How Many CFM Does the Ventilation Fan Need?
Take 0.03 cfm for each square foot of floor area, add 7.5 cfm times the number of bedrooms plus one, then subtract a small credit for natural leakage. In the Energy Commission's example, a three-bedroom, 1,800 square foot townhouse totals 84 cfm and its fan must move 82. The fan runs continuously or on a schedule averaged over three hours, and an ECC-Rater measures the airflow.
- Choose your report
- Upload your plans
- Pay securely
- Permit-ready report by email
The formula
Section 150.0(o)1C gives the total as Qtot = 0.03 x floor area + 7.5 x (bedrooms + 1), counting at least one bedroom. The fan rate, Qfan, is that total less a credit for the air that leaks in unaided. The leakage rate defaults to 2 air changes an hour at 50 pascals, so no blower door test is needed. Verified tighter leakage changes the credit. A balanced system earns a larger credit than an exhaust-only or supply-only one. Where Qfan comes out under 10 cfm, no fan is required.
Which fans qualify
Exhaust, supply, balanced and central fan integrated systems are all allowed. A whole-dwelling fan, or any local exhaust fan that runs continuously, must be rated at 1 sone or less. Bathroom fans can do the job: the compliance manual works an example with two continuous bathroom fans delivering 90 cfm against a 98 cfm requirement, and answers it by upsizing a fan or switching to a balanced system. The fan switch carries a label telling the owner to leave it on.
Running it
- Open windows do not count (Section 150.0(o)1A).
- A fan that runs only part of the time must be sized up so the three-hour average still meets the requirement. A system needing 80 cfm that runs one hour in three must deliver 240 cfm.
- An air handler may not run continuously to supply it.
The test
An ECC-Rater measures the airflow with a flow hood or grid under Residential Appendix RA3.7. For a balanced system the supply and exhaust figures are averaged: an HRV delivering 80 cfm and exhausting 90 against a requirement of 86 averages 85 and fails. Rated airflow and installed airflow differ, so leave a margin. The installer completes CF2R forms MCH-27 and MCH-32, and the rater's CF3R form is MCH-27.
Which projects
The requirement applies to new dwelling units, including detached ADUs and attached ones that share no floor or ceiling with another unit, but not junior ADUs. It also reaches additions of more than 1,000 square feet to an existing dwelling. Smaller additions, alterations and room additions are not required to add one, though a component that already met these rules must keep meeting them. The bathroom fan rules are on bathroom exhaust fans, and the background is on whole-house ventilation.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
No. Section 150.0(o)1A says window operation is not a permissible way to provide the whole-dwelling ventilation airflow.
It may run on a schedule if the average over any three-hour period meets the required airflow. ASHRAE 62.2 does not allow averaging over a longer period, so a fan that is off for part of the day must be larger.
Yes. The compliance manual allows demand-controlled local exhaust to be integrated with the dwelling ventilation system. A fan that runs continuously must be rated at 1 sone or less, and its delivered airflow is measured by an ECC-Rater.
Explore Ventilation
- MERV 13 Filter Sizing for Title 24
- Is Your Range Hood Title 24 Compliant?
- Does Title 24 Require a Whole House Fan?
- Whole-House Ventilation Fan Sizing Under Title 24
- Whole-House Ventilation Requirements
View all Ventilation guides → · HVAC and Water Heating · Title 24 guide