Power Law Model of Airflow For Blower Door Testing
Building leakage is described by a power law equation of flow through an orifice. The orifice flow equation is typically expressed as
- =Airflow (m3/sec)
- = Air Leakage Coefficient
- = Pressure Differential (Pa)
- = 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 (QFan) and airflow through the building envelope (QBuilding).
It is assumed in blower door analysis that mass is conserved, resulting in:
Which results in:
Fan airflow is determined using CFan and nFan values that are provided by the blower door manufacturer, and they are used to calculate QFan. 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: CBuilding and nBuilding. 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, nBuilding 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.
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