Whole house fans (WHF) have been around for decades, but the 2022 California Energy Code now requires them to be tested in Climate Zones 8 through 14 (1).

Map of california with different zones outlined respective to their climates

In 2022, VCA tested hundreds of these fans in newly constructed housing and encountered many issues with getting WHFs to meet code. As with most HERS tests, sticking to the basics are key. Here’s what we have learned:

Have the correct testing equipment.
California has a list of approved test equipment. Below are two setups of approved testing equipment. The first is the setup VCA used and the second is a “do-it-yourself” version assembled with existing HERS equipment. The capture hood must fully cover the inlet grille and the equipment should measure the amount of airflow for which the WHF is rated. For example, the “Flow Grid” is on the approved list, but it can only measure 2,000 CFM or less. If your WHF moves more than 2,000 CFM of air, you will need different test equipment.

Two whole house fan set ups

Whole house fans are designed to circulate air in an entire home or building. The fan removes hot air from the building and draws in cooler outdoor air through windows and other openings.

Adhere to the recommended testing procedure.
To pass this required HERS test (when a WHF is installed), you must install a WHF capable of moving at least 1.5 cubic feet per minute (cfm) per square foot of the home’s conditioned floor area (1.5 cfm/ft2 or greater). Additionally, CA Energy Code (2) requires at least one square foot of attic vent free area for each 750 cfm of rated WHF airflow (3).

Accurate testing can be tricky.
Testing issues tend to be similar across projects. Often, the biggest challenge is that a “no pass” is the result of multiple issues requiring fixes by several trades, and the builder may not be able to just point to a single vendor for a quick fix. One of VCA’s projects with a WHF required a conference call with every related trade and the energy modeler to find a solution after 14 houses failed. The California Energy Commission (CEC) has a published test procedure for airflow, found here and elaborated on below.

Issues that can prevent the WHF from meeting Code include:

  • The WHF backdraft damper box (which prevents air from leaking into the house) is installed over the stairs, making it difficult or unsafe to test. The testing equipment needs roughly a three-foot2 flat surface to rest on (not a staircase).
  • Electricians may have installed the controls incorrectly and the fans are not programmed to an adequate speed.
  • There is not enough attic ventilation. The WHF has a pressure sensor, and when it senses too much pressure in the attic, it slows down. Too much pressure can be caused by an insufficient quantity of gable vents, the vents could be covered by insulation, or perhaps the roofer did not cut the penetrations properly.
  • WHF backdraft dampers can get stuck or an errant mounting screw can be prevent the damper from opening enough when the fan is running.
  • There is typically an acoustic duct that connects the backdraft damper box to the fan, and this can be damaged or smashed.
  • The attic may not be properly sealed. For example, the project is missing fire sprinkler caps, or the attic lid is missing the gasket, causing the air from the WHF entering the attic to mix back into the conditioned space. This throws off the test results and can also disrupt the even coverage of the thermal insulation in the attic.


Preconstruction coordination, as early as possible

  • Set up a meeting with the WHF manufacturer and architect and to ensure there are enough gable vents and the damper box is not positioned over the stairs.
  • Set up a meeting with the roofer, insulators, WHF manufacturer, HERS rater, and all onsite supervisors to review what is needed. The list above could be a starting point.
  • Include site supervisors to ensure there are no covered cable vents, duct damage, or other contributing issues.
  • Ensure the HERS contract includes checking for these issues during rough inspections.

During construction, before testing the WHF

  • Ensure the site supervisor checks the following:
  • Gable venting
  • Acoustic ducts
  • Correct wiring for the controls
  • Installation of fire sprinkler caps (escutcheons)
  • Installation of gaskets on the attic access
  • Damper blockage

Whole house fans contribute to energy savings and make life more comfortable, but they must be installed correctly to be efficient and satisfy the Energy Code. If you’re thinking about installing them in your next project, certain manufacturers provide useful charts that can help with modeling (4).

If help is needed to understand the Whole House Fan requirements, or if your systems need to be tested to ensure your occupants are comfortable and happy, reach out to VCA Green for more information.

Contributing Writer: Richard Swingle, HERS Rater

Moe Fakih, Principal
VCA Green

1- Residential Appendix RA3.9
2- Section 150.1(c)12
3- As determined by the Energy Commission’s database of certified appliances (MAEDBS), (PG & E, et. al., 2021)
4- Pacific Gas and Electric Company, Southern California Edison, San Diego Gas & Electric Company, Los Angeles Department of Water and Power, and Sacramento Municipal Utility District. (2021, March). Evaluation of Test Procedures for Whole House Fan Airflow Measurement. 2022 Title 24, Part 6—Technical Report, p. 2.