Sizing and Regs
Building Regs Part F intermittent extract: 15 L/s bathroom + shower room, 30 L/s utility, 6 L/s WC. Continuous extract (dMEV — decentralised Mechanical Extract Ventilation): 8 L/s bathroom continuous. New builds and material changes default to continuous (better moisture control). Windowless en-suite: 15 L/s minimum + 15-minute over-run timer after light switched off. With window: extract still recommended; relying on window alone fails in winter (closed for warmth). Trickle vents in window frames provide background flow but not extract; not a substitute.
Fan types and ducting
Axial fan (in-line with duct on bathroom wall/ceiling): cheap (£35–£185), easy install, fine for short runs <3m. Centrifugal fan: more powerful, handles bends and long ducts (3–10m), £85–£385. Inline fan (located in loft mid-duct): quietest at the bathroom, very powerful, premium choice (£185–£550). Humidistat control (Vent-Axia Lo-Carbon Silent, Manrose Quiet Series): starts at 65% RH, runs to 55% — eliminates user-switched failures. Overrun timer 15 minutes (sometimes 30) after light off. Ducting: 100mm rigid plastic duct preferred (flexible plastic concertina restricts flow by 30%+); short runs only of flex acceptable at termination. Terminate at external wall with backdraft damper grille; not into roof void.
MVHR integration and noise
MVHR (Mechanical Ventilation with Heat Recovery): whole-house system extracts from wet rooms + kitchens, supplies fresh air to bedrooms + living, recovers 80–92% heat from extracted air. Bathroom extract integrated — no individual fan needed. Premium spec on retrofit £4,800–£12,500; new build mandatory route for high-spec EPCs. Noise rating critical en-suite: en-suite adjacent to bedroom needs <22dB at extract grille (whisper-quiet); standard fans 30–45dB are too loud (wakes partner). Acoustic boots on ductwork and inline fans achieve <22dB. Bedroom-adjacent bathroom extract: never centrifugal fan on bathroom wall — always inline in loft.
