Make-up Air Units and Their Role in Building Ventilation

Make-up air units are an essential part of a building’s ventilation system and are responsible for supplying outdoor air to buildings. These units provide fresh air to the interior of the building, which helps keep the environment healthy and comfortable for occupants. In this post, we’ll explore how make-up air units work, their role in ventilation, and why they are important.

What is Make-up Air?

Make-up air is a crucial component in any ventilation system, as it prevents the build-up of indoor air contaminates and helps prevent negative pressure in buildings. By replacing the extracted warm or cooled air removed by exhaust fans, make-up air units help maintain comfortable interior temperatures and proper airflow balance within buildings.

“Make-up air”, typically consisting of 100% outdoor air, is different by definition from “ventilation air” in that make-up air is specifically designed to provide “make-up” for the amounts of air exhausted. Ventilation air refers to the air flow volume that ensures adequate air changes within an area to maintain healthy conditions.  Sometimes ventilation air is used as a part of the make-up for exhaust fans; however, the terms are not interchangeable.

Providing adequate make-up air to offset exhaust airflows ensures that the space is properly ventilated, providing optimal temperature and humidity comfort levels for its occupants. Make-up air systems ensure that the introduction of air for make-up purposes is done in a controlled manner, through the proper air handling equipment, rather than just drawing it in through any windows, doors, or other leakage spots in a building in an uncontrolled manner. 

If you have ever entered a restaurant and had a difficult time opening the front door due to air pressure, you have experienced a building with an improperly operating make-up air system.  

Additionally, make-up air minimizes the potential for back-drafting of non-sealed combustion appliances such as water heaters and furnaces.  This is a serious concern for any building with natural draft equipment, which relies on the availability of free flowing make-up air to establish a draft for the proper exhausting of the combustion flues.  Natural draft combustion equipment in an area with inadequate make-up air is a very dangerous condition due to the inability of the equipment to get rid of the toxic combustion gasses.  Gas fired appliances, such as clothes dryers also require sufficient make-up air to operate properly.

Overall, proper installation of make-up air units is essential in ensuring a more healthy and comfortable indoor environment.

What is a Make-up Air Unit (MAU)?

A make-up air unit is a dedicated piece of equipment that brings in outside air in order to “make up” for any loss due to exhausting operations, such as commercial kitchens, industrial processes, and fireplaces.  

These units are particularly important because they help control the amount of CO2, humidity, and air contaminate levels inside the building; this is accomplished by controlling the level of ventilation based on the needs of the exhausted equipment.  Make-up air units are typically selected based on the total amount of exhaust in the area served, plus a small additional amount to ensure that the area remains under a slight positive pressure. This positive pressure is provided to ensure that uncontrolled infiltration does not occur, which adversely impacts occupant comfort levels and indoor humidity.

Without an adequate make-up air flow, buildings can become overly dry, overly humid, or have significant negative pressure. This lack of balance leads to poor indoor quality and potential health concerns, especially in commercial facilities where high volumes of people gather on a daily basis. Investing in a properly designed MAU system provides the assurance that the right quantity and quality of air will be delivered at all times.

Common Applications of MAUs

Restaurants

Make-up air units are a powerful, efficient way to provide restaurant kitchen staff with the ventilation they need to work safely and effectively. Most models come equipped with multiple settings that allow restaurants to customize airflow speed, temperature, and humidity based on their unique needs.

Additionally, many units feature energy-saving technology, making them an ideal choice for businesses seeking to reduce overhead costs without sacrificing the quality of air circulation or convenience. Maintenance is relatively simple as well, because most make-up air units are constructed with long-life stainless steel heat exchangers and are equipped with standard HVAC filters to remove particles quickly and cleanly, preventing the accumulation of dirt, while simultaneously maintaining indoor air quality standards. 

Hospitals

Hospitals have a significant number of exhaust systems to maintain infection control standards and to exhaust potentially hazardous materials. Providing clean air indoors is especially important for the health and well-being of patients and staff. Make-up air units are essential to hospital environments in order to provide ventilation and replace air exhaust inside a building in a temperature and humidity controlled manner.

The use of proper make-up air equipment helps to control humidity levels, reduce contaminants, disperse odors, maintain proper room-to-room air pressure differentials, and remove heat gain from equipment such as sterilizers, kitchen exhaust, fume hoods, and laundry systems. The construction of make-up air units that are designed for hospitals are more specific to withstand hospital hygiene requirements and ensure an adequate flow of clean outdoor air.  

Laboratories

Make-up air units are absolutely essential in laboratories. These systems ensure that air is being replenished within the laboratory space constantly by introducing fresh air to offset the air exhausted out of the building.

This reduces the number of health and safety hazards, and improves the indoor comfort levels associated with the proper pressurization inside a laboratory building. Make-up air units are particularly helpful in laboratories; in addition to fresh air they also provide supplemental heat and humidity controls depending on the occupant’s needs.

Manufacturing Facilities

Make-up air units are critical to many industries, but they are especially important in manufacturing facilities. The air exchange provided by these units ensures a safe, healthy work environment for employees by preventing hazardous fumes and gases from accumulating. Further, the outdoor air introduced into the facility can often be filtered, heated, or cooled in order to achieve desired response times or thermal comfort levels.

Make-up Air Units – Pros and Cons

Make-up air units offer numerous benefits to buildings with continuously running ventilation systems, such as those discussed above. They help regulate the amount and location of the outdoor air coming in, while also releasing stale indoor air, preventing the buildup of carbon dioxide in crowded spaces.

MAUs can also help maintain ideal temperatures by providing supplemental heating, cooling, humidification, and dehumidification when necessary.

MAUs are often equipped with various forms of energy recovery systems, including “heat-wheels”, which extract energy from the outgoing airstream and uses it to pre-condition the incoming make-up air stream.  These units can drastically reduce the cost of bringing in the appropriate amount of outdoor air, providing significant benefits to the environment.

However, MAUs can be costly to install and require regular maintenance. The build-up of dirt or other contaminates can cause poor energy efficiency performance and poor air quality, regardless of the design criteria or controls. Filters must be maintained, and like any piece of HVAC equipment, routine maintenance including lubrication, belt replacement, or other adjustments must be completed on a regular schedule to keep the MAU system operating as intended. To ensure sufficient airflow, it is important for the time between service intervals to be appropriate for the application.

It is often thought that MAU systems can simply be turned off in an effort to conserve energy.  However, this is a false economy, because the exhaust systems will be compromised and the “make-up” air will enter the building anyway, through cracks in walls, windows, and doors.  One way or another, the air necessary to support the exhaust systems must enter the building, and it is far better for that to happen through a controlled make-up air unit.

Conclusion

All factors considered, MAUs are an invaluable component of the overall HVAC system for any building with a significant amount of exhaust volume. The benefits far outweigh any upfront costs, and the long-term operating cost of the facility can be substantially reduced with a properly designed make-up air system. 

At Schnackel Engineers, your satisfaction is our number one priority. We’re always here to answer any questions you may have. Our team of expert HVAC engineers is more than happy to provide advice and assistance whenever you need it. So, don’t hesitate to contact us any time for all your MAU and ventilation needs.