The Ultimate Guide to Makeup Air Units

In the previous article, we explained the concept of makeup air, and why it’s necessary for most indoor buildings. 

In this installment, we explain what a makeup air unit is, and explore the different types of makeup air units for different applications. We also discuss the codes and regulatory standards that mandate the use of makeup air units.

What is Makeup Air?

Makeup air is controlled outside air that replaces the air exhausted from inside the building. 

Since inside air gets expelled by various appliances like range hoods, this causes outside air to rush in. Makeup air is controlled and conditioned air so that the air that goes in is properly filtered and temperature controlled for the safety and comfort of people in the building.

What are Make up Air Units?

Make up air units are devices that control the makeup air that goes into the building. A makeup air unit filters the air for contaminants and pollutants, while conditioning it at the same time so that it is neither too hot nor too cold for the comfort of the occupants.

What are the Components of a Make up Air Unit?

A make up air unit shares many of the same parts with a fan, air handler, or air conditioner. The major parts consist of:

✅ Intake – this is the opening where the outside air comes in and gets fed into the unit.

✅ Back-draft damper – also known as gravity dampers, these control airflow to move only in one direction and stop reverse airflow from going back into the system. This allows contaminated air to move out of the building, while preventing it from flowing back inside. The damper can be simple mechanical louvers for residential makeup air units, or complex motorized units for industrial models.

✅ Filters – much like air cleaner or AC filters, these trap contaminants like pollutants, toxins, and allergens from going inside the building.

✅ Heating and/or cooling elements  – these either heat up or cool down the air coming into the building to make it more comfortable for occupants and reduce load on the central AC or furnace.

✅ Ducts and registers – these transport the processed air throughout the interior.

✅ Exhaust fan – as the name implies, it expels the used air out of the building. 

Optional features or more advanced functions can include:

• Recirculation features
• Variable volume
• Building pressure controls
• High temperature discharge
• Multi-stage or ultraviolet light filtration

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Why Are Make Up Air Units Important?

Without a makeup air unit, sealed buildings can suffer from a host of issues caused by negative interior air pressure, including:

• Contaminants and allergens
• Persistent odors
• Humidity problems
• Inefficient HVAC operation
• Higher utility costs

Makeup air units are more important than ever as buildings get tighter and more efficient. Without a makeup air system in place, such tightly sealed structures are much more susceptible to the effects mentioned above.

Make up Air Units: Codes and Regulations

Makeup air units are governed by different regulatory standards and codes, including but not limited to:

ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)

ASHRAE Standard 62.1 outlines ventilation requirements for acceptable indoor air quality in commercial buildings, specifying ventilation rates, air quality parameters, and system design guidelines.

International Mechanical Code (IMC)

The IMC provides regulations pertaining to mechanical systems, including ventilation requirements, equipment specifications, and installation guidelines for makeup air units in various occupancies and building types.

National Fire Protection Association (NFPA)

NFPA standards such as NFPA 96 specify ventilation requirements for commercial cooking operations, including the installation and maintenance of makeup air units in commercial kitchens to mitigate fire hazards.

Local Building Codes

Municipalities and cities often enact building codes and ordinances mandating the installation and operation of makeup air units in commercial, industrial, and residential buildings to ensure compliance with safety and health, as well as environmental regulations.

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Where are Makeup Air Systems Required?

Residential homes

Tightly sealed houses can result in unequal pressure, even when just opening doors. 

In addition, some powerful range hoods can cause negative air pressure in the kitchen. Other household appliances can suck air out of the house, causing unfiltered air to seep in and exacerbate gaps, holes or cracks in the exterior walls and in weatherstripping.

The most common appliances that exhaust indoor air are:

• Bathroom exhaust fans (40‐200 CFM)
• Clothes dryer (100‐225 CFM)
• Kitchen range hood (100‐1,500 CFM)

For residential units, the International Residential Code Section M1503.4 and the International Mechanical Code Section 505.2 require a make-up air unit for all domestic range hoods over 400 CFM, and the MUA should be equipped with at least one damper:

“Exhaust hood systems capable of exhausting in excess of 400 cfm (0.19 m3/s) shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.”

The minimum ventilation requirements for each area of the house are:

Space	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.	Exhaust airflow rate (CFM / sq ft)
Living areas	1st bedroom: 2Succeeding bedrooms: 1 per room	0.35 ACH, and at least 15 CFM / person	n/a	n/a
Bathroom	n/a	n/a	n/a	20/50
Kitchen	n/a	n/a	n/a	25/100
Garage	n/a	n/a	n/a	0.75

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.2 also calls for adequate makeup air for residential single-family detached and low-rise attached multifamily dwellings. Most states also have their own makeup air codes for residential buildings.

Dormitories and condominiums

In residential buildings, multiple units, entrances and hallways tend to destabilize interior air pressure equilibrium. Without a makeup air unit, odors and cooking fumes from different apartments may also find their way into the corridors as well as in neighboring units.

The revised version of ASHRAE 62.2 (see above) now covers mid-rise and high-rise residential occupancies, which includes most dormitories and condominiums. In addition, new condominium developments are shifting to providing makeup air directly into individual units, instead of supplying air through the hallway. Based on IMC 2009 Section 601.2, corridors should not serve as supply, return, exhaust, relief or ventilation air ducts for condominium units.

Hotels and Resorts

Based on IMC 403.1, the minimum ventilation for commercial hospitality buildings like resorts, motels and hotels are:

Space	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Bedroom	10	5	0.06
Lobby / prefunction	30	7.5	0.06
Multipurpose assembly hall	120	5	0.06

In terms of air shifts, the hotel room is recommended to have at least 3 l/s per sqm of supply air, while reception areas should have 15-20 l/s per person.

Commercial kitchens

An industrial makeup air unit is required for restaurants and commercial kitchens to ensure proper exhaust and ventilation for kitchen workers. Based on IMC Section 508.1:

“Makeup air shall be supplied during the operation of commercial kitchen exhaust systems that are provided for commercial cooking appliances. The amount of makeup air supplied to the building from all sources shall be approximately equal to the amount of exhaust air for all exhaust systems for the building.”

In addition, section 508.1.1 (Makeup air temperature) of the IMC specifies the indoor air temperature:

“The temperature differential between makeup air and the air in the conditioned space shall not exceed 10°F (6°C).”

Since the IMC requires that total makeup air should be equal to the exhaust, commercial kitchens typically get 80% of the makeup air from their MUA, and only about 20% from the building’s HVAC unit. This makes makeup air systems indispensable for commercial kitchen operations.

When not specified by code, these may be used to determine the minimum kitchen hood exhaust:

Type	Equipment	CFM/Ft.2 of Hood
Light appliances	KettleOvenRange	50
Medium appliances	FryerGriddle	75
Heavy-duty appliances	CharbroilerElectric Broiler	100

Note that some local codes may require 100 CFM/Ft.2 of hood area for wall-style hoods.

In addition, the National Fire Protection Agency has additional requirements for commercial kitchen applications as per NFPA 96:

• A grease trap or grease collector 
• Ventilated roof curb
• Clean-out Port
• Hinge Kit
• 40” minimum distance from the roof deck to the top of exhaust fan windband
• 18” minimum distance from the roof deck to top of curb
• 10-foot minimum distance between the supply fan intake and all exhaust fans
• In cases where the 10-ft distance is not possible horizontally, there should be at least 3-ft vertical difference between the intake and exhaust (exhaust should be taller)

For other areas of the dining establishment, IMC 403 has the following guidelines:

Classification	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Bar / cocktail lounge	100	7.5	0.18
Fast food / cafeteria	100	7.5	0.18
Restaurant (dining room area)	70	7.5	0.18

Office and corporate spaces

Makeup air is essential for office spaces, considering that employees spend at least 8 hours per weekday sharing the same interior space. ASHRAE 62.1 requires 4-10 air changes per hour for office spaces, depending on the size and occupancy. 

In addition, IMC 403 requires the following:

Area	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Main entry lobby	10	5	0.06
Reception areas	30	5	0.06
Office spaces	5	5	0.06
Telephone / data entry	60	5	0.06

Conference rooms  / Auditoriums

A typical conference room will require between 4-10 air changes per hour, and more for larger gathering spaces. This is in accordance with ASHRAE Standard 62.1. A key consideration for these types of spaces is that the makeup air system performs its function without being distracting. 

In such spaces, the ventilation equipment is best positioned in non-sound critical areas such as a corridor or utility room, and connected via ductwork. A gravity ventilator is also recommended so that the exhaust air can pass freely through the roof. It should be noted that the gravity ventilator should be sized to accommodate the same volume of air that the inline fan exhausts.

In addition, IMC 403 requires the following:

Area	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Conference room	50	5	0.06
Auditorium	150	5	0.06

Commercial and Industrial Applications

• Makeup air systems are required to ensure adequate ventilation for workers.
• The system also removes industrial byproducts like contaminants and fumes.

IMC 401 governs the makeup air requirements for commercial spaces, while OSHA OSHA 3430-04 provides regulations for commercial and institutional buildings. 

Retail and Services

IMC 401 covers retail stores and specialty services. These are defined as commercial spaces and specialty shops that provide goods and services.

Area	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Barbershop	25	7.5	0.06
Beauty / nail salon	25	20	0.12
Mall common area	40	7.5	0.06
Dry cleaner (coin-operated)	20	15	n/a
Dry cleaner (commercial)	30	30	n/a
Laundry (coin-operated)	20	7.5	0.06
Laundry (commercial)	10	25	n/a
Pet shops (animal areas)	10	7.5	0.18
Retail store	15	7.5	0.12
Supermarkets	8	7.5	0.06
Smoking lounge	70	60	n/a

Educational Institutions

Schools and universities have no stringent requirements regarding minimum outside air. Instead, they fall under the ASHRAE standard, which specifies 15 cubic feet of air per minute per person per room.

However, certain areas like laboratories and wood or metal shops need more attention to ensure adequate ventilation and exhaust of potential fumes. 

Guidelines based on IMC 403.1:

Area	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Classrooms (ages 5-8)	25	10	0.12
Classrooms (for age 9 and above)	35	10	0.12
Daycare (age 4 below)	25	10	0.18
Lecture classroom	65	7.5	0.06
Lecture hall with fixed seating	150	7.5	0.06
Multiuse assembly hall	100	7.5	0.06
Science lab	25	10	0.18
Wood/metal shop	20	10	0.18

Amusement and Recreational Facilities

Recreational and sports buildings are also covered by IMC 403.1:

Classification	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Arcade	20	7.5	0.18
Bowling center	40	10	0.12
Casino	120	7.5	0.18
Disco bar	100	20	0.06
Gym: aerobics room	40	20	0.06
Gym: weight room	10	20	0.06
Theater lobby	150	5	0.06
Theater stage	70	10	0.06

Medical Facilities

Healthcare institutions require more stringent airflow. In addition to adequate ventilation, clinical-grade filters are used in makeup air units. Below is the recommended air supply for particular areas.

Area	Supply Air flow(l/s per m2floor area)	Supply Air Flow(l/s)
General spaces	15 per person
Therapy rooms	9
Operating rooms	14
Autopsy room	9
Dental clinic	10-15 per person

The American Institute of Architects (AIA) has guidelines for indoor air-quality standards for healthcare facilities, including ventilation rates, temperature levels, humidity levels, pressure relationships, and minimum air changes per hour. The regulations represent a consensus of different government agencies, including Department of Health and Human Services, Department of Labor, OSHA, as well as professional bodies like the ASHRAE, the American Society for Healthcare Engineering (ASHE), and accrediting organizations like the Joint Commission on Accreditation of Healthcare Organizations.

Public Buildings

These generally refer to public facilities provided by local authorities. 

Building	Occupant density (per 1,000 sq. ft)	CFM per person	CFM per sq ft.
Courtroom	70	5	0.06
Legislative chamber	50	5	0.06
Library	10	5	0.12
Museum (children’s)	40	7.5	0.12
Museum gallery	40	7.5	0.06
Religious buildings	120	5	0.06
Sports spectator areas	150	7.5	0.06
Transportation platforms	100	7.5	0.06

Smoking vs. Non-smoking areas

Spaces dedicated for smoking have a dramatic effect on the makeup air required, as outlined by the Chartered Institution of Building Service Engineers, CIBSE Guide B2 (Ventilation and Ductwork). The greater the number of the smokers, the higher the supply air per person:

Smoking level	Percentage of occupants who smoke	Supply air needed per person (l/s)
No smoking	0	8
Light	25%	16
Heavy	45%	24
Very heavy	75%	36

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Makeup Air Unit Sizing

In indoor ventilation, the airflow is measured in Cubic feet per Minute (CFM). This refers to the airflow that passes through the space in one minute.

To determine the size of the makeup air unit you need:

1. Total the CFM rating of all the blowers and exhaust fans in the space
2. Add 10% so there will be positive pressure.
3. The total CFM + 10% represents the minimum rating you need for your makeup air unit.

If you wish to size blowers or exhaust fans, this is determined by dividing the volume of the space with the number of minutes per air change:

Room volume (L x W x H)

Minutes per air change 

= 

Blower or fan size in CFM

Note that the use of ductwork results in higher static pressures and increases the resistance in the system. Both these factors should also be considered when sizing an exhaust fan.

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What About Air Conditioners with Integrated Makeup Air Units?

Some air conditioner models come with built-in makeup air functions. These provide the capability of makeup air without the expense and huge footprint of a separate makeup air unit.

Packaged terminal air conditioner (PTAC)

A PTAC is a standalone AC/heater that is installed through a wall. PTAC units are self-contained, meaning they do not rely on ducts to operate. This makes them a great option for those who want to heat or cool a single room versus an entire home or building.

Friedrich’s FreshAire line of PTAC units combines air conditioning and makeup air in one compact package.

Vertical Terminal Air Conditioner (VTAC)

As the name suggests, a VTAC is a vertical heating and cooling unit that is usually installed in a space hidden from view, such as a closet. A VTAC can control the temperature of a single room or multiple rooms in a building.

While PTAC and VTAC units with makeup air offer lower cost and convenience, they can only cover small rooms, maxing out at around 52 CFM. This makes them ideal for residential applications or smaller spaces, where a dedicated makeup air unit doesn’t make financial sense.

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Our Recommended Makeup Air Units

For residential makeup air

The International Residential Code requires make-up air be provided for all kitchen hoods capable of exhausting 400 or more cubic feet per minute. These makeup air units are capable of servicing most home range hoods and household appliances. Some also possess inverter technology that allows them to be more efficient than standard AC units.

Note however that makeup air PTAC units are not recommended for use within two miles of coastal areas.

Friedrich PVH12K3FC 12000 BTU Class FreshAire Series with Inverter Technology PTAC Air Conditioner with Heat Pump

Friedrich’s FreshAire line of PTACs features a fully integrated make-up air system and can be used in both commercial and residential applications. They are the only PTACs that meet ASHRAE 62.1-2013 requirements delivering up to 52 CFM of conditioned, MERV 8 filtered make-up air.

This model offers 12,000 BTU of cooling with 11,500 BTUs reverse cycle heat pump as well as back-up electric resistive heat. It boasts an EER of 11.5, thanks to the use of Inverter Technology. Aluminum end plates and DiamonBlue Advanced Corrosion protection help protect against harsh weather elements and prolong unit life. 

GE AZVS15DMXWA 15000 BTU Class Zoneline PTAC Air Conditioner with Heat Pump

The new GE Zoneline series equipped with a factory-installed makeup air system. It will deliver Variable Speed Make-up Air Delivery, up to 50 CFMs, with a default setting of 35 CFMs. In addition to providing fresh, outdoor air through a makeup air solution, it also dehumidifies the incoming air when the relative humidity is above 50%. 

This particular unit has a nominal cooling capacity of 15,000 BTUs and a nominal heating capacity of 14,000 BTUs. Electric heat is power cord dependent. It is a fixed speed unit with hi or low settings.

Similar to central heating systems, the GE Zoneline Vertical Heat Pump incorporates the use of a reverse cycle defrost system. This allows the heat pump to operate at lower outdoor temperatures without frost accumulation on the outdoor coil. This allows the PTAC to operate to 0°F. When the temperature is below 25°F, the electric heater will provide the heat with its Heat Boost Option. 

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VTAC with Integrated Makeup Air Unit

GE AVVL12DMXWA 12,000 BTU Class Zoneline Ultimate V12 VTAC with Heat Pump

The Zoneline VTAC unit is equipped with a factory-installed makeup air system. It will deliver 30 – 70 CFMs (without a filter) and 25 – 70 CFMs (with MERV13 filter; RAA13V) of continuous makeup air.

It offers a rated cooling capacity of 12,500 BTUs (@95°F outdoor temp.) with a cooling range capacity of 5,200 – 16,200 BTUs. The heat pump offers 11,200 BTUs (@47°F outdoor temp.) with a heating BTU range of 4,500 – 13,000. The unit has an excellent COP of 3.5 @47°F. It also meets new AHRI Standards and boasts 12.0 EER2, 16.5 SEER2 and 7.1 HSPF2 ratings. This is all thanks to the use of inverter technology for better energy efficiency. It is also WiFi ready with GE’s Smart HQ WiFi.

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Makeup Air Unit vs Makeup Air Module: What’s the Difference?

Some makeup air units are not composed of a single unit; instead, they are composed of modular sections for adding heating or cooling functions. These are called makeup air modules.

Makeup air modules are add-on sections designed to meet different needs, space constraints, or operating requirements. They are typically used in larger-sized makeup air units meant for commercial or industrial use.

What are the Different Types of Makeup Air Modules?

Makeup air modules come in different designs, which differ mainly in how the air is treated or cooled. Here are the main types:

Makeup Air Fan

Also known as an untempered makeup air unit or fan box, this is the simplest type of MUA. Basically it consists of an intake fan bringing in air to the building without any heating or cooling equipment.

Such models are ideal for locations with a consistent temperate climate, or where specific indoor conditions are not required. They have the least footprint, acquisition cost, and operating cost.

Direct Gas Fired Heat

A direct-fired make-up air heater uses either natural gas or propane to heat the air before circulating it into the building. They are considered to be the most efficient type of makeup air system, since 100% of the heat generated gets delivered into the interior.

On the other hand, the direct gas fire design does lead to a limited amount of gas byproduct getting mixed into the air. This can be negated with proper filters and safety equipment built into the system. In addition, only up to 85% of the air can be recirculated as mandated by ANSI standards.

Direct gas fired MUAs can be further classified into different models, each with their own BTU capacity and air volume capacity:

• Compact low CFM Direct Fire Heater – smallest size, but lowest air volume capacity
• Modular Direct Fired Heater – highest BTU rating, can be configured into V-bank or evaporative cooler intake
• Horizontal Industrial Direct Fired Heater – available in regular or recirculating versions
• Vertical Industrial Direct Fired Heater –  available in regular or recirculating versions

In-Direct Gas Fired Heat

As the name suggests, these use an indirect method of heating similar to a home furnace. The gas flame is contained in metal pipes known as a heat exchanger. The air is then heated by passing it through the exchanger before circulating it into the building. 

While the design is less efficient compared to direct fire MUAs, it prevents any gas byproducts from being mixed with the air. The heat exchanger is connected to an exhaust flue that safely discharges these products. This results in clean, dry air that is ideal for heating spaces susceptible to mold. The air can also be recirculated up to 100%.

However, the complexity of the design results in the largest footprint among all types of makeup air units, as well as the highest acquisition cost.

Non-gas fired Makeup Air Units

Apart from propane or natural gas fired MUAs, make up air heaters can be powered by:

• Steam
• Hot water
• Electricity

Each of these models have their own pros and cons, but in general they possess lower BTU and air volume capacities than a gas-fired MUA.

DX Cooling

DX stands for direct expansion cooling. This model uses a series of refrigerant coils to cool the air supply. As the air passes through the coils, the refrigerant absorbs the heat and gets recirculated through the system. The condenser can either be built into the unit, or set separately as a modular add-on.

Chilled Water Cooling

Also known as chiller units, these are very similar to DX cooling. The only difference is that chillers use cold water to cool the air. The water is run through coils fed by a separate water chiller module. Chiller units are ideal for large air handling units that need to service huge indoor spaces.

Evaporative Cooling

Evaporative coolers don’t just cool the air, they provide humidity by mixing moisture into the air supply. They are commonly used in locations with hot and dry climates to restore interior humidity while providing cooler indoor temperature.

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Conclusion

Makeup air units are a cornerstone of modern ventilation systems. They offer improved air quality, conditioned air, and prevent a host of health and safety concerns caused by negative indoor air pressure. 
As building designs get tighter and more efficient, so too do makeup air systems become more crucial as an essential part of the building’s ventilation system. Check out our selection of AC units with integrated makeup air systems to ensure safety and comfort for you and your loved ones.