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Part F Extract Fan Requirements
Fan sizing, noise limits, duct derating, and timer requirements for domestic extract ventilation, referencing Approved Document F: Ventilation, Volume 1 - Dwellings (2021 edition, effective 15 June 2022).
Minimum Fan Extract Rates by Room
| Room | Intermittent (l/s) | Continuous Low (l/s) | Continuous Boost (l/s) |
|---|---|---|---|
| Kitchen (adjacent to hob) | 30 | 13 | 30 |
| Kitchen (remote from hob) | 60 | 13 | 60 |
| Utility room | 30 | 8 | 30 |
| Bathroom (with bath/shower) | 15 | 8 | 15 |
| WC (no bath/shower) | 6 | 6 | 6 |
Noise Limits for Extract Fans
| Room Type | Max. Noise Level dB(A) | Applies To |
|---|---|---|
| Bedrooms | 30 dB(A) | Continuous systems (MEV/MVHR) |
| Living rooms | 35 dB(A) | Continuous systems |
| Kitchens | 35 dB(A) | Continuous running mode |
| Bathrooms/WCs | No specific limit | Short-duration intermittent use acceptable |
Duct Length Derating Guide
Fan performance must be assessed at the installed system resistance. The following shows typical derating for a 100 mm diameter duct system.
| Duct Configuration | Approx. Resistance (Pa) | Typical Flow Loss |
|---|---|---|
| 1 m straight + grille | 10-15 | 5-10% |
| 2 m straight + 1 bend + grille | 20-30 | 15-25% |
| 3 m straight + 2 bends + grille | 35-50 | 25-40% |
| 4 m+ straight + 3 bends + grille | 50-70 | 35-50% |
| Each additional 90掳 bend | ~10 | Equiv. ~1.5 m straight duct |
Timer and Control Requirements
| Requirement | Detail |
|---|---|
| Bathroom overrun timer | Minimum 15 minutes after switching off |
| Boost mode activation | Manual switch, PIR sensor, or humidity sensor |
| Continuous fan isolation | Must not be easily switched off by occupants (no local switch on continuous systems) |
| Commissioning | All fans must be tested/commissioned and flow rates recorded |
Related Calculators
Use the Extract Fan Sizing Calculator for full sizing, or the Part F Ventilation Rates page for the complete ventilation strategy overview.
Always check the latest edition of Approved Documents on the Planning Portal (planningportal.co.uk) as regulations are updated periodically.
Next up: related pages
Frequently Asked Questions
AD-F 2021 recommends that extract fans in bedrooms should not exceed 30 dB(A) and in living rooms/kitchens 35 dB(A) for continuous-running systems. For intermittent fans, slightly higher noise levels are acceptable as they only run for short periods. These are measured at 1 m from the fan with background noise below 20 dB(A). Noise above these levels may cause occupants to disable the fan.
Every metre of ductwork and every bend reduces the effective airflow from a fan. As a rule of thumb, each 90-degree bend is equivalent to approximately 1.5 m of straight duct. A fan rated at 15 l/s with 1 m of duct may only deliver 10 l/s with 3 m of duct and 2 bends. Always check the manufacturer's performance curve at the installed system resistance, not just the free-air flow rate.
Yes. For System 1 (intermittent extract), bathroom and shower room fans must have an overrun timer of at least 15 minutes after the light or occupancy trigger switches off. This ensures moisture is properly extracted after bathing. The fan can be triggered by the light switch, a pull cord, a PIR sensor, or a humidity sensor.
Yes. Humidity-controlled fans are an acceptable alternative to manually switched fans with timers. They must be capable of achieving the minimum extract rate (e.g. 15 l/s for a bathroom) when humidity rises above a set threshold. They are particularly useful for System 3 (continuous MEV) where the fan speed increases automatically when moisture is detected.
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Last updated: April 2026
Verified against UK standards 路 estimates only, confirm with your supplier.