Make-Up Air Units Explained

HVAC KNOW IT ALL | | Categories: direct fired make up air , indirect fired make up air , make up air unit , mau , mua


Make up air units or MUA as they are commonly referred to play a vital role in your condominium. The building MUA unit is generally located at the top of the building, either in the mechanical room or out on the roof. The function of the MUA unit is in the an initialism; make up air to the building that gets exhausted from kitchen, bathroom and or dryer exhaust systems. The building ventilation and the MUA system need to work together to ensure the building pressure is maintained. Too much MUA in a building and noise becomes a common complaint; too little MUA and complaints of smells in the hallways are more prevalent. The MUA needs to ensure the hallways are pressurized which helps keep cooking odors etc. localized to each suite.


Air Balancing

One item that is often overlooked with MUA systems is the air balancing portion. Over the years it’s not uncommon to have tenants make adjustments to the hallway diffusers that have an adverse effect on the overall system. The system should be checked and rebalanced every so often to ensure each floor is receiving the proper amount of air. The airflow is measured in Cubic feet per Minute (CFM) The total CFM of the MUA system is recorded and compared to the nameplate rating. The balancing and adjusting of every hallway grill on each floor is carried out and recorded to ensure the proper airflow is being delivered.


Hallway Temperatures

The majority of MUA systems temper the air in the winter to ensure icy cold air is not being delivered to the hallways. Some MUA systems are designed to provide cooling in the summer as well. One complaint I hear all the time is that the hallways temperature doesn’t feel the same as my unit. Hallways do not need to be kept at 23C (74F) in the winter time, 20C (68F) is more than an adequate temperature; the hallways are not living spaces. It is important to remember that the amount of gas required to heat up the outside air from -10C to a comfortable hallway temperature is very significant. The difference between a house furnace and a MUA is that the MUA is always trying to heat outside air, -10C in this case,  compared to a home furnace heating up the return air coming back  around 20C.  


Regular Preventative Maintenance

I can’t stress enough the importance of regular preventative maintenance. The MUA filters in many cases may require changing every month, if you only have Bi-monthly inspections then every two months is adequate. The MUA belts, motor and components need to be inspected as well. I find the inlet dampers on a lot of MUA units get neglected and should be lubricated twice a year. As with any gas-fired appliance a major inspection should be carried out on an annual basis where the major components such as the burners, ventor motor, heat exchanger etc. are thoroughly inspected. This service should be scheduled for the summer.


Variable Frequency Drives

You have probably heard the term Variable frequency drives (VFD) a lot in the last 10 years. These are devises that are installed on pieces of equipment to slow the operation of a motor or pump. With respects to MUA units a VFD drive can pay for itself in just a few years. The function of the VFD on a MUA unit is to slow down the motor and deliver less air. The VFD is typically set up on a timer to provide a percentage of the full CFM the building requires. There are certain times of the day that require less make up air. The peak demands for air will be first thing in the morning when residents are getting ready for work and after work when residents get home. Laundry machines with dryers, showering, cooking for the most part take place at these times of day. During the day and overnight when residents are either at work or sleeping there is typically not a lot of exhausting appliances operating. When the air flow is reduced during this non-peak operating hours there are significant gas savings. When the MUA unit delivers less air it means there is less air that needs to be heated and when the outdoor air is at -10C a tremendous amount of gas is consumed to reach our set point temperature. As with any type of energy savings there is a fine line that needs to be walked. There are limits to the amount of air that can be reduced with respects to the overall building requirements and the MUA design specifications, I always recommend consulting a professional to ensure these items are satisfied.     

Here is an example and video tour of a direct fired make up air


Check out the link to my YouTube channel for more tips, tricks, and troubleshooting videos and check out the The HVAC Know It All podcast here or on your favourite podcast app.  Happy HVACing...


Article By Derek Kernick


Bio: Responsible for the leadership and management of the operations and field service technicians. Ensure the revenue and profit objectives are achieved. Coach and develop employees in the office and the field to maximize their performance, provide feedback and or corrective action. Develop plans and align work to meet contractual obligations and ensure service work is completed in the time allotted. Interview and hire new employees and ensure on-going training is provided. Provide technical support to field staff. Meet with engineers on site to review mechanical retrofits and help provide solutions. Meet with property managers and board members to review contracts and manage large projects.

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