Make Up Air Units: A Guide to Different Models for Different Needs

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

In this installment, we discuss what a makeup air unit is, and explore the different types of makeup air systems for different applications.

What is Makeup Air?

Makeup air is controlled outside air that replaces the air exhausted from inside the building. As air inside gets expelled, this results in negative pressure and outside air rushes in to fill the void.

What are Make up Air Units?

Make up air units are devices that are designed to compensate for the pressure difference and ensure an adequate supply of filtered air for safety and comfort.

Without makeup air systems, sealed structures 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

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

What are Makeup Air Modules?

Some makeup air units are not composed of a single unit, rather they use modular sections for adding heating or cooling functions. This is particularly true for larger-sized models or industrial units. These modules are designed to meet different needs, space constraints, or operating requirements.

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.

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: 2 Succeeding 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	Kettle Oven Range	50
Medium appliances	Fryer Griddle	75
Heavy-duty appliances	Charbroiler Electric 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 m2 floor 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

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.

Air Conditioners with Integrated Makeup Air

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.

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.

GE AZ65H07DBM 7000 BTU Zoneline PTAC with Heat Pump

This PTAC unit is conveniently equipped with a factory-installed makeup air system, offering both cooling and makeup air functions in one unit. It can deliver 25 – 50 CFMs of continuous makeup air while providing up to 7,000 BTU of cooling power. In addition to fresh air, it also dehumidifies the incoming air when the relative humidity is above 55% and the outdoor temperature is above 50°F.

GE’s Zoneline units are the quietest PTACs on the market at reducing outside noise when compared to leading competitors, with an impressive sound transmission class of 29.

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GE AZ65H09DBM 9000 BTU Zoneline PTAC Heat Pump – Makeup Air

Like the previous model, this GE unit combines the functions of a PTAC and makeup air system in one. It can deliver 25 – 50 CFMs of continuous makeup air at a default setting of 35 CFMs, while providing 9,000 BTUs of cooling capacity. 

The unit can be paired with GE’s optional pair occupancy sensing technology, which senses when someone comes into the room and automatically turns the make-up air module on

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GE AZ45E09DBM 9000 BTU Class Zoneline PTAC Air Conditioner with Electric Heat and Makeup Air – Power Cord Included

This PTAC and MUA combo can deliver 25 – 50 CFMs of continuous makeup air at a default setting of 35 CFMs. Like the rest of GE’s Zoneline series, it boasts the quietest PTAC operation in the market. This is accomplished by a large cross-flow barrier that optimizes airflow for a low, balanced sound that is hardly noticeable. A heavy-duty, baked-on mastic barrier covers the back of the unit for superior sound insulation.

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For larger spaces

Friedrich PVH12K3FB 12000 BTU Class FreshAire Series with Inverter Technology 11.5 EER PTAC Air Conditioner with Heat Pump – 20 Amp – 230 Volt

Friedrich’s FreshAire line features a fully integrated make-up air system in a PTAC unit. This model has 11,800 BTUs of cooling and heating, as well as back-up electric resistive heat. It also boasts inverter technology that allows it to achieve an impressive EER of 11.5.

On the makeup air side, it can deliver up to 52 CFM of conditioned, MERV 8 filtered make-up air. Friedrich’s FreshAire series are the only PTACs that meet ASHRAE 62.1-2013 requirements.

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GE AZ65H12DBM 12000 BTU Class Zoneline PTAC Heat Pump – Makeup Air – Power Cord Included

Part of GE’s quiet Zoneline series, this PTAC delivers 12,000 BTU of cooling and 10,200 BTUs of heating, while providing up to 50 CFM of continuous makeup air. Features include a 3-stage heating design, electronic 7-step temperature limiting, automatic indoor frost control, and quick heat recovery.

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GE AZ45E12DBM 12,000 BTU Class Zoneline PTAC Air Conditioner with Electric Heat and Makeup Air – Power Cord Included

This PTAC unit is capable of up to 11,700 BTUS of cooling, and 25-50 CFM of continuous makeup air delivery. It has a higher EER of 11.7 while maintaining GE’s class-leading sound transmission level of 29, the quietest on the market. 

This updated unit now comes in a sleeker design and updated DC fan motors that reduce noise by dialing in the exact airflow needed for each heating and cooling function. 

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Friedrich PVH12K3FB 12000 BTU Class FreshAire Series with Inverter Technology 11.5 EER PTAC Air Conditioner with Heat Pump – 20 Amp – 230 Volt

Another new addition to Friedrich’s FreshAire series, this PTAC inverter unit provides 11,800 BTUs of cooling or heating, enough to service 450 – 550 sq. ft. It can deliver up to 52 CFM of continuous makeup air, processed through a MERV 8 filter.

Friedrich units are the only PTACs that meet ASHRAE 62.1-2013 requirements. 

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VTAC unit

GE AZ9VH12DBM 12000 BTU Class 11.6 EER V12 Zoneline Inverter VTAC with Heat Pump and Make-Up Air

GE’s Zoneline VTAC series has excellent COP (Coefficient of Performance) ratings. This is the measurement of the efficiency of the heat pump output compared to electric resistance heat. In addition to comparing heat pump output versus electric heat, this number also allows a direct comparison to other heat pumps with the same range capacity. This model boasts a heat pump capacity range of 5,000 – 16,000 BTUs, and a COP of 3.4 at 47°F.

The factory installed makeup air system delivers 37-90 SCFMs (without a filter), and 18- 70 SCFMs when equipped with a MERV13 filter. In addition to providing fresh air, it dehumidifies incoming air when the relative humidity is above 55% and outdoor temperature is above 50°F. 

This unit is also equipped with an inverter-driven Variable Speed Compressor, which results in an excellent 11.6 EER and 75% sensible heat ratio rating. Overall footprint is only 24″ x 29″, compared to a standard PTAC unit which can take up over 42″ of floor space.