Trickle Vents and Extract Fans

Trickle vents are small, controllable openings, usually in window frames, that let a small, steady stream of fresh air into a room without opening the window, while extract fans in kitchens and bathrooms pull stale, moist air out. Together they create a simple background system: fresh air enters through vents in living rooms and bedrooms, flows through the home via gaps under internal doors, and leaves through fans in wet rooms. That through-flow is the whole idea, and it only works if vents, door gaps and fans are all present and working.

Extract fans vary in control: some run on a manual switch or a light switch, some on a humidistat that senses moisture and runs until humidity falls, and some run continuously at a low trickle rate with a boost when needed. Humidistat and continuous fans handle moisture more reliably than a fan that depends on someone remembering to switch it on during a shower.

Positioning decides whether the system feels like fresh air or like a cold draught. Trickle vents above a window, where incoming air mixes with warm room air before reaching occupants, feel less draughty than a vent at sitting height, and vents should not be placed directly above a bed or a desk where the incoming stream is felt constantly. Extract fans should sit near the moisture source, above a shower or near a hob, with a duct route that is short, straight and properly terminated outside, not into a loft or cavity where moisture will condense.

Noise and maintenance decide whether the system stays in use. A fan that is loud, vibrates or runs all night will be switched off or disconnected, and a vent clogged with dust or painted shut provides no ventilation at all. Choosing quiet fans, accessible filters and vents that can be cleaned without tools makes the difference between a system that works and one that is bypassed.

Check every vent for blockage, paint, tape or closed settings, and test every fan for airflow, noise and control behaviour: does it start with the light, with humidity or continuously, and does it boost when needed. Note the duct routes if they are visible, and check whether any duct terminates in a loft, which is a common fault that causes hidden condensation.

Also check internal door gaps: a tightly carpeted door with no undercut can block airflow from a bedroom vent to a bathroom fan, so the whole system stalls. A small undercut or a transfer grille may be needed, and that detail should be part of any ventilation assessment.

In the EU, trickle vent and extract fan provisions follow national building regulations, with member states differing on required vent areas, fan flow rates and whether humidistat control is required in wet rooms. Some countries favour continuous mechanical extract as a baseline, while others accept intermittent fans with background vents, so the local baseline should be checked rather than assumed.

In the UK, Approved Document F sets vent areas and fan rates for England, with devolved equivalents elsewhere, and it distinguishes intermittent and continuous extract systems. Product standards for fans, including noise and efficiency, and building regulation notification for electrical work in bathrooms, add further layers that a competent installer should cover.

A frequent mistake is taping trickle vents shut to stop a draught, which removes the fresh air inlet the extract fan depends on, so the fan pulls air from wherever it can, often a chimney or a neighbouring room. Another is fitting a powerful kitchen fan with a long, convoluted duct that loses most of its airflow to resistance, so the fan is noisy but ineffective.

A third mistake is placing a humidistat fan where steam from a shower never reaches it, or setting its threshold so high that it rarely triggers, leaving moisture to condense on cold walls. Fan position and settings should be tested after installation, not just assumed from the box.

Keep the vent and fan specifications, the control settings, the duct layout where known, and a maintenance note for cleaning vents and fan filters. Those papers help a later occupant restore a system that has been switched off or blocked, rather than replacing it unnecessarily.

Ask a qualified installer: what vent area and fan rate are proposed for each room, how are fans controlled, where do ducts terminate, and how should vents and fans be cleaned. Room-by-room answers show a designed system, not a generic fan in each wet room.