How Does a Make-Up Air Unit Work?
Providing a safe and healthy environment for employees continues to be a priority for manufacturing leaders, especially in facilities where manufacturing conditions create a potentially harmful environment for those working inside. Many manufacturing and industrial facilities that are plagued with poor indoor air quality typically do not have adequate ventilation. This lack of proper fresh air can create a variety of poor conditions such as high concentrations of harmful contaminants, hazy indoor conditions, improper process or product quality, OSHA safety violations, or even severe employee illness.
However, if your exhaust fans don’t also replace the air they take out of your building, you can very quickly end up with negative pressure, uneven heating or cooling, and even poor indoor air quality. This is where make-up air comes in handy. Utilized properly, make-up air units can bring in outside air while acclimatizing it to your building for safe, comfortable, quality indoor air. Learn more about how make-up air units work in this guide by the experts at Cambridge Air Solutions.
Why is Make-Up Air Needed?
The first step in solving your Indoor Air Quality issues for your manufacturing environment is a properly sized mechanical exhaust system, preferably located nearest to the source of indoor contaminants. The rate of exhaust airflow or amount of fans may vary due to the process or the type of contaminant. As the exhaust fans capture, contain, and expel these harmful gases, a deficit in fresh air is created.
If you don’t replace that air, you end up with:
- Negative building pressure (doors are hard to open, outside air gets sucked in through cracks)
- Drafts and cold or hot spots
- Poor indoor air quality and humidity issues
- Combustion safety risks; fuel-burning appliances (water heaters, boilers, unit heaters) may not draft properly if the building is starved for air
What Does a Make-Up Air Unit Do?
Make-up air units supplement the normal exhaust fan process by also intentionally bringing outside air into a building or workspace while simultaneously conditioning it. This helps to further lower the concentration of harmful elements created by industrial processes, and replace the air removed to maintain high-quality indoor air. This also helps balance building pressure and reduces humidity issues.
Components of Make-Up Air Units
Most MUAs share the same core pieces:
- Outdoor air hood and filters
- The unit draws in 100% outside air.
- Intake hood and filters keep out debris, insects, and larger particulates.
- Dampers
- Motorized dampers open and close to control how much outdoor air enters.
- Often tied to building pressure sensors or exhaust fan status.
- Fan/Blower
- A supply fan pulls air through the unit and pushes it into the duct system or directly into the space.
- Many units now use VFD's (variable frequency drives) to ramp fan speed up or down based on demand.
- Heating section (and sometimes cooling/tempering)
- Common heating options
- Direct-fired gas: Burner is in the airstream, very efficient, often used in industrial spaces.
- Indirect-fired gas: Burner heats a heat exchanger, the air does not contact combustion gases.
- Hot water/steam coil: Tied to a boiler system.
- Electric heaters: Less common for large volumes due to energy costs
- Some units also have:
- DX cooling or chilled water coils: To cool or "temper" hot outside air in summer.
- Discharge section / ductwork
- Conditioned air is discharged through ductwork or a discharge plenum.
- Air can be distributed across the space or targeted to problem areas (doors, production, lines, etc.)
- Controls and sensors
- Temperature sensors (outdoor and supply air)
- Building pressure sensors (to maintain neutral or slightly positive pressure)
- Airflow switches, freeze protection, safety limits
- Control panel with BMS integration or standalone controller