Make-Up Air for Commercial Kitchens: Why It Matters

Every commercial kitchen exhaust system removes thousands of cubic feet of air per minute. That air has to come from somewhere. If you don't provide a designed path for replacement air — called make-up air (MUA) — the kitchen creates a vacuum. Doors won't open, the hood stops capturing smoke, energy bills spike, and your HVAC system fights a losing battle. Make-up air isn't optional. It's what makes your exhaust system actually work.

What Make-Up Air Actually Does

When an exhaust hood removes air from the kitchen, the building needs to replace it. Without a dedicated make-up air system, that replacement air enters the building through every crack, gap, and opening it can find — exterior doors, windows, gaps in the building envelope, and even plumbing vents. This uncontrolled infiltration is:

• Unconditioned (hot in summer, cold in winter)
• Unfiltered (dust, pollen, exhaust fumes from the parking lot)
• Unpredictable (you can't control where it enters or how much)

A make-up air unit provides a controlled, conditioned supply of replacement air delivered directly to the kitchen — typically through a duct system near the hood or through ceiling diffusers. This keeps the building pressure balanced, the hood working efficiently, and the HVAC system from being overwhelmed.

The 80% Rule: How Much Make-Up Air You Need

The standard design practice is to supply make-up air equal to 80% of the exhaust volume. The remaining 20% comes from "transfer air" — air drawn from the dining room and adjacent spaces through the kitchen's swinging doors and pass-through windows.

Some codes and jurisdictions require different ratios. The IMC states that the total supply and make-up air must approximately equal the exhaust air to maintain building pressure. Some engineers design for 85-90% MUA in tightly sealed buildings where transfer air paths are limited.

The transfer air must also be accounted for in the dining room HVAC design. If the kitchen pulls 1,000 CFM of transfer air from the dining room, the dining room's HVAC needs to supply an additional 1,000 CFM to maintain its own pressure balance. This is frequently overlooked, leading to uncomfortable dining rooms.

What Happens Without Adequate Make-Up Air

The consequences of undersized or missing make-up air are immediate and measurable:

Negative Pressure

When exhaust exceeds supply, the kitchen goes negative. You can feel it — the pressure difference makes exterior doors hard to open (or impossible for some employees) and interior doors slam shut. In severe cases, the negative pressure is strong enough to pull exhaust gases back down inactive flue vents, creating carbon monoxide hazards.

A properly balanced kitchen should have slightly negative pressure (0.02-0.05 inches water gauge) relative to the dining room — just enough to keep cooking odors from migrating to guests, but not so much that it creates operational problems.

Reduced Hood Efficiency

An exhaust hood works by creating a capture velocity at the hood face — typically 50-100 feet per minute. When the kitchen is severely negative, the supply of air to the hood face becomes turbulent and inconsistent. Smoke and grease escape the hood canopy, the kitchen fills with haze, and grease deposits accumulate on surfaces throughout the space.

Energy Waste

Without MUA, your building's HVAC system tries to compensate for the air being removed. In winter, this means pulling freezing outdoor air through every crack — your heating system runs nonstop. In summer, hot humid air floods in, overwhelming the cooling system. Facilities without proper MUA typically see 30-50% higher HVAC energy costs compared to properly balanced kitchens.

Pilot Light and Burner Issues

Extreme negative pressure can starve gas burners of combustion air, causing pilot lights to blow out, burners to burn yellow (incomplete combustion), and in worst cases, carbon monoxide production. This is a safety hazard that health departments and fire marshals will flag immediately.

Types of Make-Up Air Systems

1. Direct-Fired MUA Units

The most common type for commercial kitchens. Outdoor air passes through a gas-fired burner that heats it before delivering it to the kitchen. No heat exchanger — the flame contacts the air directly, which is 92-95% efficient.

• Pros: Highest efficiency, lowest operating cost, fast response to temperature changes
• Cons: Cannot cool air (heating only), requires gas connection
• Cost: $5,000-$15,000 for the unit depending on CFM capacity
• Best for: Northern climates where heating the make-up air is the primary concern

2. Indirect-Fired MUA Units

Similar to direct-fired but uses a heat exchanger to separate combustion gases from the supply air. Less efficient (80-85%) but produces cleaner air — important in spaces with stringent indoor air quality requirements.

3. Rooftop MUA Units with Cooling

These provide both heating and cooling for the make-up air. Essential in southern climates where outdoor air in summer is 95°F+ with high humidity. Delivering unconditioned 95°F air into a kitchen that's already 100°F from cooking equipment is counterproductive.

• Cost: $10,000-$25,000+ (significantly more due to refrigeration components)
• Best for: Southern and western US, any location with 90°F+ summers

4. Short-Circuit (Front-Face Discharge) Supply

Make-up air is delivered through a perforated plate or diffuser built into the front face of the hood itself. The air enters at low velocity directly in front of the hood opening, where it's immediately captured by the exhaust. This "short circuits" the air path — reducing the net CFM that the building's HVAC system needs to handle.

• Pros: Reduces effective load on building HVAC, improves hood capture
• Cons: Must be carefully balanced — too much velocity disrupts the thermal plume
• Typical supply: Up to 50-60% of exhaust CFM through short-circuit, remainder through conventional MUA

Code Requirements

Both the IMC and UMC require that make-up air be provided for commercial kitchen exhaust systems. Key code points:

• Make-up air must be provided at a rate approximately equal to the exhaust rate
• Make-up air must be conditioned (heated, at minimum) to avoid discomfort and condensation
• The system must be interlocked with the exhaust fan — when exhaust runs, MUA runs
• Outdoor air intakes must be located away from exhaust outlets (minimum 10 feet separation per most codes)
• In seismic zones, ductwork must be braced per local amendments

Installation Considerations

Where and how make-up air enters the kitchen matters as much as the volume:

• Air velocity: MUA should enter the kitchen at low velocity (50-75 FPM) through diffusers. High-velocity jets of air disrupt the hood's capture zone and blow cooking fumes into the kitchen.
• Location: Supply diffusers should be positioned to create a general flow pattern toward the hood, not directly into the hood face (which causes turbulence) or behind the cook (which creates uncomfortable drafts).
• Temperature: MUA should be delivered at 65-72°F year-round. Delivering unconditioned 40°F air into a kitchen creates cold spots and condensation on cooking surfaces.

Size Your Ventilation System

Getting the make-up air balance right starts with knowing your exhaust CFM requirements. Our calculator sizes the entire ventilation system — exhaust CFM based on your equipment and hood type, plus the matching make-up air requirement — so you can get accurate numbers for equipment selection and contractor bids.

Try the Kitchen Exhaust CFM Calculator →