Whole-House Ventilation Fan Sizing Under Title 24
Add 0.03 cfm per square foot of floor to 7.5 cfm times bedrooms plus one. In an existing house an addition triggers a whole-dwelling fan only when it adds more than 1,000 square feet, and then existing plus new floor area counts. A 2,600 square foot four-bedroom home needs 115.5 cfm before the leakage credit.
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First question: does the addition trigger a fan?
Section 150.2(a)1Ci excuses an addition that adds 1,000 square feet or less to an existing dwelling. Above that, airflow is based on the whole dwelling unit, existing plus addition. A new dwelling unit built as an addition is sized on its own floor area. Kitchens and bathrooms still need local exhaust either way.
Two worked examples
- 1,500 square feet plus 900. The addition is under the line, so no whole-dwelling fan is required.
- 1,500 square feet plus 1,100, four bedrooms afterwards. Qtot = 0.03 x 2,600 + 7.5 x (4 + 1) = 78 + 37.5 = 115.5 cfm.
Taking the leakage credit
Qfan = Qtot less F x Qinf x Aext. For the second house, a two-story detached home with 9-foot ceilings has 23,400 cubic feet. At the default 2 ACH50, leakage at 50 Pa is 780 cfm. With a weather factor of 0.43 (Table 150.0-D, March AFB) and a height of 18 feet, Qinf is about 23.9 cfm. An exhaust fan uses F = Qinf / Qtot, about 0.21, so the credit is about 5 cfm and the fan moves roughly 110.6 cfm. Under 10 cfm, no fan is needed.
Choosing the fan
- Window opening never counts, and a continuously running air handler may not serve as the ventilation fan.
- A fan running two hours in every three must deliver about 166 cfm to average 110.6.
- A fan that runs continuously is rated 1.0 sone or less, and the wall switch carries the leave-on label.
- The ECC-Rater measures at the grilles, so choose a rating above the target to cover duct losses.
If you are replacing a fan in the existing house instead, it must be rated for airflow and sound and, where airflow is required, at no less than that figure. The formula is also on the IAQ fan page and the background on whole-house ventilation. Addition rules by size are on 1,000 square foot additions and larger ones, balanced options on HRV vs ERV, and reports at residential.
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
The equation counts bedrooms in the dwelling unit, and above 1,000 square feet the floor area is that of the whole dwelling. Count the bedrooms the home has once the addition is finished.
No whole-dwelling fan, because it adds 1,000 square feet or less. Local exhaust in each kitchen and bathroom still applies.
Explore Ventilation
- Whole-House Ventilation Requirements
- Title 24 Bathroom Exhaust Fan Requirements
- Can Bathroom Exhaust Fans Serve as Whole-House Ventilation?
- Can Existing HVAC Serve a New Home Addition?
- Can My Furnace Fan Provide Whole-House Ventilation? CFI Under Title 24
View all Ventilation guides → · HVAC and Water Heating · Title 24 guide