leakage area (or equivalent leakage area) of the house
  • Effective leakage area between adjacent multifamily units
  • Effective leakage area of a buffer space

Limitations:

  • Localized pressurization caused by different systems within the house will influence the leakage area of a house. The uniform pressurization of the house by a blower door makes it impossible to see these effects.
  • The distribution of leakage area in the building envelope can have a large effect on the infiltration rate of the house. The blower door test does not provide any information about where the leaks are located, or how large they are on average. Assumptions about leak distribution can be validated to some extent by comparing blower door results to tracer gas results under known weather conditions.
  • Blower door measurements must be made during very still outdoor conditions. Breezy days can result in inaccurate measurements, and blower door measurements should never be made on windy days. To limit the effects of wind, at least four pressure lines of equal length should be routed to the four sides of the home's exterior and placed such that any breeze would blow perpendicular to the tubes' openings.

References:

  1. 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
  2. The Energy Conservatory. Minneapolis Blower Door Operation Manual. Minneapolis, Minnesota: The Energy Conservatory, January 2001.
  3. Judkoff, R. A Computer Program to Calculate Effective Leakage Area from Blower Door Data, and to Calculate Natural Infiltration Rate as a Function of Building and Weather Specific Variables, Solar Energy Research Institute (now the National Renewable Energy Laboratory), November 1986.
  4. The Energy Conservatory.TECTITE Version 2. Minneapolis, Minnesota: The Energy Conservatory, 1999.
  5. Sherman, M. and Modera, M. ASTM STP904, Comparison of Measured and Predicted Infiltration using the LBL Infiltration Model, West Conshohocken, Pennsylvania: American Society for Testing and Materials, April 1984.
  6. Hendron, R.; Hancock, E. and McDowell, T. Building America Field Test and Analysis Report for Burlingame Community, Aspen, CO. June 2008.