What Size Extract Fan Do I Need?

Minimum Extract Rates (Approved Document F)

• Kitchen (intermittent): 30 l/s adjacent to hob, or 60 l/s if cooker hood
• Kitchen (continuous): 13 l/s
• Bathroom (intermittent): 15 l/s
• Bathroom (continuous): 8 l/s
• Utility room (intermittent): 30 l/s
• WC/cloakroom (intermittent): 6 l/s

Intermittent vs Continuous

• Intermittent: Runs when needed (linked to light switch or humidistat). Higher flow rate required.
• Continuous (trickle): Runs at low speed all the time with boost when needed. Lower base rate but uses more energy overall.

Duct Length Matters

Fan performance drops significantly with duct length. A fan rated at 15 l/s at zero duct length may only deliver 8–10 l/s through a 3m duct with bends. Always check the fan’s performance curve against your actual duct run.

• Short duct (under 1m): Standard fan is fine
• Medium duct (1–3m): Choose a fan rated 20–30% above requirement
• Long duct (3m+ or multiple bends): Use an inline/centrifugal fan

Overrun Timer

Building Regulations require bathroom fans to have a minimum 15-minute overrun after the light is switched off. This ensures adequate moisture removal after showering or bathing.