Extract ventilation tackles moisture and pollutants at source: it pulls air out of the 'wet' rooms where steam, cooking moisture and odours are produced, and fresh air enters from elsewhere (trickle vents, leakage, or transfer from other rooms) to replace it. It's the most common mechanical ventilation in UK homes and the baseline strategy in Building Regulations Part F.
The three types of extract
| Type | How it runs | Typical use |
|---|---|---|
| Intermittent extract | On-demand (switch, light, humidity, occupancy) | Bathrooms, WCs, kitchens — boosts during use |
| dMEV (decentralised continuous) | Runs continuously at a trickle, boosts on demand | Per-room continuous extract in wet rooms |
| MEV (centralised continuous) | One central fan, ducted to several wet rooms | Whole-house continuous extract |
Intermittent extract fans
The familiar bathroom or kitchen fan that runs when you switch it on (or when the light's on, or when humidity/occupancy triggers it). Simple and cheap, but it only ventilates when active, so moisture generated between runs isn't removed — and many are under-used or wired to switch off too soon.
dMEV — decentralised continuous extract
A low-wattage fan in each wet room that runs continuously at a low trickle rate, boosting automatically (often on humidity) during showers or cooking. Because it ventilates around the clock, it controls background humidity far better than an intermittent fan, at very low running cost. It's a popular, effective upgrade for moisture and mould control.
MEV — centralised continuous extract
A single central fan unit (often in a loft or cupboard) ducted to extract continuously from all the wet rooms at once, with boost. It's a whole-house continuous extract strategy — tidy and effective — but, like all extract systems, it recovers no heat: the warm extracted air is simply thrown away and replaced by cold incoming air.
Humidity-sensing and smart controls
Modern extract fans increasingly self-regulate — ramping up when humidity rises (a shower) and idling when the room is dry — which matches ventilation to need, saves energy and improves moisture control without relying on occupants remembering to switch them on. Humidity-responsive dMEV is often the sweet spot of cost, effectiveness and simplicity for retrofit moisture control in a not-yet-airtight home.
Extract and Part F
Building Regulations Part F sets minimum extract rates for wet rooms (with higher rates for intermittent vs continuous operation) and minimum whole-dwelling ventilation. Extract ventilation, combined with background ventilators (trickle vents), is the standard route to compliance in many homes. As airtightness increases, Part F pushes toward continuous systems, because intermittent extract plus leakage no longer guarantees adequate background ventilation in a tight house.
The key limitation — no heat recovery
Where extract fits
Extract ventilation — especially humidity-sensing dMEV — is a sound, cost-effective moisture-control strategy for homes that aren't being made highly airtight. It's simple, reliable and targets moisture at source. As a retrofit deepens toward airtightness, the strategy should shift to MVHR; until then, good continuous extract is far better than relying on intermittent fans and open windows.
Part F extract rates — the numbers
Part F sets minimum extract rates for wet rooms, and they differ between intermittent and continuous operation. For intermittent extract the figures are familiar: roughly 30 litres per second for a kitchen (15 l/s if ducted over a hob with a cooker hood), 15 l/s for a utility room, and 6 l/s for a bathroom or shower room, with a WC typically covered by 6 l/s or adequate background ventilation. Continuous systems run at lower trickle rates with a boost, sized on the whole-dwelling minimum ventilation rate, which scales with the number of bedrooms and the floor area. The practical point for a homeowner is that an extract fan is not adequate simply because it spins — it has to move the required volume of air, against the resistance of its ducting, to actually control moisture. A powerful-sounding fan strangled by a long, kinked duct can fall well short of its rated flow.
| Room | Minimum extract rate |
|---|---|
| Kitchen (over hob / cooker hood) | 30 l/s (or 15 l/s if hood-ducted) |
| Utility room | 30 l/s |
| Bathroom / shower room | 15 l/s |
| WC | 6 l/s |
Why extract systems under-perform
Most disappointing extract installations fail not on the fan but on everything around it. The commonest causes are: long, undersized or flexible concertina ducting that throttles the airflow far below the fan's rated figure; sharp bends and crushed runs in loft voids; blocked or restrictive external grilles; and the absence of any make-up air path, so a sealed room simply cannot let the extracted volume be replaced. Intermittent fans are also routinely defeated by behaviour — switched off too quickly by an over-run timer, or never used at all. A humidity-sensing continuous fan removes the behavioural failure, but the ducting and make-up air still have to be right. This is why commissioning matters: measuring the actual flow rate at the grille, not assuming the fan delivers its box figure, is the only way to know an extract system genuinely works.
