Makeup air unit and commercial kitchen ventilation balance

The makeup air unit, and why the balance is the whole job

A makeup air unit, the MAU, is the supply fan and heating section that puts back the air a commercial kitchen exhaust hood pulls out. A Type I grease hood over a cookline moves a lot of air, often several thousand CFM, and every cubic foot of it leaves the building through the roof fan. That air has to come back from somewhere. The MAU is the somewhere you design on purpose, instead of letting the building find its own makeup through door gaps, elevator shafts, and whatever else it can pull through.

Think of the kitchen as a balance. Exhaust out, makeup in. When the two match the way they should, the kitchen sits at a slight, controlled negative to the dining room, the hood captures grease and smoke, the gas appliances draft up their vents, and the back door opens with one hand. Break the balance and every one of those goes wrong at once, usually on the busiest night.

The MAU is not a comfort unit for the dining room and it is not optional trim on the hood package. It exists because the hood exists. Size it to the exhaust, temper it for the climate, tie it to the exhaust fan so it can never be off while the hood runs, and balance it with a gauge instead of a feel. Everything else in this guide is a consequence of those four moves.

Why does the kitchen door slam and the hood spill smoke?

The kitchen door slams, or you have to lean on it to open it, because the exhaust fan is pulling more air out of the building than anything is putting back. The room goes negative to the outdoors, and that negative tries to pull air in through every gap it can find, the doors first. The same negative that fights the door is what starves the hood.

Here is the chain. The exhaust fan is rated to move its CFM against a certain static pressure. Starve it of makeup air and the room static climbs negative, the fan moves less than its rating, and the capture velocity at the hood lip drops off. Now the smoke and grease that should go up the duct rolls out the front of the hood into the cook's face. Field experience and the manufacturers both put real numbers on it: a kitchen run hard negative can lose a large fraction of its rated exhaust, on the order of up to 30 percent, while the staff swears the fan is broken.

The worst case is not comfort and it is not capture. It is combustion. A negative kitchen can backdraft an atmospheric water heater or a gas appliance, pulling flue gas and carbon monoxide back down the vent and into the space where people work. That is the failure that puts someone in the hospital, and it traces straight back to makeup air that was never provided or never sized. This is why the code treats makeup air as a requirement, not a nicety.

Start with the exhaust: the Type I hood and its CFM

Size the exhaust before you size the makeup, because the exhaust sets the number everything else follows. A Type I hood handles grease and smoke from cooking and is the one over your fryers, griddles, ranges, and charbroilers. A Type II hood handles heat and steam only, over a dishwasher or a steam kettle, and it is a different animal with no grease duct and no suppression. The makeup conversation is almost always about Type I.

The exhaust CFM comes from the hood style, the duty of the appliances under it, and the capture the design needs. A light-duty canopy over an oven moves far less air than a heavy-duty canopy over a charbroiler, because the broiler throws a hot, fast thermal plume that takes more air to catch and contain. Capture and containment is the hood's entire purpose, and it depends on the exhaust rate and the hood overhang together. Too little overhang or too little exhaust and the plume escapes the edges no matter how much makeup you supply.

Pull the exhaust rate from the hood manufacturer's listing and the project drawings, not from a rule of thumb, because a listed hood has a tested exhaust rate for the appliances it covers. The grease filters, the welded duct, the cleanouts, and the suppression all belong to a deeper inspection, and the hood suppression guide covers them in depth. For the makeup conversation, the exhaust CFM is the single input you carry forward into every number that follows.

How much makeup air does a kitchen need?

A kitchen needs makeup air roughly equal to what it exhausts. The IMC puts it plainly: the makeup air supplied from all sources has to be approximately equal to the total exhaust for the building. That is the code floor, and it is written about the whole building, not one piece of equipment. Get the building roughly neutral to the outdoors and you have solved most of the problem.

The design then splits that total. A common approach supplies about 80 to 90 percent of the hood exhaust from the dedicated MAU ducted to the kitchen, and lets the rest come from the conditioned space as transfer air through the pass window and the door from the dining room. That last slice is exactly what holds the kitchen slightly negative to the dining room. Some designs run the MAU lower, around 70 percent, and lean harder on transfer air and the dining room rooftop units. The split is a design decision, not a fixed law.

The number that triggers all of this in the IMC is 400 CFM. A hood exhausting more than that has to be provided with makeup air. Above roughly 800 CFM a dedicated, interlocked MAU usually earns its place over trying to pull makeup through a louver or a wall opening, because at those flows the negative needed to draw makeup passively gets large enough to fight the doors and the fan. Confirm the threshold and the method against the adopted code and the engineer's design before you commit the equipment.

Why the kitchen stays slightly negative to the dining room

The kitchen is held slightly negative to the dining room on purpose, so grease, smoke, and cooking odor stay on the kitchen side and do not drift out to the guests. Air moves from higher pressure to lower. Keep the dining room a touch positive relative to the kitchen and the air that leaks across the pass window and the door flows the right way, into the kitchen and up the hood, never the reverse.

Get this backwards and you smell it in the dining room. A kitchen pushed positive, usually from too much makeup air, blows cooking smell and grease-laden air out into the seating and the front entrance. Customers notice within a week, and so does the grease film that ends up on the dining room ceiling tiles and the light fixtures.

The balance the trade aims for is slight, not strong. Hard negative fights the door and backdrafts the appliances. Positive spills into the house. The window between them is narrow, often measured in hundredths of an inch of water column, which is the whole reason makeup air gets commissioned with a manometer instead of a guess. The target is two relationships at once: roughly neutral to the outdoors overall, and just negative to the space the diners sit in.

Tempering the makeup air

Tempering is heating, and in hot climates cooling, the makeup air so you are not dumping raw outdoor air on the cooks. In January a 3000 CFM MAU blowing 10 degree F air into the kitchen turns the line into a walk-in freezer with a stove in it, and the staff will prop the back door to get warm and defeat the entire system. So a commercial MAU carries a heating section, gas-fired in most units, that brings the supply up to a workable discharge temperature.

The IMC ties a number to this. The temperature difference between the makeup air and the conditioned space is held to about 10 degrees F unless the building HVAC is sized to carry the extra heating and cooling load. In practice many kitchens set the MAU discharge around 65 to 70 degrees F so the air lands close to neutral, neither warming the line nor chilling it. Confirm the setpoint against the design and the local code, because the differential limit is the part inspectors actually check.

Hot, dry climates flip the problem. There the makeup air gets cooled, often by direct or indirect evaporative cooling, which suits a high-airflow makeup load better than mechanical refrigeration because the air is changing out constantly and does not need to be driven deep cold. The lesson is the same in both climates. Condition the makeup so it is usable at the line, or the staff will find a way around it and the balance goes with them.

Direct-fired or indirect-fired makeup air?

Direct-fired and indirect-fired describe how the MAU heats the air, and the difference is where the products of combustion go. A direct-fired unit fires a gas burner straight into the airstream, so the flame and its combustion products mix into the supply air. An indirect-fired unit fires into a heat exchanger and only the heat crosses into the air, while the flue gas leaves separately through a vent.

Direct-fired wins on efficiency. With no heat exchanger to lose energy across, close to all of the fuel's heat reaches the space, so the operating cost is lower and the unit is cheaper up front. The cost is that carbon dioxide, water vapor, and trace combustion products, including nitrogen oxides and carbon monoxide, go into the kitchen with the air. The listing and the code cap how much, and some jurisdictions restrict direct-fired makeup over open food.

Indirect-fired keeps the combustion separate, so the supply air stays clean of flue products, at a lower thermal efficiency and a higher price. For a commercial kitchen where the air passes over open food, the cleaner indirect approach is often what gets specified, and the FDA Food Code requirement that makeup air not contaminate food pushes the same direction. Which type is allowed and which is required is a code and design call, so verify both against the AHJ and the project specification before you select the unit. Do not assume the cheaper direct-fired unit is acceptable just because it is on the shelf.

How and where the makeup air gets introduced

Where you put the makeup air matters as much as how much you supply. The clean approach is a dedicated MAU ducted into the kitchen and released near the cookline, low and slow, so it sweeps toward the hood without blasting across the capture zone. Done right the cooks feel a gentle replacement of air, not a draft, and the hood pulls the supply up off the appliances the way it is supposed to.

The mistake that kills capture is dumping makeup air straight into the hood's capture area, or aiming a high-throw supply diffuser at the front face of the hood. The hood is trying to pull a slow, even thermal plume up off the cooking surface. Hit that plume with a jet of supply air and you blow the smoke right back out the front into the room. Throw velocity is the enemy of capture, and a supply register in the wrong spot undoes a correctly sized exhaust.

Some hoods are built to introduce makeup at the hood itself. A short-circuit or front-discharge hood brings a portion of the makeup down at the inside front face, and an air-curtain style hood does a version of the same. These can work, but they are easy to get wrong, and untempered makeup delivered right at the hood face is a cold-draft and capture complaint waiting to happen. The simplest, most forgiving design keeps the bulk of the makeup as tempered room supply, placed away from the hood lip, with transfer air filling the rest.

The interlock: the fans run together or not at all

The MAU and the exhaust fan are interlocked so they run together, every time, with no way to have the hood on and the makeup off. The IMC requires the mechanical makeup air system to start and operate automatically with the exhaust system. That is not a convenience feature. It is the control that prevents the negative-pressure failure the whole system exists to avoid.

The usual control proves it both directions. Turn the hood switch on and the exhaust fan and the MAU energize together. The MAU should also not be able to run while the exhaust is off, or you pressurize the kitchen and push air out the front door. On variable systems the two modulate together so the makeup tracks the exhaust as it ramps up and down through the day.

The failure to look for is an interlock that got defeated. Someone wires the MAU to a separate switch for the morning warm-up, or a control relay fails and the exhaust runs alone for weeks while the staff fight the door and blame the building. Prove the interlock at commissioning and again at every service. Kill power to the MAU, confirm the exhaust drops out or alarms, and confirm the room does not go hard negative with the hood still running. An interlock you did not test is an interlock you do not have.

The exhaust fan, the MAU fan, and the static they fight

The exhaust fan on a grease hood is almost always an upblast roof fan, hinged so it tips up for duct cleaning, with the motor kept up out of the greasy airstream and the discharge thrown up and away from the roof surface. It is sized for the exhaust CFM against the total static of the hood, the grease filters, and the welded duct run to the roof.

The MAU has its own fan and its own static to overcome: the intake louver and damper, the filters, the heating section, and the supply duct run to the kitchen. That total is the external static pressure, the ESP, the unit has to push against while still delivering its rated airflow. A MAU selected with no margin on ESP will fall short of its CFM the moment the filters start to load, and the kitchen drifts negative as it does, slowly, so nobody connects the spilling hood to the dirty filter.

The two fans have to be matched in the field, not just on the schedule. The exhaust sets the target, the makeup is then set to land the building pressure where it belongs, and both are read at real operating static with the filters in place and the dampers where they sit in normal operation. A fan curve on paper is a starting point. The gauge on the running system is the truth, and the truth shifts as the system fouls.

The grease exhaust duct as a moving static load

The grease exhaust duct is welded, liquid-tight steel, pitched back to the hood or to a drain so grease does not pool in a low spot, with the cleanout openings the cleaning contractor needs to reach the whole run. It carries the exhaust from the hood plenum to the upblast fan, and it is the path a grease fire travels, which is why its construction, clearances, and cleaning are governed hard by NFPA 96.

For the makeup conversation, the duct matters because it is static pressure that changes over time. Every foot of it, every transition, and a loaded grease film add resistance the exhaust fan has to pull through. A duct that has not been cleaned narrows, the static climbs, the exhaust falls off, and the carefully set balance walks negative over months as the grease builds. A kitchen that captured fine in the spring and spills by the fall usually has a fouled duct, not a failed fan.

The grease side, the cleaning interval, the access panels, and the duct's fire-rated construction belong to the hood and exhaust cleaning scope, and the hood suppression guide covers them. Here, treat the duct as the variable static that quietly moves your balance if nobody cleans it on a schedule. When you commission the makeup, you are setting it against a clean duct, and the owner needs to know that the balance depends on keeping it that way.

Setting and proving the kitchen balance

Setting the balance means measuring the exhaust CFM, measuring the makeup CFM, and reading the building pressure, then adjusting until the kitchen sits where the design wants it. You cannot do it by feel, and you cannot do it from the schedule alone. The whole job is a numbers job and it needs a gauge.

Read the exhaust and the supply airflow with the methods a balancer uses: a flow hood or a duct traverse on the supply, and a traverse or the fan's rated curve cross-checked against measured static on the exhaust. The blowercfm tool turns a velocity traverse into a CFM number on the spot so you are not doing the arithmetic on a greasy notepad. Then read the building pressure with a sensitive manometer, kitchen to dining room and kitchen to outdoors, working in the range of hundredths of an inch of water column. The door test is the field sanity check. With everything running, the back door should pull closed gently and open with normal effort, not slam and not fly open.

This is air balancing applied to a kitchen, and it follows the same discipline as a full test, adjust, and balance, covered in the air balancing guide. The kitchen-specific part is that the target is a pressure relationship, slightly negative to the dining room and roughly neutral to the outdoors, not just design CFM at each outlet. Two kitchens can both hit their design airflow and only one of them captures, because only one of them got the pressure relationship right.

The commissioning readings

Commissioning a kitchen makeup system comes down to a handful of readings, taken with everything running at design and again at any reduced speed the controls allow. Take them, write them down, and you have a baseline the owner can be held to and the next tech can compare against.

Read the exhaust CFM at each hood, the makeup CFM from the MAU, the resulting makeup as a percent of exhaust, the building pressure kitchen-to-dining and kitchen-to-outdoors, and the MAU discharge temperature in both heating and, where the unit has it, cooling. Confirm the interlock fires both fans together and that killing the MAU drops the exhaust or alarms. Confirm the back door opens and closes with normal effort under full operation.

The reading people skip is the discharge temperature under real heating, in cold weather or with the burner forced on. A MAU can pass an airflow check in mild fall weather and still blow cold in January, because the burner stage or the discharge controller was never actually proven against a call for heat. Force the heat, watch the discharge climb to setpoint and hold, and record it. That single reading is what prevents the winter callback that arrives the first hard freeze after the building opens.

Winter cold complaints and the temper setpoint

Winter is when an undersized or mis-set makeup system gets exposed. The complaint is always the same. The kitchen is freezing, the staff propped the back door to get some relief, and now the hood is spilling because the propped door changed the pressure the whole balance depended on. The root cause is usually makeup air that is too cold, too much, or simply not tempered to the right discharge.

Check the discharge setpoint first. If the MAU is blowing 50 degree F air across the cooks, raise it toward the 65 to 70 degree F range the design likely intended, within the load the heating section can actually make on the coldest design day. A unit that cannot hold setpoint on a design winter day was either undersized or has a burner or staging problem, and that is a real finding to write up, not a comfort preference to wave off.

Watch for the defeat. A propped door, a register taped over, a hood switch left off during prep, all of it is a sign the system is failing the people who work under it. A staff that has learned to defeat the controls will keep doing it until the comfort problem is solved. Fix the cold air at the source and the door stays shut on its own, and the balance you set holds through the season.

Demand-control kitchen ventilation and the energy cost

Demand-control kitchen ventilation, DCKV, varies the exhaust and the makeup together based on how much cooking is actually happening, instead of running both flat out from open to close. Sensors at the hood, temperature and often optical, tell a controller how hard the line is cooking, and variable-frequency drives ramp the exhaust and the MAU down through the slow hours and back up for the rush.

The savings are real because makeup air is expensive to condition. Every CFM the exhaust pulls is a CFM the MAU has to heat or cool, so turning both down at three in the afternoon when nothing is on the broiler cuts fan energy and tempering energy at the same time. The energy code drives this. ASHRAE 90.1 generally requires an energy measure on kitchens above 5,000 CFM of total exhaust, and DCKV is the common way to meet it, with a typical turndown to around 50 percent of full airflow.

The makeup-side rule does not change at part load. As the exhaust modulates, the makeup has to track it so the building pressure stays put. A DCKV job that ramps the exhaust without ramping the makeup just relocates the negative-pressure problem to part load, where it is harder to catch because it only shows up at certain hours. Confirm the control sequence keeps the two locked together across the whole operating range, not just at full speed.

Cafeterias, institutional kitchens, and data center cafeterias

A small restaurant with one hood is the simple case. The same physics scales up and gets harder in a large cafeteria, an institutional kitchen, or the kitchen serving a corporate campus or a data center, where multiple hoods over several cooklines run on a schedule that swings hard between meal service and idle.

The bigger the total exhaust, the bigger the makeup load and the more the building pressure has to be managed as a system, not hood by hood. Multiple hoods can share an exhaust and a makeup system, or run as separate zones, and the controls have to keep the whole kitchen's makeup tracking the whole kitchen's exhaust through every combination of hoods on and off. That is exactly where DCKV and zoned controls earn their place, because a fixed-volume system sized for all hoods running wastes a fortune when only one line is cooking.

A campus or data center cafeteria adds a wrinkle. The kitchen is a small, high-intensity ventilation load sitting inside a building tuned for something else entirely, often a building with its own tight pressure requirements. The makeup air for the kitchen cannot be allowed to upset the pressure relationships the rest of the building depends on. Treat the kitchen as its own pressure zone, balanced to roughly neutral with the spaces around it, so the cookline does not pull conditioned air out of the dining room, the corridors, or the spaces beyond.

Keeping it running after turnover

The makeup system the owner inherits at turnover needs maintenance, and the items are not exotic. The MAU has filters that load up and choke airflow, belts that wear and slip, a heating section that drifts out of tune, and an intake damper and linkage that stick or seize. Any one of them quietly moves the building pressure off where it was commissioned.

The MAU filter is the one that bites first. As it loads, the MAU airflow falls, the kitchen drifts negative, and the hood starts to spill, all without anyone touching the exhaust. A belt-driven MAU fan slipping on a glazed belt does the same thing more abruptly. The fix is a maintenance schedule that changes the filters and checks the belt tension on an interval, not a reaction after the kitchen is already spilling smoke at dinner.

The balance itself is a maintenance item people forget. Appliances get swapped for higher-duty equipment, a hood gets added, the exhaust duct fouls, and the balance that was right at opening day is wrong two years later. A periodic re-check of the building pressure and the makeup percentage, on the same gauge used at commissioning, catches that drift before it becomes a comfort or a safety complaint. Hand the owner the commissioning numbers at turnover so the re-check has something real to compare against, instead of starting from scratch every time.

What to document

The record is what makes the next service or the next inspection fast instead of a guess. Capture the readings at commissioning and at each service, on the same basis, so drift shows up as a number on a page instead of a vague complaint from the chef.

Document the exhaust CFM, the makeup CFM, the makeup as a percent of exhaust, the building pressure relationships, and the MAU discharge temperature, along with the interlock test and the door test result. A system that captures the readings below gives the owner something to defend at inspection and the next tech something to compare against when the kitchen starts to spill.

Common mistakes

• No makeup air, or an undersized MAU, so the building goes negative and the hood spills smoke into the kitchen.
• Makeup air over 100 percent of exhaust, pushing the kitchen positive so the hood loses capture and odor blows into the dining room.
• Cold, untempered makeup air dumped on the cookline, so the staff prop the door and defeat the balance.
• The MAU not interlocked with the exhaust, so the hood can run with no makeup and the room goes hard negative.
• Makeup air discharged into the hood capture zone or aimed at the hood face, blowing the plume back out the front.
• Setting the balance on a clean duct and clean filters and never re-checking it as both foul.
• Ignoring the backdraft risk: a negative kitchen pulling flue gas back down an atmospheric appliance vent.

Field checklist

Standards and references

The framework lives in the mechanical code and the fire standard, and they split the job. The International Mechanical Code, in its commercial kitchen makeup air section, covers the makeup side: that makeup has to be provided for hoods above the threshold, that it has to be approximately equal to the exhaust, that it must start and operate automatically with the exhaust, and that its temperature stays within about 10 degrees F of the space unless the HVAC is sized to carry the load. The exact section numbers and figures shift between code cycles and local amendments, so confirm them against the edition the jurisdiction actually adopted before citing them on a submittal.

NFPA 96 is the standard for the grease side: the hood, the welded grease duct, the upblast exhaust fan, the clearances, and the cleaning. It governs the exhaust the makeup is replacing, and the hood suppression guide covers it in depth. The energy code, ASHRAE 90.1 and the IECC, drives demand-control kitchen ventilation on larger kitchens, generally those above 5,000 CFM of exhaust.

Above all of it sit the hood and MAU manufacturer's listings and the engineer's design. A listed hood has a tested exhaust rate, a MAU has a rated airflow and discharge temperature, and the project specification sets the pressure targets. Where a listing or a spec is tighter than the code minimum, it governs. Verify everything against the adopted code, the AHJ, and the equipment listings, not a rule of thumb carried in from the last job.

Units, terms, and conversions

Kitchen ventilation borrows units from air balancing and combustion both, so the same system gets described in CFM, inches of water column, and degrees across three different sheets in the same submittal.

Airflow is in CFM, cubic feet per minute, for both exhaust and makeup. Building pressure is in inches of water column, written in. w.c. or in. wg, and the numbers are small, living in the hundredths. Discharge temperature is in degrees Fahrenheit. The makeup-to-exhaust relationship is given as a percent of the exhaust the makeup is replacing.

MAU (makeup air unit)

The supply fan and heating section that replaces the air a kitchen hood exhausts

CFM

Cubic feet per minute, the airflow unit for both exhaust and makeup

in. w.c. / in. wg

Inches of water column, the unit used for building and duct static pressure

ESP

External static pressure the fan overcomes while delivering its rated airflow

Type I hood

A grease hood over cooking appliances, with a welded grease duct and suppression

Type II hood

A heat-and-steam hood with no grease, over dishwashers or steam kettles

Direct-fired

Burner fires into the airstream; high efficiency, combustion products in the supply

Indirect-fired

Burner fires into a heat exchanger; clean supply air, lower efficiency

DCKV

Demand-control kitchen ventilation, varying exhaust and makeup with the cooking load

Transfer air

Air drawn from the conditioned space to make up part of the exhaust

Related tools

Calculators and readiness checks for this work

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