Machine used during air leakage testing
A blower door is a machine used to perform a building air leakage test. It can also be used to measure airflow between building zones, to test ductwork airtightness and to help physically locate air leakage sites in the building envelope.
There are three primary components to a blower door: a calibrated, variable-speed blower or fan, capable of inducing a range of airflows sufficient to pressurize and depressurize a variety of building sizes; a pressure measurement instrument, called a manometer, to simultaneously measure the pressure differential induced across the face of the fan and across the building envelope, as a result of fan airflow; and a mounting system, used to mount the fan in a building opening, such as a door or a window.
Airtightness testing is usually thought of in residential settings. It is becoming more common in commercial settings. The General Services Administration (GSA) requires testing of new US federal government buildings.
A variety of blower door air tightness metrics can be produced using the combination of building-to-outside pressure and fan airflow measurements. These metrics differ in their measurement methods, calculation and uses. Blower door tests are used by building researchers, weatherization crews, home performance contractors, home energy auditors, and others in efforts to assess the construction quality of the building envelope, locate air leakage pathways, assess how much ventilation is supplied by the air leakage, assess the energy losses resulting from that air leakage, determine if the building is too tight or too loose, determine if the building needs mechanical ventilation and to assess compliance with building performance standards.
History.
In Sweden blower door technology was first used to measure building air tightness in 1977. The earliest implementation in Sweden used a fan mounted in a window, rather than a door. By 1979, similar window-mounted measurement techniques were being pursued in Texas, and door-mounted test fans were being developed by a team at Princeton University to help them find and fix air leaks in homes in a Twin Rivers, New Jersey housing development.
In Canada the initial concept similarly involved the use of a "blower window", which was first utilized by G. T. Tamura in Ottawa, Canada, as part of a Division of Building Research study to test houses in Ottawa in 1967–1968 and was published in 1975.
In Canada, a team at the National Research Council of Canada's Division of Building Research (NRC/DBR) in Saskatchewan, advanced the published work of Tamura in 1975 and went from a blower window to a blower door concept used in the construction of the Saskatchewan Conservation House in 1977. The Saskatchewan group was actively involved in the development of the blower door in 1977-78 and published their findings in 1980. They made available their flow nozzle to interested companies including one from Minneapolis. Harold Orr, who had been in Ottawa in 1967 when Tamura was conducting his work, continued to work on blower door technology after Tamura published his paper. Tamura's blower window concept from 1967, preceded the Swedish work in 1977 by a decade. The first blower door was further used to test the airtightness of the Saskatchewan Conservation House built in 1977, which was tested at 0.5 ach at 50 Pa.
These early research efforts demonstrated the potential power of blower door testing in revealing otherwise unaccounted for energy losses in homes. Previously, air leakage around doors, windows and electrical outlets was considered to be the primary leakage pathway in homes, but Harrje, Dutt and Beya used blower doors to identify thermal bypasses. These bypasses were air leakage sites, such as attic utility chases, that accounted for the largest percentage of air leakage energy loss in most homes. Use of blower doors in home energy retrofitting and weatherization efforts became known as "house doctoring" by researchers on the east and west United States coasts.
The blower door first became commercially available in the United States in 1980 under the name Gadsco. Harmax started to sell units in 1981, followed closely by The Energy Conservatory in 1982.
While these blower door-testing efforts were useful in identifying leakage pathways and in accounting for otherwise inexplicable energy losses, the results could not be used to determine real-time air exchange in buildings under natural conditions, or even to determine average annual air exchange levels. Sherman attributes the first attempt at doing this to Persily and Kronvall, who estimated annual average air exchange by:
formula_1
formula_2 = Natural Air Changes per Hour [1/h]
formula_3 = Air Changes per Hour at 50 pascal [1/h]
Further physical modeling efforts allowed for the development and validation of an infiltration model by researchers at Lawrence Berkeley National Laboratory (LBNL). This model combined data derived from blower door tests with annual weather data to generate time-resolved ventilation rates for a given home in a specific location. This model has been incorporated into the ASHRAE Handbook of Fundamentals (1989), and it has been used in the development of ASHRAE Standards 119 and 136. Other infiltration models have been developed elsewhere, including one by Deru and Burns at the National Renewable Energy Laboratory (NREL), for use in whole-building performance simulation.
How blower door tests work.
A basic blower door system includes three components: a calibrated fan, a door panel system, and a pressure measurement device (manometer).
Test setup.
The blower door fan is temporarily sealed into an exterior doorway using the door panel system. All interior doors are opened, and all exterior doors and windows are closed. HVAC balancing dampers and registers are not to be adjusted, and fireplaces and other operable dampers should be closed. All mechanical exhaust devices in the home, such as bathroom exhaust, kitchen range hood or dryer, should be turned off. Pressure tubing is used to measure the fan pressure, and it is also run to the exterior of the building, so that the indoor/outdoor pressure differential can be measured. The exterior pressure sensor should be shielded from wind and direct sunlight. The test begins by sealing the face of the fan and measuring the baseline indoor/outdoor pressure differential. The average value is to be subtracted from all indoor/outdoor pressure differential measurements during the test.
Test procedure.
The blower door fan is used to blow air into or out of the building, creating either a positive or negative pressure differential between inside and outside. This pressure difference forces air through all holes and penetrations in the building enclosure. The tighter the building (e.g. fewer holes), the less air is needed from the blower door fan to create a change in building pressure. Typically, only depressurization testing is performed, but both depressurization and pressurization are preferable. Different values for blower door metrics are to be expected for pressurizing and depressurizing, due to the building envelope's response to directional airflow. The smallest fan ring that allows the fan to reach the maximum target indoor/outdoor pressure differential should be used. A multi-point test can be performed either manually or using data acquisition and fan control software products. The manual test consists of adjusting the fan to maintain a series of indoor/outdoor pressure differentials and recording the resulting average fan and indoor/outdoor pressures. Alternatively, a single-point test can be performed, where the blower door fan is ramped up to a reference indoor/outdoor pressure differential and the fan pressure is recorded. Often the blower door hardware converts fan pressure measurements directly to fan airflow values.
Power law model of airflow.
Building leakage is described by a power law equation of flow through an orifice. The orifice flow equation is typically expressed as
formula_4
formula_5 =Airflow (m3/s)
formula_6 = Air leakage coefficient
formula_7 = Pressure differential (Pa)
formula_8 = Pressure exponent
The C parameter reflects the size of the orifice, the ∆P is the pressure differential across the orifice, and the n parameter represents the characteristic shape of the orifice, with values ranging from 0.5 to 1, representing a perfect orifice and a very long, thin crack, respectively.
There are two airflows to be determined in blower door testing, airflow through the fan (codice_1) and airflow through the building envelope (codice_2).
formula_9
formula_10
It is assumed in blower door analysis that mass is conserved, resulting in:
formula_11
Which results in:
formula_12
Fan airflow is determined using codice_3 and codice_4 values that are provided by the blower door manufacturer, and they are used to calculate codice_1. The multi-point blower door test procedure results in a series of known values of Qn, Fan and ∆Pn, Building. Typical ∆Pn, Building values are ±5, 10, 20, 30, 40 and 50 pascal. Ordinary least squares regression analysis is then used to calculate the leakage characteristics of the building envelope: codice_6 and codice_7. These leakage characteristics of the building envelope can then be used to calculate how much airflow will be induced through the building envelope for a given pressure difference caused by wind, temperature difference or mechanical forces. 50 Pa can be plugged into the orifice-flow equation, along with the derived building C and n values to calculate airflow at 50 pascal. This same method can be used to calculate airflow at a variety of pressures, for use in creation of other blower door metrics.
An alternative approach to the multi-point procedure is to only measure fan airflow and building pressure differential at a single test point, such as 50 Pa, and then use an assumed pressure exponent, codice_7 in the analysis and generation of blower door metrics. This method is preferred by some for two main reasons: (1) measuring and recording one data point is easier than recording multiple test points, and (2) the measurements are least reliable at very low building pressure differentials, due both to fan calibration and to wind effects.
Air density corrections.
In order to increase the accuracy of blower door test results, air density corrections should be applied to all airflow data. This must be done prior to the derivation of building air leakage coefficients (formula_13) and pressure exponents (formula_14). The following methods are used to correct blower door data to standard conditions.
For depressurization testing, the following equation should be used:
formula_15
formula_16 = Airflow corrected to actual air density 
formula_17 = Airflow derived using formula_18 and formula_19
formula_20 = Air density inside the building, during testing
formula_21 = Air density outside the building, during testing
For pressurization testing, the following equation should be used:
formula_22
The values formula_23 and formula_24 are referred to as air density correction factors in product literature. They are often tabulated in easy to use tables in product literature, where a factor can be determined from outside and inside temperatures. If such tables are not used, the following equations will be required to calculate air densities.
formula_20 can be calculated in IP units using the following equation:
formula_26
formula_20 = Air density inside the building, during testing
formula_28 = Elevation above sea level (ft)
formula_29 = Indoor temperature (F)
formula_21 can be calculated in IP units using the following equation:
formula_31
formula_21 = Air density outside the building, during testing
formula_28 = Elevation above sea level (ft)
formula_34 = Outdoor temperature (F)
In order to translate the airflow values derived using formula_18 and formula_19 from the blower door manufacturer to the actual volumetric airflow through the fan, use the following:
formula_37
formula_38 = Actual volumetric airflow through the fan 
formula_39 = Volumetric airflow calculated using manufacturer's coefficients or software
formula_40 = Reference air density (typically 1.204 for kg/m3 or 0.075 for lb/ft3)
formula_41 = Actual density of air going through the fan formula_20 for depressurization and formula_21 for pressurization
Blower door metrics.
Depending on how a blower door test is performed, a wide variety of airtightness and building airflow metrics can be derived from the gathered data. Some of the most common metrics and their variations are discussed below. The examples below use the SI pressure measurement unit Pascal (pa). Imperial measurement units are commonly water column inches (WC Inch or IWC). The conversion rate is 1 WC inch = 249 Pa. Examples below use the commonly accepted pressure of 50pa which is 20% of 1 IWC.
Airflow at a specified building pressure.
This is the first metric that results from a Blower Door Test. The airflow, (Imperial in Cubic Feet / minute; SI in liters / second) at a given building-to-outside pressure differential, 50 pascal (Q50). This standardized single-point test allows for comparison between homes measured at the same reference pressure. This is a raw number reflecting only the flow of air through the fan. Homes of different sizes and similar envelope quality will have different results in this test.
Airflow per unit surface area or floor area.
Often, an effort is made to control for building size and layout by normalizing the airflow at a specified building pressure to either the building's