Make-Up Air During Kitchen Fires

A common request we receive is to shut off the make-up air unit during a kitchen fire, under the belief that doing so prevents feeding the flames. However, Evolve Mechanical Solutions strongly recommends that the make-up air remain ON during a fire.

Why?

Because make-up air plays a critical role in supporting the kitchen exhaust system. It helps remove smoke, flames, and combustion byproducts from the kitchen, maintaining visibility and improving safety for occupants and first responders.

NFPA-96 Guidance

The only scenario where make-up air should be shut off is when it is supplied internally to the hood, such as in the case of a bypass hood. This is outlined in NFPA 96 (2014), Section 8.3.2:

8.3.2 When the fire-extinguishing system activates, makeup air supplied internally to a hood shall be shut off.

The NFPA 96 Handbook further explains:

“This requirement is an attempt to reduce a ‘fanning’ effect on a fire.”

This guideline applies only to internally supplied hoods, not to systems where make-up air is introduced elsewhere in the kitchen.

What Happens If You Turn Off Make-Up Air During a Fire?

Let’s break it down with a simple example:

  • Restaurant Size: 650 sq. ft.

  • Ceiling Height: 10 ft

  • Total Volume: 6,500 cubic feet

  • Kitchen Exhaust Fan Capacity: 6,500 CFM (cubic feet per minute)

This means the exhaust fan is trying to remove all the air in the restaurant every 60 seconds. If the make-up air is turned off, there’s no source to replace that volume. The result?

  • The kitchen becomes starved for air

  • Exit doors may be difficult or impossible to open due to the air pressure difference between the interior and exterior of the restaurant

  • External static pressure rises sharply in the kitchen exhaust fan

  • Exhaust airflow drops dramatically—often to a fraction of its rated capacity

  • Smoke and flames remain trapped, reducing visibility and increasing the risk of injury or asphyxiation

Conclusion

Turning off the make-up air during a fire can severely impair smoke removal and make the situation more dangerous. Unless your system is a bypass hood or similar configuration covered under NFPA 96 Section 8.3.2, the make-up air should remain ON during a fire event.

James Hicks
Principal | Evolve Mechanical Solutions
✉️ james@evolvesolutions.ca
📞 236.777.3487
🏢 #200-116 W. Hastings St., Vancouver

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