Make Up Air Systems for Commercial Kitchens

Why a Commercial Kitchen Needs a Make Up Air System

A commercial kitchen needs a make up air system because every cubic foot of air an exhaust hood removes has to come back into the building from somewhere. If no one plans where it comes from, it arrives through door gaps, the drive-thru window, flue vents and the dining room, and it usually arrives cold, hot or dirty.

A single 12-foot canopy over a fryer and charbroiler line can easily exhaust several thousand CFM. Without a matched supply, the building goes strongly negative. The symptoms are familiar to any operator who has lived with them:

• Exterior doors that are hard to open or slam shut on their own.
• Water heater and appliance flues that backdraft, leaving combustion odor in the space.
• Hoods that spill smoke because the exhaust fan cannot move its design airflow against the building suction.
• Grease and odor migrating into neighboring tenant spaces in strip centers and food halls.

The mechanical code requires make up air to be supplied while the kitchen exhaust runs, in an amount approximately equal to the exhaust. How that air is heated, filtered and delivered is where the engineering judgment comes in. The exhaust side is covered on our commercial kitchen exhaust system page; this page is about the supply side.

Balancing Make Up Air Against Hood Exhaust and Building Pressure

The goal is a kitchen that sits slightly negative to the dining room and adjacent spaces, so cooking odor flows toward the hoods and not out into the seating area. A common approach is to supply most of the exhaust volume, often 80 to 90 percent, through the dedicated make up air unit and let the remainder arrive as transfer air from the dining room, where the rooftop HVAC unit has already conditioned it.

That transfer air only works if the front-of-house rooftop units bring in enough outdoor air to cover it. If they do not, the building still runs negative even though the kitchen balance looks fine on paper.

A simple balance check

Exhaust rates themselves come from the appliance duty classes and hood style. Our hood sizing guide walks through how those exhaust numbers are built, and the make up air schedule follows directly from them.

Tempered vs Untempered Make Up Air

Untempered make up air is outdoor air delivered at whatever temperature it is outside; tempered make up air is heated, and sometimes cooled, before it enters the kitchen. The choice depends mostly on climate, the hours the kitchen runs and what the local energy and mechanical codes allow.

Untempered units are simple: a filter section, a supply fan and a motorized intake damper. They make sense in mild climates, or where the supply is discharged so close to the hood that the cooks barely feel it. In a northern winter, though, untempered air drops the kitchen temperature, chills the line, and tends to get shut off by staff, which defeats the whole system.

Tempered units add a heat source and usually a discharge temperature controller. Some projects also add cooling through a DX coil or evaporative section, which helps comfort on a hot line but adds cost and refrigerant or water service. Many jurisdictions limit how much unconditioned or overconditioned air a kitchen can use, so the energy code is part of the selection.

Direct-Fired and Indirect-Fired Gas Make Up Air Units

Gas-heated make up air units come in two basic types: direct-fired, where the burner sits in the airstream, and indirect-fired, where a heat exchanger keeps combustion products out of the supply air.

Direct-fired units

A direct-fired unit burns natural gas or propane right in the supply airstream. Nearly all of the fuel energy ends up in the air, which makes these units efficient and quick to respond. The combustion products are diluted into a large volume of outdoor air, and the units are built and listed to keep those products within limits. Because of that, direct-fired equipment is generally intended for 100 percent outdoor air and should not be set up to recirculate indoor air.

Indirect-fired units

An indirect-fired unit heats the air through a heat exchanger, and the flue gases leave through a vent. Efficiency is lower than direct-fired, but the supply air never contacts combustion products. Indirect units are often chosen where the owner, the engineer or the jurisdiction prefers that separation, or where the unit also has to handle some return air. Where gas is not available, electric heat or a hot water coil can do the job, though electric heat at kitchen airflows often means a larger electrical service.

Make Up Air Discharge Options at the Hood

Where and how make up air enters the kitchen matters as much as how much of it there is. Supply air blown across the face of a hood, or straight down onto the cooking line, can push smoke out of the capture zone even when the exhaust rate is correct.

Front face discharge

A front face plenum mounts on the front of the canopy and discharges air horizontally into the room, away from the hood edge. It is popular because it delivers air close to the cooks without a separate ceiling layout. The discharge velocity has to stay low and the air should be directed outward, not down across the hood opening.

Perforated perimeter supply

A perforated plenum along the front or sides of the hood releases air downward at low velocity over a wide face. When the velocity is kept low, this pattern tends to support capture rather than disturb it. Many hood manufacturers test their perimeter supply options with specific hood models, and the published limits are worth following.

Ceiling diffusers

Ceiling diffusers spread make up air through the kitchen like conventional HVAC supply. Use low-velocity, perforated-face or displacement-style diffusers, and keep them away from the hood face. Four-way slot diffusers placed directly in front of a canopy are a classic cause of spillage.

Short-circuit (internal supply) hoods

Short-circuit hoods inject unconditioned air inside the canopy so it is exhausted almost immediately. They were sold as an energy saver, but field and lab testing over the years showed that injecting a large share of supply inside the hood often hurts capture. Some codes and jurisdictions restrict the approach. We generally do not recommend it, and where an existing kitchen has one, we look at capture performance before reusing it.

Interlocking Make Up Air With the Exhaust Fan

The make up air unit must be interlocked with the kitchen exhaust so the two run together, and the controls should be arranged so supply cannot run when the exhaust is off. The code calls for the interlock, and practical experience backs it up: a supply fan running alone pressurizes the kitchen and pushes cooking odor and heat into the dining room.

Typical control arrangements include:

• A hood control panel that starts exhaust and supply together from one switch.
• Automatic exhaust start on heat sensing in the hood, required by code in some cases when appliances are on.
• Demand control kitchen ventilation, where exhaust and supply speeds track cooking load through temperature or optical sensors and variable frequency drives.
• Shutdown of the make up air on fire suppression system discharge, while the exhaust fan generally keeps running unless the fire protection design calls otherwise.

The fire suppression system typically shuts off fuel to the appliances and the make up air, but leaving the exhaust running helps clear smoke. The exact sequence comes from the suppression system design and the AHJ, so it belongs on the control drawings. See our page on commercial kitchen exhaust fans for the fan side of these controls.

Rooftop Placement, Filtration and Service Access

Most kitchen make up air units sit on the roof on a curb or rail, with a supply duct dropping to the hood plenums or ceiling diffusers. A few placement details prevent most of the problems we see later.

• Separation from the exhaust fan. The intake must be located away from the grease exhaust discharge so the unit does not pull greasy air back in. The required separation is set by the mechanical code and should be measured on the roof plan, with the prevailing wind in mind.
• Away from other exhaust and plumbing vents. Toilet exhaust, sewer vents and appliance flues near the intake bring odor straight into the kitchen.
• Filters. Outdoor air filters keep insects, cottonwood and dust out of the duct. They clog faster than most people expect.

Supply ductwork from the make up air unit is ordinary sheet metal duct, not grease duct, and should be insulated where it carries tempered air through unconditioned space. It must never connect to the grease exhaust system.

Planning a Make Up Air System for a New or Existing Kitchen

The best time to plan make up air is when the hood schedule is being set, because exhaust and supply are two halves of the same calculation. On new projects we size the unit from the hood exhaust schedule, select the heating type with the mechanical engineer, and coordinate discharge style with the hood layout.

Existing kitchens are harder. When an operator adds a fryer or extends a hood, the exhaust goes up and the old make up air unit rarely keeps pace. Before adding exhaust, we look at:

1. The nameplate and actual measured airflow of the existing make up air unit.
2. Remaining heating capacity at the new airflow.
3. Whether the existing diffusers can take more air without blowing on the hood.
4. The roof structure and gas service if a larger unit is needed.

The design is done with the mechanical code, NFPA 96 and ASHRAE 154 in mind, and the authority having jurisdiction has the final say on the details. If you have a hood schedule or existing drawings, send them for a make up air quote and an engineer will review the balance with you.