Why Is My Kitchen Humid Even With Extraction On?

How to trace a humid kitchen, step by step

Work through these in order. Most persistent humidity comes down to one or two of them, and you can check the first few yourself before calling anyone out.

1. Confirm there is real make-up air, not just extraction. A canopy can only remove as much air as it can pull back in. If your only source of replacement air is a cracked-open fire door or gaps around the delivery entrance, the fan is fighting the room, moisture-laden air lingers, and the extraction quietly under-performs against its rated figure. Look for a dedicated supply grille or plenum feeding fresh air toward the cooking line.
2. Feel for negative pressure at the doors. Open the door between the kitchen and a cooler adjacent space; if it drags hard against your hand or whistles as air rushes in, the room is starved of make-up air. Under DW/172 a mild negative pressure is intentional and correct, but a strong pull tells you supply and extract are badly out of balance and humidity will stay trapped.
3. Inspect the baffle and grease filters. Grease-clogged baffles choke the volume of air the canopy can actually move, so steam from pans rolls out into the room instead of up into the hood. Filters are the single most common cause of a system that looks fine but performs poorly, and they are the easiest thing to put right - lift each one out, hold it to the light, and clean or exchange any you cannot see through.
4. Deal with the warewasher separately. A pass-through or conveyor dishwasher dumps a large slug of hot, wet air every time the hood lifts, and a general canopy several metres away will never catch it. DW/172 expects warewashers without built-in condensers to have their own dedicated extract or a condensate hood directly over the machine. If yours vents straight into the room, that alone can account for the fog.
5. Check the fan is actually pulling its rated volume. A slipping drive belt, a grease-laden impeller, a seized damper, or a speed controller wound down to save noise will all cut airflow without switching anything off. If the extraction has grown louder or weaker lately, that is a strong clue the fan is straining - our guide on why your extraction has got louder walks through the mechanical causes.
6. Look for blockages further up the duct. Heavy grease deposits, a collapsed flexible section, a stuck backdraught shutter, or a bird nest at the cowl all reduce the volume reaching outside, even when the fan itself is healthy. If the canopy feels weak across its whole length rather than at one filter, suspect the ductwork or the discharge point.
7. Rule out the room fighting itself. A separate air-conditioning unit set to recirculate, a heater near the pass, or a second extract fan competing for the same make-up air can all upset the balance. Turn off anything non-essential and see whether the humidity eases, then reintroduce items one at a time.

Why balance matters more than raw fan power

It is tempting to assume a humid kitchen just needs a bigger extract fan. In practice, fitting more extraction to a room that already lacks make-up air usually makes things worse - the deeper the negative pressure, the harder outside air has to force its way in through whatever gaps it can find, and the less the canopy achieves for the extra energy it burns. You end up louder, draughtier and no drier.

The reason comes down to what humid air actually is. Boiling stockpots, steamers, combi ovens on a wet setting and the warewasher all release water vapour into the room. Extraction removes that vapour only if the moist air is captured close to source and carried away, and only if an equal-and-slightly-less volume of drier air is arriving to take its place. Break that loop - by starving the supply, choking the filters, or letting the warewasher vent into the open - and the same water simply cycles around the room, condensing on the coolest surfaces it can find. That is the sweat you see on windows, the drip from ceiling grilles and the wet patch behind the pass.

Humidity is, in the words of the DW/172 guidance itself, difficult and expensive to control, which is exactly why the design has to be right rather than simply powerful. A well-balanced system tempers the incoming make-up air so it is not a cold draught in winter, positions supply grilles so fresh air sweeps across the cooking line toward the canopy, gives wet equipment its own local extract, and keeps every filter and duct run clean so the rated airflow is the real airflow. Get those four things aligned and a kitchen that felt tropical becomes workable within a single service.

The maintenance side is not optional, either. Grease is the enemy of airflow and the reason a system that was balanced on the day it was commissioned drifts out of balance within months. Filters that are cleaned on a schedule, ductwork cleaned to TR19 Grease intervals, and a fan checked for belt tension and impeller fouling will hold the design airflow far longer than a system left until someone notices the fog.