What Is Make Up Air?

Understanding Make Up Air and its Benefits for Indoor Air Quality and Comfort

Make up air units are designed for use in commercial & industrial building spaces to provide building HVAC and efficiently establish and maintain a slightly positive air pressure condition in the building by delivering heated or cooled outdoor air. Slight pressurization is desired for increased comfort through reduction of unwanted air infiltration from building seams and cracks or from windows and doors.  Typically make-up air is used in large, open industrial and commercial spaces, and the supply air can be distributed with or without ductwork.

Loading Dock Make Up Air

Cure Negative Air Pressure

Besides providing efficient heating for a building, make-up air can correct these problems associated with negative air pressure:

  • Exterior doors, not opening/closing.
  • Poor combustion and back-drafting exhaust issues with other heaters in the building which have atmospheric burners (boilers, unit heaters, infrared heaters).
  • Underperforming exhaust fans or fume extraction.
  • Underperforming dust collection systems.
  • Cold air infiltration in winter, cold spots near doors, windows, and walls.

Improve Building Performance

Make-up air improves building performance - Building exhaust and fume extraction system performance can improve when make-up air is used.  Slight building pressurization helps exhaust systems work at optimum performance and solves common facility performance struggles associated with negative air pressure spaces.

Make-up air improves comfort - Providing fresh outdoor air boosts indoor air quality (IAQ) and pressurization facilitates even building temperature and reduces air stratification.

Factory Make-Up Air Building Pressurization

Make Up Air

Direct-Fired 4000-Series  |  Direct-Fired 2000-Series  |  Indirect-Fired-8500D Series  |  Split Condensers 3200-Series

Industrial buildings virtually always have ventilation and exhaust equipment.  Due to the nature of work being accomplished in industrial facilities, ventilation and exhaust systems are required to properly maintain a clean and safe indoor air quality, maintain worker comfort and effectively operate building processes.

As ventilation and exhaust systems remove air and contaminants from the building, air pressure will find equilibrium and air will then enter the building in an amount to equal the flow rate of exhaust air whether or not there is a planned system for the air’s replacement.

What is the meaning of the term “Make-Up Air”?

Loading Dock Make Up Air

Make-Up Air is Replacement Air

Make-Up Air units are designed to replace exhaust air by bringing in fresh outdoor air into the space and heating or cooling it to the desired condition and discharge the conditioned or tempered air into the building.   At the same time when properly designed, a make-up air system provides building pressure thus eliminating negative building pressure and the problems caused by negative pressure.   Make-Up Air (MUA) systems are the preferred HVAC and IAQ design solution in industrial spaces because all industrial spaces use ventilation and exhaust, so make-up air (replacement air) is always needed.  Incorporating heating and/or cooling into the make-up air system reduces or eliminates the need for supplemental building heating and cooling, thus reducing overall HVAC equipment and energy costs.

Without replacement air (make-up air), building exhaust struggles to draw air into the building through every crack, door and window possible at a rate equal to the air it exhausts.

Factory Make-Up Air Building Pressurization

The air being drawn into the building to replace exhausted air has barriers to overcome.  Due to this resistance, the ventilation and exhaust systems are not able to operate to overcome these barriers to air availability and performance drops below rated air flow rates.  The decrease from the designed exhaust flow rate can be significant.  Ventilation and exhaust systems no longer move the correct amount of air needed. The resultant indoor environmental condition quickly becomes undesirable.

The drawing in of outdoor air into the building creates “a negative building pressure”.

Buildings under “negative pressure” suffer a host of performance problems:

  • Reduced process performance or quality.
  • HVAC problems.
  • Poor Energy Efficiency.
  • Visibly reduced indoor air quality (IAQ).
  • Safety concerns.
  • Uncomfortable temperature and humidity.

When a building is in a negative air condition, air contaminants are not properly cleared and purged through exhaust, often noticed by a haze in the air.  This haze (air contaminants) can cause safety, health and manufacturing process problems. Air temperature and humidity becomes uneven as an influx of cold outside air chills the perimeter of the building in winter (or draws in hot, humid air into air conditioned spaces in summer). Workers are subjected to drafts, workspace temperatures are not uniform, and the building heating/cooling system most likely gets overtaxed. Although the air temperature may eventually mix to acceptable condition as it moves toward the building interior, this is an ineffective, uncomfortable and uncontrolled way of transferring heat to the air and usually results in greater fuel usage.

Industrial Make Up Air Rapid Make Up Air Unit

Benefits of Make-Up Air for industrial buildings:

  • Automatic management and control of Indoor Air Quality (IAQ), Temperature and building pressure.
  • Boost worker comfort and productivity, consistent space temperature, eliminate cold drafts.
  • Outdoor Air/Fresh Air code compliance.
  • 100% efficient direct-fired combustion for low operating cost.
  • Reduce overall heating and ventilating cost. Make-up air corrects multiple building comfort, compliance and mechanical HVAC and ventilation performance failures.
  • Eliminate negative air pressure in the building.
  • Improve performance of building exhaust systems & eliminate haze and indoor air polluting particulates.
  • Augment existing ventilation systems and assists dust collector operation.
  • Remedy dangerous carbon monoxide and byproduct back-drafting of vent chimneys of other building HVAC units.
  • Prevent untempered building air infiltration and drafts.
  • Diffuse and eliminate unwanted process odors.


Negative Pressure Inches of Water Adverse Conditions Which May Result
0.01 to 0.02 Worker Comfort-High velocity drafts through doors, window and crack openings.
0.01 to 0.05 Natural Draft Inoperable-Ventilation through exhaust ventilators, flow through stacks with natural draft greatly reduced.
0.02 to 0.05 Carbon Monoxide Exposure-Back drafting of atmospheric burners will take place in water heaters, unit heaters, furnaces and other combustion equipment not provided with induced draft.
0.03 to 0.10 Mechanical Ventilation Greatly Reduced-Airflow amounts reduced in propeller fans and exhaust system.
0.04 to 0.10 Doors Difficult to Open - Employee injury may result from non-checked slamming doors.
0.10 to 0.25 Local Exhaust Ventilation Reduced- Centrifugal fan fume exhaust flow reduced. Exhaust hood source capture zones greatly reduced.


The markets and building types we serve are generally focused in industrial and commercial building segments:

  • Factories & Manufacturing
  • Manufacturing Process
  • Distribution Centers
  • Warehouses
  • Freight and Logistics Hubs
  • Aircraft Hangars
  • Repair & Maintenance Facilities
  • Military Operations and Warehousing
  • Government & Public Works Facilities
  • Waste Water Treatment Plants (WWTP)
  • Ventilated Parking Structures
  • Large Volume, Open Space Buildings

Warehouse Make Up Air and Exhaust System

RAPID® Heavy Duty Construction & Premium Features:

  • Welded Structural Steel Rigid Cabinet Framing
  • Welded Sheet Metal Cabinet Casing
  • Welded Drum & Tube Heat Exchangers
  • High Airflow Capacity - Up to 150,000 CFM Direct-Fired; Up to 60,000 CFM Indirect-Fired
  • DDC and Networked Controls Options
  • Add Cooling - Pair with Rapid® Split Condensers for complete industrial heat/cool systems
  • Combine & Integrate Best-in-Class HVAC Technologies for a total building solution (Make-Up Air, Infrared & Dedicated Outdoor Air Systems) with VIRTUAL WALL®.

Commercial & Industrial Applications

We manufacture both direct fired & indirect fired make-up air units to meet commercial and industrial heating, cooling and ventilation requirements ranging from 1,000 to 150,000 CFM.  Our manufacturing capabilities allow semi-customization for additional heat sources: electric, steam or hot water (and cooling coil options).  Our HVAC systems have controls options to best suit the specific use of each building or to enhance and support the manufacturing processes of industrial applications.   Building temperature and pressurization can be controlled by a direct digital controller (DDC), allowing communication with building management systems via BACNet, Modbus, N2 and LONworks.

Warehouse Make Up Air