Use one unit throughout: m³/h, CFM or another consistent volumetric-flow unit.
Industrial Exhaust System Guide
Make-Up Air For Industrial Exhaust Fans: Balance, Pressure And Interlocks
Make-up air replaces air removed by exhaust. Size and control it from the complete air balance and the required room pressure—not from a universal percentage of fan nameplate airflow.
Make-Up AirOutside Replacement Air
QuantityFrom The Full Air Balance
PressureProject-Defined Target
ProofFlow Or Pressure, Not Power
Direct Answer
What Does Make-Up Air Do For An Exhaust Fan?
Make-up air gives the building a deliberate, clean path for replacing the air that an industrial exhaust system removes.
Without an adequate path, the exhaust fan must work against increasing building negative pressure. Doors can become difficult to open, air can enter through dirty or unsafe locations, natural-draft appliances can be affected, and the fan may deliver less airflow than the duty requires.
More supply is not automatically better. Too much or poorly distributed air can destroy intended negative containment, push contaminants toward cleaner rooms, create cross-drafts at a hood or travel directly from an inlet to an outlet. Quantity, distribution and control must be reviewed together.
Method and source links reviewed 3 October 2026 · This guide does not set a universal supply-to-exhaust ratio or room-pressure value
Do Not Mix The Functions
Make-Up Air, Ventilation, Circulation And Exhaust Are Different
A single air-handling system may serve more than one duty only when each duty is calculated and verified.
| Airflow Function | Primary Job | What It Does Not Prove |
|---|---|---|
| Make-up air | Bring outside air in to replace process exhaust and building exfiltration. | It does not by itself prove the occupied-space outdoor-air requirement or contaminant capture. |
| Outdoor-air ventilation | Provide clean outdoor air according to the applicable indoor-air-quality design basis. | It may be smaller than process exhaust and therefore may not balance the building. |
| Transfer air | Move air from an adjacent room or zone through a defined opening or duct path. | It is not outside air, and it is unsuitable when the source zone is not clean enough. |
| Air circulation | Redistribute air inside the same space for mixing, comfort or temperature uniformity. | It neither replaces exhausted air nor creates outdoor-air exchange. |
| General exhaust | Remove mixed room air for a defined heat, moisture, odor or dilution objective. | It does not replace source capture where a hood or enclosure is needed. |
| Local exhaust | Capture a contaminant at or near its source before it spreads through the room. | Room ACH alone does not prove hood capture or safe discharge. |
Conservation Of Air
Calculate The Building Balance Before Selecting A Percentage
At a steady operating state, air entering the control volume must equal air leaving it.
The reference may be outdoors or an adjacent cleaner space. The correct sign and range come from the project design basis.
| Balance Input | How To Establish It | Common Error |
|---|---|---|
| Total operating exhaust | Measure or establish every hood, general exhaust fan and process mode at installed pressure. | Adding catalog free-air values that the installed system cannot deliver. |
| Dedicated make-up air | Use delivered airflow after filters, coils, louvers, dampers and duct losses. | Using supply-fan nameplate flow without a field measurement. |
| Transfer air | Measure the intended flow from adjacent clean zones and record door or damper state. | Assuming every open doorway provides clean, repeatable air. |
| Infiltration and exfiltration | Assess envelope leakage and verify the resulting room pressure under the real operating state. | Treating uncontrolled leakage as a stable engineered supply path. |
| Operating diversity | List minimum, normal, maximum and fault combinations of exhaust and supply equipment. | Balancing only one fan combination while the process has several modes. |
Why There Is No Universal “80% Rule”
A fixed percentage ignores transfer air, envelope leakage, multiple exhaust modes, filter loading, seasonal density and the actual pressure objective. Use a preliminary ratio only as an explicitly labeled scenario; commission the final balance with measured airflow and differential pressure.
Diagnostic Evidence
Symptoms Of Too Little, Too Much Or Poorly Distributed Air
Symptoms identify what to measure; they do not replace airflow and pressure testing.
| Observed Condition | Possible System Effect | Verify Before Changing Setpoints |
|---|---|---|
| Doors pull inward or are hard to open | Excessive negative pressure or an undersized/blocked replacement-air path. | Room differential pressure, damper position, louver free area and filter pressure drop. |
| Exhaust volume falls as the building closes | Added building resistance moves the exhaust fan to a lower-flow operating point. | Exhaust airflow and fan inlet/static pressure with controlled openings open and closed. |
| Drafts enter through docks, cracks or combustion flues | Air is using unintended paths; contaminants or combustion products may be drawn indoors. | Flow direction at openings, appliance requirements and intake availability. |
| Odor or dust migrates into cleaner rooms | The process room may be neutral/positive when negative containment was intended. | Room-to-room pressure, supply modulation and exhaust proof in every operating mode. |
| Hood capture becomes unstable | A nearby diffuser or doorway creates a cross-draft across the capture zone. | Local air velocity, smoke visualization where appropriate and diffuser direction. |
| Supply air travels directly to exhaust | Short-circuiting leaves occupied or process zones poorly served. | Inlet/outlet placement, distribution pattern and zone measurements. |
Location And Distribution
Bring Clean Air In Without Defeating The Exhaust System
The intake location protects air quality; the supply pattern protects capture and room balance.
Choose A Clean Intake
Review exhaust discharge, loading docks, parking and idling zones, plumbing vents, combustion outlets, cooling towers and process emissions. Apply the separation required by the project jurisdiction and equipment.
Protect The Opening
Account for weather hood, screen, louver, filter, damper and actual net free area. Each component adds pressure loss that belongs in the supply-fan duty.
Distribute At Low Disturbance
Keep supply jets from crossing a hood face or pushing contaminants through the breathing zone. Use the room geometry and occupied zones to choose the discharge pattern.
Avoid Short-Circuit Paths
Do not let fresh air move directly from an inlet to the nearest exhaust opening while bypassing the process or occupied area that it is meant to serve.
Do Not Copy One Global Separation Distance
Required intake-to-discharge separation depends on local code, discharge type, contaminant, wind, building geometry and the adopted standard. Record the governing source and edition for the actual site.
Controls And Fault Proof
What A Make-Up Air And Exhaust Interlock Should Verify
The sequence must prove the airflow condition that protects the process—not merely that a motor received power.
Enable The Required Exhaust Mode
Start the exhaust combination required for the active process. Where capture is safety-critical, define whether the process may start before exhaust proof is established.
Prove Exhaust Performance
Use the commissioned signal appropriate to the system—such as hood or fan static pressure, airflow or damper position. A motor run contact cannot detect a broken belt, closed damper or blocked path.
Enable And Prove Make-Up Air
Coordinate the supply command with exhaust, then verify supply-fan pressure or airflow and filter condition. Avoid a sequence that leaves the room at an unintended pressure during changeover.
Modulate To The Pressure Objective
Use a room differential-pressure sensor located away from doors, windows and the supply discharge. Tune the control loop against real building response, not a copied setpoint.
Alarm A Failed Condition
Provide a clear alarm for low exhaust proof, low supply proof, abnormal room pressure, blocked filters or a failed damper. State which fan, zone and action the alarm concerns.
Define The Safe Fault Response
Decide from the process risk whether a fault stops the process, holds exhaust on, closes a damper, changes fan speed or requires evacuation. Do not apply one shutdown sequence to every application.
Field Acceptance
Make-Up Air Commissioning Checklist
Record the state of the whole system so the result can be repeated after filter loading, seasonal change or maintenance.
| Commissioning Check | Record | Acceptance Question |
|---|---|---|
| Operating modes | Every exhaust and supply combination, process permissive, door and damper state | Are normal, reduced, maximum and fault modes explicitly defined? |
| Delivered exhaust airflow | Measured flow or commissioned pressure at each critical hood/fan | Does capture remain adequate at the installed system resistance? |
| Delivered make-up air | Measured supply volume after filters, coils, louvers and ducts | Does the actual supply support the intended balance in every mode? |
| Room differential pressure | Room versus corridor/outdoors, sensor location and door state | Is the sign stable and within the project-defined range? |
| Distribution and cross-drafts | Diffuser direction, local velocities and observations at hoods/doors | Does incoming air reach the zone without defeating capture? |
| Intake and discharge | Locations, contamination sources, prevailing conditions and applicable separation rule | Is clean air protected from re-entrainment? |
| Controls and alarms | Setpoints, delays, proof signals, alarm messages and tested fault response | Does each simulated failure produce the documented safe action? |
| Baseline for maintenance | Clean-filter pressure, fan speed, airflow, pressure and instrument details | Can future technicians distinguish filter loading or equipment drift from normal operation? |
Primary References
Definitions, Design Logic And Control Evidence
These sources establish principles and terminology. The governing code and qualified design review still come from the project location and process.
ANSI/AMCA Standard 99-25: defines make-up air as outside air brought into a building to replace process exhaust air and building leakage. It also distinguishes a packaged make-up air unit.
Open The AMCA Standards HandbookOSHA Technical Manual, Section III, Chapter 3: explains that exhausted air must be replaced, and that an undersized replacement-air path can create negative pressure and reduce exhaust flow.
Open The OSHA Ventilation ChapterUFC 3-410-04 Industrial Ventilation: provides a detailed U.S. Department of Defense design example covering replacement-air quantity and distribution, room pressure, sensors, alarms and interlocks. Its mandatory provisions apply to its stated DoD scope, not every industrial project worldwide.
Open The Official WBDG UFC PageReplacement-Air Questions
Industrial Make-Up Air FAQ
Short answers for system planning; the balance, sequence and checklist above show how to verify them.
What is make-up air for an industrial exhaust fan?
It is outside air intentionally brought into the building to replace air removed by process or general exhaust and air lost through exfiltration. AMCA's definition distinguishes this replacement role from simple recirculation.
How much make-up air is required?
There is no universal percentage. Account for total exhaust, dedicated supply, clean transfer air, leakage and the intended room pressure, then confirm the result with measured airflow and differential pressure.
Is make-up air the same as outdoor-air ventilation?
No. Make-up air balances exhausted air. Outdoor-air ventilation follows the applicable indoor-air-quality method. One air system may contribute to both, but the design must demonstrate both duties separately.
Should the exhaust and make-up air fans be interlocked?
Where loss of either airflow defeats pressure or contaminant control, coordinated operation and a defined fault response are needed. Use an appropriate airflow, pressure or damper proof; a motor-on signal alone does not prove performance.
Can an open door be the replacement-air path?
An open door can carry air, but it changes with use, weather and pressure and may admit contaminants or disrupt capture. Use a defined, controllable path when the system needs repeatable operation.
Why does exhaust airflow fall when the building is closed?
As the building becomes more negative, the fan sees additional resistance. Its operating point can shift to lower flow unless a practical replacement-air path relieves that pressure.
Turn The Air Balance Into A Fan Brief
Send the exhaust duty and operating modes, space relationship, intended pressure, clean-air source, intake/discharge locations, duct and filter losses, electrical supply, controls and required documents.