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What Percentage of Exhaust Air Should Be Replaced by Makeup Air?

2026-10-26

In any ventilation system, air removed by an exhaust fan must be replaced. If it is not, the room will develop negative pressure. This can reduce exhaust performance, make doors hard to open, cause drafts, and pull unfiltered air into the workspace.

The replacement air is called makeup air.

The General Rule: 80%–90%

For many industrial and commercial kitchen exhaust systems, the makeup air volume is commonly designed at 80%–90% of the total exhaust air volume.

The remaining 10%–20% is usually allowed to enter through natural infiltration, such as gaps around doors, windows, and building openings. This small amount of infiltration helps maintain a slight negative pressure, which is useful for keeping oil mist, smoke, dust, or fumes inside the controlled area.

For example, if the exhaust fan removes 10,000 m³/h, the mechanical makeup air may be designed at about 8,000–9,000 m³/h.

Why Not Always Replace 100%?

Replacing exactly 100% of exhaust air is not always necessary or ideal.

If the supply air equals or exceeds the exhaust air, the room may become neutral or even slightly positive. That can allow contaminated air to escape into adjacent areas. On the other hand, if makeup air is too low, the room becomes excessively negative. This can reduce hood capture efficiency, increase fan load, and create uncomfortable airflow conditions for operators.

Therefore, 80%–90% is often used as a practical balance between:

Stable exhaust performance
Controlled negative pressure
Energy consumption
Operator comfort
Prevention of cross-contamination

When a Different Ratio May Be Needed

The 80%–90% rule is a common reference, but it is not universal.

Key Design Tips

Confirm local codes first. Standards for makeup air can vary by country, industry, and application.
Measure total exhaust accurately. Include all hoods, ducts, and fans that operate simultaneously.
Account for infiltration. Older or leaky buildings allow more uncontrolled air entry than sealed modern buildings.
Condition the makeup air when needed. In hot, cold, or humid climates, untreated makeup air can affect worker comfort and energy costs.
Link makeup air to the exhaust system. The makeup air fan should normally start and stop together with the exhaust fan to avoid pressure imbalance.

Bottom Line

For most industrial exhaust and kitchen ventilation applications, designing makeup air at 80%–90% of the exhaust air volume is a reliable starting point. However, the final percentage should always be confirmed by the actual exhaust volume, building tightness, local regulations, and whether the space needs slight negative, neutral, or positive pressure.

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