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Evaluation of Test Procedures for Whole House Fan Airflow Measurement

Measure Description

When performance compliance requires installation of a whole-house fan (WHF), the ventilation airflow rate and fan efficacy must be field verified in accordance with the procedures in Reference Residential Appendix RA3.9. These procedures include measurement of airflow and efficacy to confirm that the values entered in compliance software are consistent with the measured values.

At the request of Energy Commission, the Statewide CASE team conducted a field study of five WHF installations and developed a technical report. This field study was conducted to satisfy two main objectives:

  1. Evaluate a New Attic Pressure Matching Procedure. Evaluate the accuracy and practicality of a WHF airflow measurement procedure that uses a fan flowmeter instrument ducted to the WHF inlet. This procedure uses the fan flowmeter to measure airflow while matching the attic-to-indoor air pressure differential observed when the WHF is operating.
  2. Reevaluate the Existing Blower Door Pressure Matching Procedure. Reevaluate the accuracy of the RA3. WHF Airflow Rate Measurement procedure that uses a fan flowmeter mounted in a blower door apparatus. This technique first sets the indoor-to-outdoor pressure differential to minus 10 Pa ±5 Pa using the WHF, then matches that pressure differential using the blower door.

Both test procedures were evaluated against the RA3. Powered Flow Capture Hood test method using the only flow capture hood certified by the Energy Commission.

The primary advantage of the newly proposed test method is that it does not require the use of a certified powered flow capture hood that is designed for WHF airflow measurements, and that it may use any adequately sized duct to connect the fan flowmeter to the WHF intake (provided the intake is fully covered). Additionally, this procedure yields important information about attic pressure and the proper installation of attic vents. The new attic pressure matching method was shown to be accurate relative to the powered flow capture hood method.

Both the proposed attic pressure matching method and the existing RA3. Powered Flow Capture Hood method directly measure airflow at the WHF intake. They do not account for air recirculated between the pressurized attic and conditioned space. Thus, they can report higher air exchanges than houses experience under normal WHF operation.

The report makes the following recommendations:

  • Add the new attic pressure matching method to the Reference Appendices.
  • Retain the existing blower door test method but allow measured airflows to be adjusted upwards by a factor of 1.16 for two-story houses and 1.09 for one-story houses to account for recirculated air and to reconcile differences with direct measurement methods.
  • Consider eliminating the non-powered flow hood method due to inaccuracies from induced airflow resistance and limited ability of instruments to measure typical WHF airflows.
  • Consider reducing the arbitrary 67 percent degradation factor for non-HERS-verified whole house fans since it is unlikely that even improperly installed fans will suffer substantially reduced airflow, and verification does not ensure optimal use by building occupants.
  • Recommend that the 1 ft2 per 750 cfm vent requirement be removed. It is not typically enforced and enforcement by HERS Raters is challenging, time consuming, and unsafe. This will effectively improve compliance since it is likely most builders only adhere to California Building Code attic vent requirements.

Relevant Documents

Measure proposals, supporting documents, and other outside references will be made public as they become available.

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The Statewide CASE Team values input from all stakeholders engaged in the Title 24, Part 6 code change process. We encourage the open exchange of code change comments and concerns.

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