All You Need to Know and More about Make Up Air Units

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Last updated: 
March 20, 2024

Make Up Air Units: A Comprehensive Guide

What are Make Up Air Units?

Make up air units are designed to supply conditioned air, with controlled temperature and humidity, into a building to replace air that has been removed by exhaust systems or combustion. The key feature of these units is that they bring in fresh outside air through ventilation heating or cooling equipment, rather than recycling or recirculating the existing indoor air .

When to Use Make Up Air Units

Make up air units are commonly used in large buildings, such as office spaces or apartment blocks, that require zoning. Zoned energy use is efficient and allows individuals to control their environment, temperature, and humidity. This is particularly attractive for building owners who rent out their properties, as it enables tenants to adjust their energy consumption and reduce costs .

Hospitals, paint shops, factories, and restaurant kitchens also frequently use make up air units, as these facilities cannot afford to recycle indoor air due to potential contamination or health risks. In residential settings, make up air units can be beneficial for homes with large kitchens that require ventilation or for households with young children where frequent air refreshment is necessary .

Location and Installation

Although often associated with rooftop units, make up air conditioner units can also be placed in mechanical rooms for easier access and maintenance. Hospitals often opt for this placement to allow for quicker emergency repairs and simpler cleaning procedures. The size of these units can be substantial, as they include compressors, controllers, pumps, gas lanterns, vents, and exhaust tanks .

Budgeting and Pricing

The price of a make up air unit is based on its design and model. Traditionally, the cost is calculated using a "per square foot of space" system, with the assumption of standard unitary equipment and minimal outside air. When ventilation requirements call for a dedicated unit, the cost per square foot may increase. However, deals can be made, and it is essential to research and compare competing system types to find the best fit for your needs and budget. Consider both the initial costs and operating costs when planning your purchase .

The Necessity of Air Replacement

Simply ventilating a space is not always sufficient. Depending on the activity, size, and requirements of the area, a make up air unit that replaces air may be necessary for several reasons:

Proper Exhaust Hood Operation

Exhaust hoods, also known as extractor hoods, cooking canopies, or baskets, are devices with built-in mechanical fans that capture and remove airborne grease, hot dry air, fumes, gases, smoke, water vapors, odors, steam, and heat generated by cooking. Some hoods bring in fresh outside air, while others recycle and filter the air before reintroducing it into the room. Make up air units ensure that exhaust hoods operate properly by preventing negative air pressure and static pressure, which can diminish the air flow from exhaust fans .

Elimination of Cross Drafts

Cross drafts, which are high-speed air currents between doors and windows, can interfere with the proper functioning of exhaust hoods and carry contaminated air from one space to another. This is particularly problematic in hospitals, where infected air must not disperse between sections, or in restaurants, where cooking odors should not invade dining areas. Make up air units help eliminate cross drafts by balancing the negative pressure created by exhaust systems .

Ensuring Fluent Natural Draft Stack Operation

The stack effect refers to air movement inside and outside of buildings due to air buoyancy, caused by differences in air density. Make up air units are crucial for removing combustion products like carbon monoxide, keeping pilot lights in burners alive, regulating temperature, and controlling corrosion damage in stacks. Proper air replacement can prevent serious fires caused by the stack effect, such as the King's Cross Fire .

Reducing Cold Drafts and Pressure Differentials

Make up air units help reduce cold drafts, which can cause discomfort to occupants and increase heating and energy costs. Additionally, they eliminate high pressure differentials that make doors difficult to open or close, potentially causing injuries and safety hazards for workers .

When considering a make up air conditioning unit, it is essential to evaluate your budget, needs, and the safety reasons involved. By understanding these factors, you can determine if a make up air unit is the best solution for your space.


  1. The Importance of Make-Up Air

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