Make Up Air Units: A Guide to Different Models for Different Needs Posted on August 24, 2022 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. Contents Toggle What is Makeup Air?What are Make up Air Units?What are the Components of a Make up Air Unit?What are Makeup Air Modules?What are the Different Types of Makeup Air Modules?Where are Makeup Air Systems Required?Smoking vs. Non-smoking areasMakeup Air Unit SizingAir Conditioners with Integrated Makeup AirOur Recommended Makeup Air Units 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: Total the CFM rating of all the blowers and exhaust fans in the space Add 10% so there will be positive pressure. 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. Price: $1,729.00 GE AZ65H07DBM 7000 BTU Zoneline PTAC with Heat Pump - Makeup Air - Power Cord Included For your convenience, we are including a required power cord of your choice with this unit. Choose either a standard power cord (included) or a hardwi... View Product 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 Price: $1,759.00 GE AZ65H09DBM 9000 BTU Zoneline PTAC Heat Pump - Makeup Air - Power Cord Included For your convenience, we are including a required power cord of your choice with this unit. Choose either a standard power cord (included) or a hardwi... View Product . 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. Price: $1,569.00 GE AZ45E09DBM 9000 BTU Class Zoneline PTAC Air Conditioner with Electric Heat and Makeup Air - Power Cord Included For your convenience, we are including a required power cord of your choice with this unit. Choose either a standard power cord (included) or a hardwire kit (... View Product 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. Price: $1,599.00 Friedrich PVH12K3FB 12000 BTU Class FreshAire Series with Inverter Technology 11.5 EER PTAC Air Conditioner with Heat Pump - 20 Amp - 230 Volt Friedrich FreshAire Series PTAC units are the most advanced PTAC units. The FreshAire line features a fully integrated make-up air system while still offering... View Product 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. Price: $1,779.00 GE AZ65H12DBM 12000 BTU Class Zoneline PTAC Heat Pump - Makeup Air - Power Cord Included For your convenience, we are including a required power cord of your choice with this unit. Choose either a standard power cord (included) or a hardwire ki... View Product 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. Price: $1,599.00 GE AZ45E12DBM 12,000 BTU Class Zoneline PTAC Air Conditioner with Electric Heat and Makeup Air - Power Cord Included For your convenience, we are including a required power cord of your choice with this unit. Choose either a standard power cord (included) or a hardwire ... View Product 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. Price: $1,599.00 Friedrich PVH12K3FB 12000 BTU Class FreshAire Series with Inverter Technology 11.5 EER PTAC Air Conditioner with Heat Pump - 20 Amp - 230 Volt Friedrich FreshAire Series PTAC units are the most advanced PTAC units. The FreshAire line features a fully integrated make-up air system while still offering... View Product 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. Price: $3,012.00 GE AZ9VH12DBM 12000 BTU Class 11.6 EER V12 Zoneline Inverter VTAC with Heat Pump and Make-Up Air - Power Connection Kit Included - 208/230 Volt For your convenience, we are including a Universal Direct Connect Power Supply with this unit. Choose either a 15 amp, 20 amp or 30 amp hardwire kit. &n... View Product Mickey Luongo Mickey is the resident heating and air conditioning expert with over 15 years of experience in the industry.