Question 1

What noise level should a bathroom fan not exceed?

Accepted Answer

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.

Question 2

How does duct length affect fan performance?

Accepted Answer

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.

Question 3

Do bathroom fans need timers?

Accepted Answer

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.

Question 4

Can I use a humidity-controlled fan instead?

Accepted Answer

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.

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

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 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° bend	~10	Equiv. ~1.5 m straight duct

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

Part F Extract Fan Requirements

Minimum Fan Extract Rates by Room

Noise Limits for Extract Fans

Duct Length Derating Guide

Timer and Control Requirements

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Frequently Asked Questions

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Part O Overheating Limits

Part E Acoustic Ratings

Part F Ventilation Rates

Part L U-Value Targets History

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