Articles
Passive Ventilation
Apr
2010
As the movement towards sustainable, energy efficient houses increases so does the interest is passive ventilation systems. They are increasingly seen as an essential component of a sustainable home, but what is passive ventilation and how does it work?
Why Ventilate?
Ventilation of any sort only became an issue in UK homes after the mid 1940’s. Until then house were largely ventilated by default – draughts around windows and doors, movement in and out of the house and the odd air-brick provided all the ventilation needed. After the mid 1940’s air conditioning became more prevalent, construction methods improved (for general, low-cost housing) and gas-fired central heating began to be installed. These all led to the need to consider how much fresh air was needed in a house and how it was to get there. But as late as the 1960’s, an air-brick close to the gas boiler and the ability to open a window was still considered sufficient. It is not until we get close to the 21st century that ventilation “systems” start to become the norm. The main issue now is the realisation that uncontrolled ventilation means uncontrolled heat loss – anathema to energy efficient house design. This then led to the development of 3 main passive ventilation “systems”; natural, passive stack and wind effect ventilation.
Natural ventilation
Natural ventilation is less a ventilation “system” and more a matter of designing the house appropriately – low level inlets, high level outlets, both positioned to work with solar heat to allow natural air-flows to move through the house. Often extract fans will still be needed in wet rooms but otherwise a well designed house will allow air warmed by the sun to move through the house, taking with it the smells and excess moisture that needs to be exhausted.
Natural ventilation systems can include some form of control to the vents (manual or automated) and specific windows and on more sophisticated systems the inlet vents will alternate between north and south – in summer allowing cooler air from the north elevation and in summer using the warmer air from the south.
Passive stack ventilation
Passive Stack Ventilation (PSV) systems are based on the 'Stack Effect' or ‘Thermal Stack’ principal – which basically means that hot air rises. A chimney over a fireplace is, in effect, a thermal stack. Anyone who has used an open fire will know that the flow of air up the chimney increases as the fire gets hotter. Simply because as the air in the chimney heats up it rise more quickly.
PSV systems ventilate the whole house by siting extracts in 'wet' rooms (kitchens, utility and bathrooms) and ducting vertically to terminals on the roof. Wind will create a pressure difference around the roof terminal which helps warm, moist air to be drawn up the ducts and replaced by fresh air coming in through inlet vents in walls or window frames. This flow of fresh air from 'dry' to 'wet' areas creates whole the house ventilation. While the concept of PSV is simple, the design of an effective system is complex and relies on a number of criteria, including room volumes, number of occupants, orientation of windows and doors, probable moisture content, geographical location and site exposure. Designing systems is not a job for the amateur.
Wind effect
These systems rely on the same principal as the old middle-eastern wind towers that push air down into the building and cowls that use the wind to draw air from the building. These systems usually provide a fair degree of control over the amount of air moving through the building and the direction of its movement, by employing moisture or pressure sensitive actuation on the vents. And they use little or no energy to do it. In more sophisticated systems the incoming fresh air can be warmed by the outgoing stale air.
Supply-Air Windows
An alternative way to warm the incoming air is to use “supply-air windows”. Essentially these are double glazed windows with a third pane of glass on the outside with an inlet at the bottom of the third pane and an outlet at the top of the double-glazed unit. Incoming air passes behind the third pane, gaining a little solar heat and a little of the heat leaving the house through the double glazing and enters the house a little warmer.
Conclusion
Passive ventilation is a new twist on a very old idea and with the drive to build ever more air-tight houses controlled ventilation is becoming critical. Passive ventilation systems have few moving parts, are relatively inexpensive and provide the clean air and control needed. Adding sophistications like automated window actuators, supply-air windows or PV powered fans help to overcome the problems inherent in systems reliant on the weather, as all passive ventilation systems are. It has to be remembered that ventilation is critical to health and the design of the ventilation system is critical to its efficacy. Whether it is natural ventilation or PSV, proper consideration needs to be given to the design to make sure it works properly.