Natural Ventilation

Introduction
Ventilation is a critical factor in determining the environmental performance of buildings, from energy use to the health and comfort of the occupants. Natural ventilation, either stand-alone or mixed mode (when coupled with a mechanical ventilation system) can provide a comfortable working environment with the potential for low energy usage. In favourable climates and building types, natural ventilation can reduce energy consumption by 10-30%.

Natural Ventilation Types Natural ventilation systems rely on pressure differences to move fresh air through buildings. These pressure differences can be caused by the wind or by the stack (buoyancy effect) created by differences in temperature or humidity.

Wind-driven ventilation
Wind causes a positive pressure build up on the windward side of the building and a negative pressure on the leeward side. This pressure difference is equalised by fresh air entering any openings on the windward side and being exhausted from the leeward side openings.

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Stack-Driven Ventilation

Stack driven ventilation is temperature induced. Warmer air is more buoyant than the colder air and will therefore rise above it creating an upward air stream. In order for a building to be ventilated adequately via stack effect the inside and outside temperatures must be different so that warmer indoor air rises and escapes the building at higher apertures, while colder, denser air from the exterior enters the building through lower level openings.

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Control Strategy A critical factor in natural ventilation is the availability of windows that the occupants can open and close at their discretion. However, naturally ventilated buildings often incorporate either high-level windows or ventilators, which are under BEMS control.When under automatic control, the windows must be designed and operated such that:

  • The air change rate is sufficient to provide satisfactory fresh air for occupant health and comfort.
  • The flow of outside air should be evenly distributed throughout the occupied zone.
  • To avoid drafts, local air velocities should be monitored and the windows should be opened/ closed accordingly.
  • To avoid rain ingress, wind speed/ direction and rain should be monitored and the windows should be opened/ closed accordingly.

In order to accomplish the above criteria, wind speed/ direction and rain sensors can be monitored by the BMS along with room temperature/air quality sensors. The control algorithms will make a decision as to the window position.

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