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mechanical extraction - passive stack ventilation

With the modern UK building standards aimed at energy efficiency, houses are becoming increasing air tight although, at the same time, they need to have a certain amount of ventilation to remove stale air and introduce fresh air. Adequate ventilation is required to maintain a healthy living environment, avoid humidity rising too high and provide enough fresh air for any combustion (gas fires and cookers etc) within the house; the requirements for ventilation are given in the Building Regulations Approved Document F.

This need for ventilation is, to an extent, at odds with the needs of energy efficiency as the heat in the air leaving the house will be wasted if the temperature of the air outside the house is below the inside temperature - using a Mechanical Ventilation with Heat Recovery system (MVHR) can recover upto 90% of the heat which would otherwise be lost.

The Building Regulations specify that air extraction is fitted in all kitchens, cloakrooms and bathrooms, and that other rooms are adequately ventilated, usually by trickle vents fitted to external doors and windows. The air extraction can be by means of either mechanical extractor fans or passive stack ventilation (PSV).

Mechanical extraction ventilation

Mechanical extractor fans can be split into 3 basic types:

  • Individual fans for each area being controlled by a separate switch. The air extracted from each fan being discharged outside the house through its individual ducting, fresh air enters the building through trickle vents etc.
  • Continuous mechanical extraction where each area is connected by ducting to a central fan which runs all the time and which expels the air from the building - the expelled air is replaced by air drawn in through the trickle vents fitted to the windows and doors, and other leakage points. This system has the advantages that the noise normally associated with individual fans is removed from the individual areas, and that, as the extraction is continuous, the ventilation is generally better. For more details see our article on Continuous mechanical extraction
  • Central extraction with heat recovery where the air is extracted through ducting from the required areas and expelled from the building after passing over a heat exchanger. The recovered heat is used to heat air mechanically drawn into the building and ducted to other areas of the house. For more details see our article on Mechanical Ventilation with Heat Recovery systems (MVHR).

Passive Stack Ventilation (PSV)

With Passive Stack Ventilation (PSV), there are no mechanical fans - each area is connected by ducting to the top of the house and air is drawn out by a combination of the air flowing over the roof and by the natural buoyancy of warm moist air. The extracted air is replaced by fresh air drawn into the building through the trickle vents.

The degree of ventilation depends largely on the movement of the external air and the external air temperature. Very little control is available although humidity controlled dampers (which need no electrical power) can be fitted in the ducting to prevent over ventilation.

For more details see our article on Passive Stack Ventilation (PSV).