Planning a Lab for Fume Cupboards: Siting and Make-Up Air
James Cox
Project Manager
Estimated reading time: 6 minutes
A fume cupboard's containment performance is not decided by the cupboard alone. Where it sits in the room, what is opposite it, how close it is to a door, and where its replacement air comes from all affect whether it holds its tested face velocity in real use. Getting these decisions right at the design stage is far cheaper than fixing them once a lab is built and fitted out.
Positioning Within the Room
Imperial College London's Code of Practice for Fume Cupboards notes that the proximity of air supply equipment, fans, open windows, and physical objects such as benches, walls, and doors can all influence fume cupboard performance. In practice, that means siting decisions come down to a few key factors.
Away from doors and busy walkways
A door opening near a fume cupboard creates a draught across the sash. Site cupboards away from doorways and routes people use regularly, not immediately next to them.
Clear of supply air diffusers
A supply air grille blowing directly onto or across the sash face disrupts the cupboard's own airflow pattern. Keep diffusers away from the cupboard face, or direct them elsewhere in the room.
Away from pedestrian cross-traffic
People walking briskly past an open sash generate turbulence in much the same way as a draught. Avoid siting a cupboard where it faces onto a main circulation route.
Protect the means of escape
A fume cupboard should never be positioned where it could obstruct an escape route or where a spill or fire risk at the cupboard would block the room's exit.
Make-Up Air
Every cubic metre a fume cupboard extracts has to be replaced from somewhere. If a lab does not have enough make-up air supplied deliberately, it will pull air from wherever it can, under doors, through gaps, or from adjoining corridors and rooms, which is unpredictable and can itself disrupt the cupboard's performance. Imperial College's guidance is explicit that adequate and stable make-up air must be provided for a fume cupboard to perform as tested.
Make-Up Air, the Key Points
- Labs are normally kept slightly negative relative to adjoining corridors and offices, so any leakage draws air into the lab rather than fumes out of it. A typical pressure differential is kept modest, generally no more than around 15 Pa, so doors remain easy to open.
- Room air velocity near the cupboard should stay low, well below the cupboard's own face velocity, so that general room air movement does not compete with or disturb the containment it is trying to hold at the sash.
- Make-up air should be supplied deliberately, through properly sized and positioned supply diffusers, rather than relying on infiltration through doors and gaps to make up the shortfall.
Ductwork and Discharge
Siting does not stop at the room. Where the extracted air is discharged matters just as much for safety and for compliance.
- Keep discharge points clear of openable windows and air intakes: a duct exhaust should be sited well away from any openable window, door, or fresh air intake on the building, commonly at least 3 m, to avoid drawing extracted fumes straight back into the building.
- Efflux velocity: the fan needs to maintain sufficient discharge velocity at the stack to carry fumes clear of the roofline rather than letting them settle back around the building, which is one reason a bleed damper is often used on banks of VAV cupboards.
Quick Reference
| Consideration | Guidance |
|---|---|
| Distance from doors and cross-traffic | Site away from doorways and busy walking routes to avoid draughts across the sash |
| Room pressure differential | Lab kept slightly negative to corridors, typically no more than around 15 Pa |
| Duct discharge to windows or air intakes | At least 3 m clearance |
These are starting points for early design conversations, not a substitute for a full ventilation design specific to your building. If you are planning a new lab or refurbishing an existing one, we can review your layout and ductwork routing before it is fixed, when changes are still cheap to make.
Planning a New Lab?
Talk to us early in the design process so siting, ductwork, and make-up air are right from the start.