Make Up Air Units Explained

Make up air units explained

What Are Make Up Air Units and How Do They Work?

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 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 must work together to maintain the building pressure.

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 receives 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 are carried out and recorded to ensure the proper airflow is 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 hallway temperatures don’t feel the same as my unit.

Hallways do not need to be kept at 23C (74F) in the wintertime.

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 an 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 devices 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 an 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.

Certain times of the day require less make up air.

The peak demand 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, and cooking, for the most part, take place at these times of the day.

During the day and overnight, when residents are either at work or sleeping, there are typically not a lot of exhausting appliances operating.

When the airflow is reduced during these 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 HVAC Know It All podcast here or on your favorite podcast app. 

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Gary McCreadie

Gary McCreadie is an HVAC tech, the creator of, the HVAC Know It All Podcast, and owner of McCreadie HVAC & Refrigeration Services

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HVAC Know It All was created by Gary McCreadie, a licensed refrigeration and gas technician, with three goals in mind:
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