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Part F - System 3

System 3: Continuous mechanical extract and background ventilators

System 3 Where can it be used?

System 3 is used mainly in new build dwellings due to the need to incorporate ductwork within the building fabric. System 3 is particularly suited to dwellings with single façade.


How does it work?

The System uses background ventilators, usually trickle ventilators fitted in windows and a centrally located continuously running mechanical extract fan with ducts running to the moisture producing areas or "wet rooms" such as kitchens and bathrooms.

The background ventilators provide the whole building ventilation and the central mechanical extract fan runs continuously to remove odours and excessive humidity.

A boost facility provides rapid extraction when necessary to remove higher levels of pollutants. As an alternative, continuously running localised fans may be used in place of a centrally located continuously running mechanical extract fan ducted to extract areas.

How do we determine ventilation rates?

The extract rates can be determined in Table 5.1a and 5.1b of the Document.

Guidance on minimum provisions for extract and whole building ventilation is set out in Table 5.2c.

What benefits does it offer?

  • Continuous ventilation at predetermined rates
  • Low noise (central extract)
  • Discreet (central extract)
  • Single penetration to external building fabric (central extract)

What difficulties could we encounter?

  • Perceived high running costs, though usually not actual
  • Noise potential, especially if ducts are poorly installed
  • Ductwork may be difficult to conceal
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