Makeup Air Systems: Ductwork Design for Replacement Air

Every cubic foot of air that an exhaust system removes from a building must be replaced. In loosely constructed older buildings, natural infiltration handled this automatically — enough air leaked in through gaps to replace what was exhausted. In modern tight construction, and in buildings with large exhaust systems, this replacement air doesn't arrive on its own. A makeup air system must supply it deliberately.

When makeup air is absent or inadequate, the results are predictable: negative building pressure, combustion appliance back-drafting, kitchen exhaust hood starvation, uncomfortable drafts at entry doors, and occupant discomfort complaints that are difficult to diagnose without understanding the air balance concept.

When Is Makeup Air Required?

Makeup air is required — either by code or by physical necessity — in these situations:

Commercial kitchens. A commercial kitchen hood exhausting 2,000 CFM or more creates a significant negative pressure in the kitchen unless replacement air is supplied directly to the hood face or the kitchen space. IMC Section 507.4.1 requires makeup air systems for hoods exhausting more than 2,000 CFM. NFPA 96 reinforces this requirement. Without makeup air, the exhaust hood operates ineffectively and the kitchen becomes uncomfortable and negatively pressurized relative to the dining room.

Industrial exhaust systems. Process exhaust — paint booths, welding exhaust, laboratory fume hoods — can exhaust thousands of CFM. Buildings with total exhaust exceeding 1,500 CFM typically require engineered makeup air systems to maintain neutral building pressure.

Tight residential construction. ASHRAE 62.2-2019 and IRC M1508 address whole-house ventilation for residences with blower door test results below 3 ACH50. These tight houses cannot rely on infiltration for fresh air and require mechanical ventilation that is effectively a makeup air system for the living space.

High-flow residential range hoods. Range hoods exhausting more than 400 CFM in modern tight houses require makeup air under IRC M1503.4. Without it, the hood may back-draft combustion appliances in the building.

Makeup Air Quantity: How Much Do You Need?

The makeup air quantity equals the exhaust quantity, adjusted for intentional building pressurization strategy. For most commercial applications, the target is neutral building pressure — makeup air equals exhaust air. For some applications, slight positive pressure is desirable:

• Clean rooms and hospitals: Slightly positive (0.02–0.05 inches WC) to prevent contaminant infiltration
• Commercial kitchens: Slightly negative in the kitchen (to prevent cooking odors migrating to dining areas) but managed through hood face velocity, not by starving the makeup air system
• Most commercial spaces: Neutral to slightly positive at building perimeter

Calculation approach:

1. Sum all exhaust system airflows (hoods, toilet exhaust, general exhaust)
2. Add fresh air requirements from ASHRAE 62.1 for occupancy ventilation
3. Subtract any natural infiltration credit (typically zero for tight buildings)
4. The result is the required makeup air supply quantity

Makeup Air Delivery Methods

How makeup air enters the space is as important as how much is supplied. Delivery method determines comfort, energy efficiency, and system effectiveness:

Direct discharge at the hood face. For commercial kitchen hoods, the most effective makeup air delivery is directly at the hood face — short-circuit air that enters the hood at the front edge without entering the kitchen space. This minimizes the thermal load on the kitchen environment (no hot or cold outdoor air blowing on kitchen workers) while satisfying the hood's airflow requirement. Duct design: a continuous slot or multiple small diffusers around the hood perimeter, supplied from a dedicated makeup air unit that tempers the outdoor air.

Ceiling distribution. For general makeup air in commercial spaces, ceiling-mounted diffusers distribute replacement air throughout the space. This approach requires tempering (heating in winter, sometimes cooling in summer) to avoid comfort problems from untempered outdoor air. Duct layout follows conventional supply duct principles with appropriate velocity targeting for diffuser selection.

High sidewall or perimeter. For spaces where ceiling distribution isn't feasible, high sidewall grilles on exterior walls provide replacement air near the exhaust zones. This minimizes the duct run from the makeup air unit to the supply points.

Direct to equipment room. In residential applications requiring makeup air for a high-flow range hood, a short duct from an exterior wall to the area behind the hood provides replacement air at the point of use with minimal duct complexity.

Makeup Air Duct Sizing

Makeup air duct systems are sized using the same principles as supply ductwork — velocity targeting and friction rate methodology. Key differences from conventional supply systems:

• Higher velocity is acceptable in some makeup air applications where noise is less critical (industrial environments, back-of-house commercial) — up to 1,500 FPM in main trunks
• Tempering equipment pressure drop must be included in the system static pressure calculation — direct-fired makeup air units add 0.25–0.50 inches WC across the burner section
• Damper sizing is critical — modulating dampers that regulate makeup air quantity with exhaust system variations must be sized for adequate travel range without hunting
• Stainless steel or aluminum should be considered for makeup air ducts in food service applications where condensation from unconditioned outdoor air can accumulate in the duct

For commercial kitchen applications, the makeup air duct from the unit to the hood typically runs 10–20 linear feet. Size the duct for 800–1,000 FPM velocity in the main run, transitioning to lower velocity at the hood face plenum before the discharge slots.

Code and Permitting Considerations

Makeup air systems in commercial applications require mechanical permits and inspections in most jurisdictions. Key code references:

• IMC Section 507.4: Makeup air requirements for commercial cooking operations
• NFPA 96: Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations — includes makeup air provisions
• IRC M1503.4: Residential makeup air for range hoods over 400 CFM
• IRC M1508: Whole-house mechanical ventilation for tight residential construction
• ASHRAE 62.1/62.2: Minimum ventilation rates that define the makeup air requirement in many commercial and residential applications

Always verify local amendments to these model codes — some jurisdictions have stricter requirements than the base model codes, particularly for commercial kitchen ventilation.