How Is Makeup Air Designed for Laboratory Exhaust Systems?

The air balance is the design's foundation document. The engineer inventories every exhaust point — fume hoods at design sash flow, snorkels, canopy hoods, biosafety cabinet connections, general room exhaust — and totals them at the realistic coincident peak, because not every device runs at maximum simultaneously but the design must handle the plausible worst case. Makeup air is then sized to that total minus the small offset that holds the lab negative to adjacent spaces. The balance is not a single number but a schedule across operating modes: full occupancy with hoods open, night setback with sashes closed, and every step between. VAV systems make this dynamic — the engineer documents the flow at each mode so the controls contractor programs the tracking correctly and the balancing contractor verifies it. Without this schedule, the system gets commissioned at one condition and drifts at all the others.

Conditioning 100 percent outside air is expensive, which is why the tempering strategy gets engineered carefully. In heating climates the makeup air unit needs substantial heating capacity — often with energy recovery from the exhaust stream as the first stage. Runaround loops and heat pipes recover heat without mixing airstreams, which matters because lab exhaust cannot be recirculated; the engineer selects the recovery technology that fits the exhaust's corrosiveness and the climate's economics. Delivery location is a containment issue. High-velocity supply jets aimed across a fume hood face will defeat the hood's capture, so the engineer uses low-velocity, high-induction diffusers positioned away from hoods, sometimes supplying through a perforated ceiling or dedicated low wall grilles. The design keeps supply air speeds low in the hood zone and verifies with smoke visualization at commissioning that makeup air delivery supports rather than disrupts containment.

A complete makeup air design keeps the lab balanced in every mode: • Exhaust inventory with coincident peak flow and per-device design rates • Makeup air sized to exhaust total minus the corridor pressure offset • Air balance schedule across full-flow, setback, and turndown modes • Tempering capacity for the climate with energy recovery on the exhaust stream • Low-velocity delivery away from fume hood faces to protect containment • Integrated controls that track VAV hood modulation with makeup air response • Pressure deviation alarming with commissioning across the full operating range