15 l/s and 30 l/s: Extractor Fan Flow Rates UK Installers Must Meet
15 l/s and 30 l/s: Extractor Fan Flow Rates UK Installers Must Meet
Approved Document F (2026) sets minimum intermittent extract rates for kitchens with an external cooker hood, higher rates where there is no hood, and specified values for utility rooms, bathrooms, and sanitary accommodation, alongside continuous “high” rates for these rooms respectively. Any intermittent fan serving a room with no openable window must also run on a 15-minute overrun after use.
TL;DR:
- Most UK dwellings require installed fans to meet either the minimum flow rates or the 4 air changes per hour purge for rooms without external walls or windows.
- Fans with flexible ducting or long runs often underperform unless rigid ducting or inline centrifugal units are used, with duct length limited to 2 meters for optimal performance.
- Building control verifies actual installed airflow at the grille during inspections, making proper duct routing, terminal placement, and overrun timers essential to pass.
- Continuous fans with humidity sensors and manual boost controls suit most situations, offering energy efficiency and better compliance than fixed-speed intermittent fans.
- Regular maintenance, including cleaning filters and ensuring unobstructed duct routes, is crucial for sustained airflow and preventing damp and mould issues.
Table of Contents
- Minimum rates at a glance: intermittent and continuous
- Intermittent versus continuous systems: what the numbers mean in practice
- Installed performance: why a fan’s free-air rating can be misleading
- How building control verifies extractor flow and common on-site failures
- Sizing and selecting a fan: practical rules of thumb for UK wet rooms
- When to book a survey: our survey-led approach to ventilation and damp
- How to calculate actual airflow based on fan specifications and room size
- Impact of fan speed control on flow rates and energy efficiency
- Noise levels associated with different flow rates and their practical implications
- Common causes of reduced flow rates post-installation and troubleshooting steps
- Effect of filter types and cleanliness on extractor fan flow rates
- Installer perspective: simple priorities that stop recurring mould and rework
- Quay Point Plastering: extractor fan assessment and remedial work
- FAQ
- Sources
Minimum rates at a glance: intermittent and continuous
Builders and homeowners checking compliance need two sets of numbers side by side, because the figure that applies depends on which system is installed.
These figures come from Approved Document F (2026), which also requires that a dwelling’s continuous extract rates sum to the whole-dwelling ventilation rate, calculated at 0.3 l/s per square metre of floor area plus an allowance based on bedroom numbers.
- Internal rooms with no external wall or window often need the greater of the table minimum or the 4 air changes per hour purge rule.
- A room that fails this check needs either a larger fan or a different ventilation strategy entirely.
Intermittent versus continuous systems: what the numbers mean in practice
An intermittent fan switches on with the light or a separate control and clears moisture in bursts, typically backed by a humidity sensor or timer. A continuous system, usually mechanical extract ventilation (MEV) or decentralised mechanical extract ventilation (dMEV), runs permanently at a low “trickle” rate and boosts to the “high” figure when a room is in use.
Continuous systems suit flats and dwellings with limited background ventilation, since they maintain air movement around the clock rather than relying on someone remembering to switch a fan on. Intermittent systems remain the default for most houses with bathrooms and kitchens that have reasonable natural ventilation elsewhere.
- Humidity sensors with manual override satisfy Approved Document F where boost control is required.
- Cooker hoods extracting to outside can sometimes be linked to the same control strategy as the kitchen’s general extract.
- Internal bathrooms without windows always need the 15-minute overrun, regardless of system type.
Pro Tip: Fit the overrun timer even where it is not strictly demanded. It costs little and removes a frequent cause of failed inspections.
Installed performance: why a fan’s free-air rating can be misleading
A fan’s stated flow rate is almost always measured in free air, with no ducting, grille or bends attached. Once fitted into a real wet room, that figure drops because of friction losses in the duct, resistance at the grille, the number of bends, and wind pressure at the external terminal. A 30 l/s fan on a long, bent, flexible run can easily deliver well under the required minimum once installed.
- Prefer rigid ducting over flexible hose wherever the run allows it, since flexible duct creates far more internal friction.
- Limit flexible duct to the final 200mm connection to the fan or terminal.
- Keep duct runs to a practical maximum of 2.0 metres without supporting design calculations.
- Site the extract terminal as high in the room as practicable, no more than 400mm below the ceiling.
- Where terminals face prevailing wind, consider rerouting to a sheltered elevation or specifying a constant-volume unit.
Approved Document F (2026) sets these installation constraints because building control now expects on-site verification of installed flow, not just a fan’s box rating.
Pro Tip: For runs longer than 2.0 metres or with more than two bends, step up to an inline centrifugal fan rather than hoping a standard axial unit will cope.
How building control verifies extractor flow and common on-site failures
Building control carries out System 1 testing on intermittent fans at completion, measuring the actual installed flow at the grille rather than accepting the manufacturer’s free-air figure. This is where many installations come unstuck.
- Underspecifying the fan for the installed conditions, often by relying on the free-air rating alone.
- Omitting the 15-minute overrun on a bathroom or WC with no window.
- Excessive duct loss from long flexible runs, tight bends or undersized spigots.
- Poor terminal siting, including units mounted low on a wall or facing directly into prevailing wind.
- Specifying a recirculating cooker hood where the regulations assume external extract.
At handover, a complete package should include a commissioning certificate, the measured flow figures in litres per second, photographs of the installed ducting and terminal, and supporting calculations for any run over 2.0 metres. Our guide to Part F ventilation covers what inspectors look for in more detail.
Sizing and selecting a fan: practical rules of thumb for UK wet rooms
Size a fan to the greater of the Approved Document F minimum for that room type or the 4 ACH purge requirement where the room has no external wall. Then build in a margin for installed losses rather than selecting a unit that only just meets the table figure on paper.
- Choose a fan whose manufacturer-stated installed performance, not just free-air rating, meets the required flow after accounting for duct length and bends.
- For long or awkward runs, an inline or centrifugal unit generally holds its flow better than a standard axial fan.
- Weigh noise and running cost against flow: a continuous low-trickle system with boost capability is often quieter day to day than a powerful intermittent fan cycling on and off.
- Call a ventilation specialist where a room combines a long duct run, no external wall, and a tight budget for noise.
Pro Tip: A standard 100mm axial bathroom fan rated above 15 l/s typically clears a short, straight run comfortably, but larger rooms or longer runs usually need an inline unit to hold the same figure once installed.
When to book a survey: our survey-led approach to ventilation and damp
Ventilation problems and damp problems are closely linked, and a fan that technically meets Approved Document F on paper can still leave a bathroom or kitchen chronically damp if the installation itself is compromised. We start every job with a visual inspection before recommending any work, documenting what we find rather than guessing at a cause.
A survey covering extractor performance typically includes a measured flow reading at the grille, a check of the duct route and terminal siting, and a written set of recommendations tied to what we actually observed. Every finding comes with photographs and a defined scope of work, and any remedial work carries a written guarantee.
How to calculate actual airflow based on fan specifications and room size
Working out the flow rate a fan needs to deliver starts with the room’s floor area and the applicable Approved Document F minimum, not with the fan’s box figure. For a standard bathroom with a window, the requirement is 15 l/s intermittent. For an internal bathroom with no window, check the 4 ACH purge calculation: multiply the room’s volume in cubic metres by 4, then divide by 3,600 to convert hours into seconds, which gives the required flow in litres per second.
Say a windowless shower room measures 2.0 metres by 1.8 metres with a 2.4 metre ceiling, giving a volume of 8.64 cubic metres. At 4 air changes per hour, that is 34.56 cubic metres per hour, or roughly 9. Since the Approved Document F table minimum for a bathroom is 15 l/s, the table figure takes priority here and the fan must be selected to deliver at least 15 l/s once installed, not the lower purge figure.
Once the target flow rate is set, compare it against the fan’s installed performance curve, not its free-air rating, after accounting for duct length, bends and terminal type. Approved Document F (2026) treats this installed figure, confirmed by on-site testing, as the one that matters for compliance.
Impact of fan speed control on flow rates and energy efficiency
A fan running on a fixed-speed intermittent control delivers one flow rate whenever it is switched on, with no adjustment for how much moisture is actually present in the room. This is simple and predictable, but it means the fan either runs at full output constantly during occupied periods or not at all.
Continuous systems with variable speed control behave differently. They hold a low background rate most of the time and step up to the “high” rate specified in Approved Document F only when triggered by occupancy, humidity or a manual boost switch. This approach tends to use less energy overall, since the fan spends most of its life at trickle speed rather than at full output.
Humidity-sensing controls with a manual override, acceptable under Approved Document F for boost-rate triggering, add a further layer of efficiency by responding to actual conditions in the room rather than a fixed timer. The trade-off is that a fan set to run too low on trickle speed, in an attempt to save energy, can fall short of the whole-dwelling continuous rate requirement, so the balance between efficiency and compliance needs checking against the specific dwelling’s calculation.
Noise levels associated with different flow rates and their practical implications
Generally, a higher flow rate from a given fan model means more noise, because moving more air through the same duct and grille increases both motor effort and air turbulence. This becomes a practical issue in bedrooms, en-suites and any room where occupants are trying to sleep or relax while the fan runs.
For continuous systems, the trickle rate is usually quiet enough to run unnoticed around the clock, while the boost or “high” rate is noticeably louder but only runs for short periods. Intermittent fans sized generously above the Approved Document F minimum to compensate for duct losses can end up louder than necessary if the installer has overcompensated rather than fixing the underlying duct run.
Choosing an inline or centrifugal unit sited away from the room itself, rather than a fan mounted directly in the ceiling or wall, often reduces perceived noise for the same flow rate, since the motor is remote from the occupied space. Where noise is a particular concern, our guide to plumbing ventilation covers practical positioning advice that applies equally to extractor fan ducting.
Common causes of reduced flow rates post-installation and troubleshooting steps
A fan that tested fine at commissioning can lose performance over time, and the causes are usually mechanical rather than a fault with the original specification. Grille and filter build-up, duct damage, and terminal blockages are the most frequent culprits.
Flexible duct that has sagged, kinked or been crushed during other building work restricts airflow considerably, even where the fan itself is running normally. A loose or disconnected duct joint lets air escape before it reaches the terminal, which can be checked by feeling for airflow at each joint while the fan runs. External terminals blocked by debris, cobwebs or bird nesting are a common and easily missed cause of reduced extract, particularly on units that run infrequently.
Troubleshooting should start with the simplest checks: clean or replace the grille and filter, confirm the duct run has not been damaged or compressed, and inspect the external terminal for blockages. If flow remains poor after these checks, the duct route and terminal siting likely need a proper reassessment against the original installation, which is where a measured on-site reading becomes useful rather than guesswork.
Effect of filter types and cleanliness on extractor fan flow rates
Many kitchen and bathroom extractor fans, particularly cooker hoods, rely on a grease or mesh filter that sits directly in the airflow path before the fan motor. A clogged filter adds resistance to the whole system, reducing the flow that reaches the room even though the fan motor itself is working at full capacity.
Grease filters in kitchen hoods need regular cleaning, generally more often than bathroom fan grilles, because cooking grease builds up faster than bathroom condensation residue. A hood that has not been cleaned for months can lose a meaningful share of its rated extract capacity purely from filter blockage, independent of any duct or terminal issue.
Where a cooker hood recirculates rather than extracting to outside, as noted earlier, it falls outside the external-extract assumptions in Approved Document F and does not count towards the 30 l/s kitchen minimum regardless of filter condition. Regular filter maintenance is a simple, low-cost way to keep an externally ducted fan performing close to its tested flow rate, and it is worth checking before assuming a fan itself has failed.
Installer perspective: simple priorities that stop recurring mould and rework
In our experience, commissioning and correct controls prevent repeat failures far more reliably than simply fitting a bigger fan. Most recurring mould complaints trace back to a duct fault or a missing overrun timer, not an undersized unit. Documenting measured flow and fitting a timed overrun on internal bathrooms are small steps that consistently avoid rework. We provide on-site checks for anyone who would rather have this inspected professionally.
— Quay
Quay Point Plastering: extractor fan assessment and remedial work
Where ventilation and damp overlap, as they often do in older bathrooms and kitchens, we start with a survey rather than a quote for a fan swap. Fixing a duct fault or control issue can resolve a damp problem that a replacement fan alone would not.
A booking leads to a site visit where we measure installed flow, check the duct route and terminal, and inspect for any related damp or mould before writing up our findings.
- A documented scope of work covering exactly what we found and what we recommend.
- A written guarantee on any remedial work we carry out.
- Clear referral to a specialist where the issue sits outside our remit.
Our extractor fan assessment service is the starting point, and you can see the full range of our work, including condensation and mould treatment, on our main services page.
FAQ
How much does it cost to run an extractor fan for an hour?
Running costs depend on the fan’s wattage and your electricity tariff, and vary enough between models and trickle versus boost settings that there is no single figure that applies to every installation. A continuous low-wattage trickle fan generally costs very little to run compared with a high-powered intermittent unit on boost.
What are the regulations for installing cooker hoods in the UK?
Approved Document F (2026) requires a kitchen cooker hood extracting to outside to achieve a minimum intermittent rate of 30 l/s, rising to 60 l/s if the kitchen has no external-extract hood at all. A recirculating hood does not count towards this external-extract requirement.
Can extractor fans prevent mould?
A correctly sized and commissioned extractor fan removes the moist air that drives condensation and mould growth, provided the installed flow actually meets the room’s requirement rather than just the fan’s free-air rating. Where mould persists despite an apparently working fan, a survey of the duct, terminal and control setup usually identifies the underlying cause.
What is an inline extractor fan?
An inline extractor fan, also called a centrifugal fan, sits within the ducting away from the room itself rather than mounted directly in the wall or ceiling. This positioning suits longer or more complex duct runs, since inline units generally hold their flow rate better over distance than a standard axial fan.