Makeup air
Outdoor air brought into a building or room to replace air removed by exhaust and to supply the airflow offset that keeps a room pressurized. In cleanrooms and laboratories a dedicated makeup air unit filters, cools, dries, or heats it before delivery.
Also called make-up air, MAU, makeup air unit, replacement air
What is makeup air?
Makeup air is conditioned outdoor air supplied to replace exhaust and leakage. In a cleanroom makeup air carries most of the moisture load: the HVAC Bench example's 700 cfm of makeup air needs about 77,000 BTU per hour of cooling on a humid design day.
Makeup air: how it works
Every cfm exhausted from a building or leaking out of a pressurized room has to be replaced, or the space goes negative and draws unconditioned air in through whatever opening it finds. A makeup air unit brings that outdoor air in on purpose and treats it first. Outdoor air in humid weather holds far more moisture than indoor air, so drying it at the makeup air unit handles the latent load in one place.
Where you meet the term
- In a cleanroom, where makeup air replaces process exhaust and the pressurization leakage of the suite.
- In a kitchen or laboratory, where large exhaust flows require matching conditioned makeup air.
- In a load calculation, where the makeup air coil can exceed the room's own cooling load.
How to check it
- Compare exhaust and makeup airflow. The balancing report should show makeup airflow close to total exhaust plus the intended pressurization offset for the space. Owner check
- Measure the makeup air leaving condition. A technician measures dry bulb temperature and dew point leaving the makeup air unit and compares them with the design, since that condition controls room humidity. Technician
Makeup air: what it is not
- Makeup air is not the same as ventilation air for occupants, although it can supply it. It is sized from exhaust and pressurization, which can far exceed occupant needs.
- A makeup air unit is not a minor add-on to a cleanroom system. Its dehumidifying coil can be the largest cooling duty in the plant.
Makeup air questions
How much makeup air does a cleanroom need?
Enough to replace all process and general exhaust plus the leakage that holds the suite above surrounding pressure. The HVAC Bench heat load example uses 400 cfm for process exhaust and 300 cfm for pressurization, 700 cfm in total.
Should makeup air be reheated before it enters a cleanroom?
Not when it mixes into recirculation air that still needs cooling, because cold dry makeup air offsets that cooling. Reheat becomes necessary at light load, when the cold air alone would overcool the room.
What happens if makeup air is less than exhaust?
The building or room goes negative and pulls in outdoor or corridor air through doors and gaps. A cleanroom suite loses its positive pressure, and exhaust hoods can lose capture.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, the bin and degree-day methods used for seasonal energy estimation, the table of heat gain from occupants by activity including light bench work at 275 BTU per hour sensible and 475 latent, the standard-air sensible and latent heat factors, and the orifice flow relationship used to estimate airflow through openings and gaps from pressure difference.
- ISO 14644-4:2022, Cleanrooms and associated controlled environments, Part 4: Design, construction and start-upSecond edition, replacing the 2001 edition, giving design, construction, and start-up guidance for new and modified cleanrooms, with checklists of requirements and performance parameters to be agreed between customer and supplier, energy management design approaches, and guidance that pressure differences between adjacent cleanrooms of different cleanliness lie in a range of about 5 to 20 pascals so doors can be opened without unintended cross-flow.