Mechanical ventilation is air change driven by a fan rather than by wind and temperature difference alone. In housing this usually means a continuous extract system: a fan unit draws air from the kitchen, bathroom and WC, while fresh air enters the living rooms and bedrooms through trickle vents or vent windows.
Why it is needed
A household puts a surprising amount of water vapour into the air: cooking, showering, laundry, breathing and houseplants together easily add several litres a day for an average family. With nowhere to go, that vapour condenses on the coldest surface — a window reveal, a cold bridge, the back of a wardrobe — and mould follows. Ventilation also removes CO₂, odours and volatile compounds. Since buildings have been built airtight, there is no longer any accidental draught ventilation to do the job.
Systems
- Continuous extract — natural supply through vents, mechanical extract. Simple and cheap, but incoming air is at outdoor temperature
- Balanced with heat recovery — mechanical supply and extract, with outgoing air pre-warming incoming air. It saves heating energy but demands filter changes and properly commissioned ductwork
- Demand controlled — CO₂ or humidity sensors set the flow rate room by room, so the system does not run at full speed all day
Points to watch
- Never switch it off. The lowest setting is background ventilation. A unit switched off at night is the single most common cause of bedroom mould
- Air has to get in. Extract without supply does not work: the fan will pull air through gaps, the letterbox or — worse — down a flue
- Maintenance. Clean the valves at least once a year and change heat recovery filters roughly every six months. Dirty filters halve the performance
- Noise. A noisy unit gets turned off, and then the system is worthless. Specify attenuators and generously sized ducts
- Ductwork. Avoid corrugated flexible duct where you can; it adds resistance, noise and dirt traps. Smooth ducts perform considerably better
- Open flues. Where there is an open-flued appliance, extract ventilation can pull combustion gases back into the room. Have the combination checked
Related terms
- Balanced ventilation
- Natural ventilation
- Indoor climate
- Airtight construction
- Condensation
- Night purge ventilation
- CO₂ monitoring
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