A kitchen installs a large hood and a powerful fan, and the room is still hot and smoky. The equipment is not faulty. The air being extracted has nowhere to come from.
What happens without make-up air
Extraction removes air from the room. If replacement air is not supplied deliberately, the room goes to negative pressure and the fan can only pull what leaks in around doors and windows.
The symptoms are consistent: doors that are hard to open, smoke rolling out from under the hood edge, burners that behave oddly, and an extraction system running at a fraction of its rated volume.
How much make-up air is needed
As a working rule, supply eighty to ninety per cent of the extracted volume as make-up air, and let the remainder come from adjacent spaces so the kitchen stays very slightly negative relative to the dining room. That small negative pressure is what keeps cooking smells out of the seating area.
Where to introduce it
Make-up air should enter low and away from the cooking line, so it does not disturb the thermal plume rising into the hood. Air blown directly at the hood face defeats the capture and pushes smoke into the room.
Sizing the hood itself
Two rules that avoid most capture problems: overhang the equipment by at least 150 mm on every open side, and mount the hood low enough — around 2,000 to 2,100 mm from the floor — that the plume is captured before it spreads.
- Size the extraction against the equipment heat load, not the hood area
- Overhang all open sides by 150 mm minimum
- Keep the duct run as short and straight as the building allows
- Provide accessible cleaning points along the duct
