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Part F - System 2

System 2: Passive stack ventilation and background ventilators

System 2Where can it be used?

System 2 is mainly used in new build houses or flats where the environmental conditions are suitable for the system to function. Each individual application has to be designed to suit the specific house design and exposure.

How does it work?

Passive stack ventilation is a non-mechanical system, as it uses background ventilators, usually trickle ventilators fitted to windows to provide the whole building ventilation and passive stack ventilation ducts installed in moisture producing areas or “wet rooms” such as kitchens and bathrooms.

The background ventilators provide supply air and the passive stack ventilation ducts remove odours and excessive humidity. The flow rates vary to provide whole building and extract ventilation depending on wind speed rates across the roof terminal that creates a low-pressure drag effect in the stack duct and the temperature difference between the internal and external air. Buoyant warm air rises up the passive ventilation stack ducts.

How do we determine ventilation rates?

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

What benefits does it offer?

  • No running costs (unless mechanical back up is required)
  • Quiet operation

What difficulties could we encounter?

  • Difficult to replicate system design for standard dwelling design formats
  • May not be effective in all conditions and may need mechanical back up at times, particularly during warm periods
  • Ductwork may be difficult to conceal
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