PASSIVE VENTILATION
A BREATH OF FRESH AIR OR A DRAUGHT IDEA?
Before building regs called for increasingly air-tight houses and mechanical ventilation became the norm, people kept cool fresh air in the house using natural methods: walls that “breath”, breezes flowing through windows, as well as large amounts of stone and brick absorbing daytime heat. These ideas were developed over thousands of years and became a standard, integral parts of building design. Today they are called "passive ventilation" and, ironically perhaps, are considered alternative technology to the “normal” mechanical ventilation systems.
There are 2 basic types of passive ventilation, which can be described as pressure and buoyancy systems. Pressure systems rely on the positive pressure on the windward side of the building causing a lower pressure on the leeward side, causing air movement through the building from the windward to the leeward side. There are obvious design implications in that vents, window openings and room layouts have to be positioned appropriately to ensure air movement is possible and in the right direction.
Whole House Passive Stack Ventilation Systems – or PSV – work on the buoyancy principal. They are recognised by BRE, included in Building Regs Part F1 and the Scottish Technical Standard 3.
Hot air is buoyant in cooler air and tends to rise. This is often called the “stack effect”, which also describes the equipment. Put simply an open fire with a chimney stack is a passive ventilation system. The chimney air is heated by the fire and the smoke carried away on the warm, rising air. In the same way a PSV system will draw warm, moist air through ducts from a bathroom or kitchen and out through the roof. Fresh air can be introduced through vents in walls or windows in habitable rooms (living room, bedroom etc.) or ducted in from the roof.
In some cases, as with the Monodraught Windcatcher, both extract air and fresh air use a single roof vent, the stale air coming out of the leeward side of the stack and fresh air introduced on the windward side. The BedZED development famously uses the Arup PSV system where a specially designed cowl and ducting system allows the fresh air being introduced to be warmed by the air being extracted – effectively passive heat recovery.
Refinements have been added to the standard PSV system to provided greater control. Systems are available that use humidity controlled extract and inlet valves which open and close according to internal conditions. So that air is only extracted from the bathroom when the bath or shower have been used.
Monodraught also offer a system with a solar PV powered fan. This provides the added benefit of more precisely controlling the volume of air being extracted and introduced, but still without the need for extra energy.
An alternative way to warm the incoming air and thereby minimise the heat loss inherent in ventilation systems 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. So that 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. Dwell-Vent Ltd offer supply-air window PSV systems with non-return valves on the air vents to prevent warm air leaving the house.
Passive ventilation is little more than a new twist on an old idea. With the drive to achieve zero carbon homes, building every more air-tight houses is becoming critical, and with it the need for controlled ventilation. Passive ventilation systems can meet the bill, usually have no moving parts and are relatively inexpensive. A basic system being little more than ducting and valves. Adding sophistications like supply-air windows or PV powered fans is a matter of personal choice but as with all these things design is critical. A well designed system will extract air from the right places and introduce the right amount of fresh air. Get the design wrong and we have either a damp, stuffy house or a cold, draughty house.