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Part F - System 4

System 4: Continuous mechanical supply & extract with heat recovery

System 4 Where can it be used?

System 4 is suitable for use in houses and flats or apartments, usually in premium developments due to the higher cost of purchase and installation.

How does it work?

A centrally located continuously running mechanical supply and extract fan extracts air via ducts from moisture producing areas or "wet rooms" such as kitchens and bathrooms to remove odours and excessive humidity. This air is passed over a heat exchanger, which transfers a high proportion of the heat of the extracted air to the incoming air, which is then distributed to the habitable rooms via ducts.

Whole building ventilation is maintained by a constant flow of air between input and extract points. A boost facility provides rapid extraction when necessary to remove higher levels of pollutants. As an alternative, every room may have an individual unit as opposed to a centrally located and ducted unit. No background ventilators are required in this System.

How do we determine ventilation rates?

Guidance on minimum provisions for extract and whole building ventilation is set out in Table 5.2d of Approved Document F (Ventilation) 2010.

What benefits does it offer?

  • Controlled airflows
  • Pre-heated incoming fresh air
  • Heat exchange may reduce heat demand
  • May be useful where external noise pollution is a problem as there are fewer building penetrations

What difficulties could we encounter?

  • Accommodating the fan unit and concealing all ducts
  • Perceived high running costs, though usually not actual
  • For the system to be effective the design target for MVHR is ≤5M3/m2/hr@50Pa
  • Noise potential, especially if ducts are poorly installed
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