Retrotec Blower Door - Manual (Page 47)

Page 47 of 87
©Retrotec Inc. 2015
2. If the fan must be installed next to a stairway or major obstruction, it is best to take measurements with a
Range Ring or Plate installed, and not the Open Range configuration.
3. Open the inside door and outside storm door as much as possible during the Door Fan test to prevent
restrictions to air flow.
5.6 Considerations when operating with high backpressure
Note: For most testing applications, backpressure is not a concern and can be ignored.
The term “backpressure" describes the pressure that the Door Fan is working against when it is running.
Backpressure is determined by measuring the Baseline pressure difference between the air directly upstream of the
fan, and the air directly exiting the fan. Typically, backpressure is simply the test pressure at which the building
airtightness measurement is being made (e.g. 50 Pascals). However, in some applications, the Door Fan could
experience backpressures that are greater than the test pressure. For example, if the Door Fan is exhausting air into
a confined area (such as an attached porch), it is possible that the porch area could become pressurized relative to
outside creating a backpressure condition that is greater than the test pressure. Although the Door Fan flow sensor
is designed to account for variations in backpressure, certain high-backpressure operating conditions can degrade
the calibration of the fan.
Retrotec Door Fans are calibrated to function in testing applications with backpressures up to 80 Pascals, with no
significant effect on accuracy. This is true for all Range Configurations (Open through L1), provided that the fan is
operated within the accepted Range Configuration. Backpressures above 80 Pa will restrict the available Range
Configurations from which accurate results can be obtained. When a Retrotec fan is used with the DM-2, or with
Retrotec software, compensation is automatically applied for the backpressure and the flow rate is not displayed
when results might be inaccurate.
Figure 30: DM-2 can't display flow reading because of back pressure.
5.7 Wind effects on test fan accuracy
Measurements are best taken with the test fans in the downwind or leeward part of the building away from velocity
pressures caused by the wind. It that cannot be done and if the wind is significant, say above 5 mph, then considerations
should be given to the winds effects.
1. When wind strikes a test fan in a blower door system, the accuracy can be affected for the following reasons:
2. Air striking the pressure pickup will alter the fan pressure reading without changing the flow rated
3. The velocity profile across the fan face will alter the relationship between Fan Pressure and flow
4. Turbulence will be added to the incoming airstream
5. A negative pressure may be created as the air passes the fan at a shallow angle.