Key Takeaways
- MUA Function: Make-up air units replenish exhausted air from kitchens, bathrooms, and dryers to maintain balanced building pressure
- Building Pressure: Proper MUA operation prevents odor migration and noise complaints by pressurizing hallways correctly
- Regular Maintenance: Monthly filter changes and annual inspections are critical for system efficiency and longevity
- Energy Savings: Variable Frequency Drives (VFDs) can significantly reduce energy costs by adjusting airflow based on building demand
What Are Make Up Air Units and How Do They Work?
Make-up air units, or MUAs, play a vital role in condominiums and commercial buildings. The building’s MUA unit is generally located at the top of the building, either in the mechanical room or on the roof.
The function of the MUA unit is in its name: it makes up the air that gets exhausted from kitchen, bathroom, and dryer exhaust systems. By replenishing the removed air, the MUA unit helps maintain balanced airflow throughout the building while ensuring proper indoor air quality levels for occupants.
The Importance of Balancing Building Pressure
The building ventilation and the MUA system must work together to maintain proper building pressure. If there is too much make-up air, noise complaints can become common as excess air forces its way through door gaps and windows. On the other hand, too little MUA can lead to complaints about smells migrating through the hallways.
The MUA system is essential for pressurizing hallways, which helps to keep odors, such as cooking smells, localized to individual suites. This positive pressure prevents the spread of odors between units and ensures a more comfortable living environment for all residents. Without proper pressurization, negative pressure can actually pull odors from one suite into common areas and neighboring units.
Air Balancing: The Often Overlooked Essential
One aspect frequently overlooked with MUA systems is the air balancing process. Over the years, it’s not uncommon for tenants to adjust hallway diffusers, which can negatively impact the overall system performance. The system should be checked and rebalanced regularly to ensure that each floor receives the proper amount of air.
Airflow is measured in Cubic Feet per Minute (CFM). During a proper air balance:
- The total CFM of the MUA system is recorded and compared to the nameplate rating
- Every hallway grill on each floor is measured and adjusted
- All readings are documented to ensure proper airflow distribution throughout the building
- Dampers are adjusted to achieve design specifications
Hallway Temperature Management
Most MUA systems temper the air in the winter to prevent icy cold air from being delivered to the hallways. Some MUA systems are also designed to provide cooling in the summer. A common complaint I often hear is, “The hallway temperatures don’t feel the same as my unit.”
However, hallways don’t need to be kept at 23°C (74°F) in the winter. A temperature of 20°C (68°F) is more than adequate since hallways are not living spaces where people spend extended time. It’s important to remember that the amount of gas required to heat outside air from -10°C to a comfortable hallway temperature is significant. Unlike a house furnace, which heats return air that’s already around 20°C, the MUA is constantly trying to heat fresh outside air, which can be as cold as -10°C or lower.
Regular Preventative Maintenance Requirements
I cannot stress enough the importance of regular preventative maintenance for MUA systems. These units work harder than most HVAC equipment and require consistent attention:
Monthly Tasks:
- Change MUA filters (or bi-monthly for less demanding applications)
- Check belt tension and alignment
- Inspect damper operation
Semi-Annual Tasks:
- Lubricate inlet dampers (critical for proper operation)
- Clean outdoor air intake screens
- Check and calibrate controls
Annual Tasks:
- Thorough inspection of gas-fired components including burners, ventor motors, and heat exchangers
- Complete system maintenance similar to commercial RTU procedures
- Schedule this comprehensive inspection during summer months for optimal timing
Variable Frequency Drives: The Energy-Saving Solution
In the past decade, Variable Frequency Drives (VFDs) have revolutionized MUA operation. These devices control and modulate the motor speed to deliver variable airflow based on actual building demand. On an MUA unit, a VFD can pay for itself in just a few years through energy savings.
The VFD is typically programmed with a schedule to provide a percentage of the full CFM that the building requires:
- Peak demand times (6-9 AM, 5-8 PM): Maximum airflow when residents use dryers, showers, and kitchens
- Low demand periods (daytime, overnight): Reduced airflow when fewer exhausting appliances are in use
When less air is delivered, less air needs to be heated, resulting in significant gas savings, especially when outdoor temperatures drop to -10°C or lower. However, it’s crucial to maintain minimum ventilation rates. There are limits to how much air can be reduced based on building requirements and MUA design specifications. Always consult a professional to ensure these guidelines are followed and that your building automation system is properly configured.
For more insights, check out the video tour below featuring an example of a direct-fired make-up air system.
