Makeup Air Requirements
Makeup air requirements are regulatory obligations to provide adequate, clean, tempered replacement air whenever air is exhausted from a workspace. This ensures exhaust systems maintain effectiveness, prevents contaminants from being drawn in from other areas, and protects workers from discomfort or hazards. Under WorkSafeBC, makeup air must be sufficient in quantity and quality, not expose workers to uncomfortable temperatures or drafts, and contain contaminants below 10% of occupational exposure limits.
On a shop floor, makeup air requirements drive ventilation design and operation. Engineers calculate total exhaust from local hoods and general fans, then size makeup air units (MAUs) to supply approximately equal airflow, maintaining neutral or slightly negative pressure. This protects exhaust effectiveness, such as hood face velocity, and prevents cross-contamination between areas like paint booths and assembly zones. Makeup air must be tempered to avoid cold drafts and filtered to ensure air quality, with interlocks so MAUs operate with exhaust systems. Examples include welding bays with roof-mounted MAUs and paint booths with dedicated supply plenums.
- Undersized or non-functional makeup air causing excessive negative pressure, reducing exhaust effectiveness and drawing contaminants from adjacent areas, violating WorkSafeBC 5.69.
- Makeup air that is too cold, hot, or creates drafts, leading to worker discomfort and system bypass, violating WorkSafeBC 5.69(2) on thermal comfort.
- Makeup air contaminated by exhaust re-entrainment or outdoor sources, exceeding 10% of OEL, due to poor intake siting or lack of filtration.
How do I determine if my shop needs dedicated makeup air units versus using existing HVAC?
If total exhaust approaches or exceeds the building's ability to supply outdoor air without causing significant negative pressure or compromising combustion equipment, dedicated MAUs are needed. WorkSafeBC 5.69 requires maintaining exhaust effectiveness and preventing contaminant draw-in; if current HVAC cannot achieve this, MAUs are required. A rule of thumb is when exhaust exceeds 400-1,000 cfm, balancing studies often indicate MAU necessity.
What constitutes 'adequate' makeup air under WorkSafeBC in quantitative terms?
WorkSafeBC does not specify a single numeric ratio. Adequacy is demonstrated by exhaust systems maintaining design flow and capture velocities, exposure levels below OELs (typically <50% for routine operation), pressure differentials not causing door slamming or contaminant ingress, and worker comfort with ASHRAE 62 outdoor air rates (15-20 cfm/person). Verification via air balancing, pressure measurements, and exposure assessments is required.
How should makeup air be integrated with exhaust systems from a control standpoint?
Makeup air devices should be interlocked with exhaust systems to operate simultaneously, as per BC Building Code. For critical exhaust like toxic gas systems, interlocks plus alarms are recommended. Control sequences modulate MAU fan speed with exhaust demand to maintain neutral pressure, and adjust heating/cooling and dampers for air quantity and comfort. Differential pressure sensors can monitor space pressure and trigger alarms if negative pressure exceeds setpoints.