Purge
Purge ventilation is the ability to quickly remove any odours or buildup of fumes in a living space. Particularly toxic fumes from cleaning, DIY, smoking or cooking. Purge ventilation is also required by Building Regulations for the ability to cool and room and remove a build-up of excess heat.
The volume of air contained in a room must be completely replaced four times in one hour.
To meet Building Regulations, windows are required to have an opening angle of at least 15 degrees and be at least a tenth of the floor area of the room. Windows that can open more than 30 degrees can be a twentieth of the room floor area.
All habitable rooms must be able to purge ventilate through either a window, door or a mechanical extraction system.
Extract
Extract ventilation is used to quickly remove any excess moisture or cooking smells before they spread to other areas of the house.
Especially critical in bathrooms and for cooking to avoid a build-up of condensation.
Extraction can only be achieved using mechanical ventilation on either an intermittent or a continual basis.
For a bathroom or utility room, the extraction rate must be 15 litres per second. For a kitchen, the extraction rate must be 30 litres per second.
Kitchen extraction cooker hoods that only filter air and don't extract externally must be 60 litres per second.
Rooms that require extract ventilation include:
- Kitchens
- Utility rooms
- Bathrooms
- Toilets
Whole Dwelling
Purge and extract are both on-demand ventilation methods to be used as and when needed. Aside from ensuring that excess moisture and fumes are cleared, a house also needs a constant flow of background ventilation to remain healthy.
Older houses will generally fall into less airtight dwellings and modern builds should be built to certain airtight standards. Passivhaus is designed on the basis of being as airtight and insulated as possible to minimise the need for heating.
If a house has an air permeability higher than below it is considered 'less airtight':
- A design air permeability higher than 5m3/(h·m2) at 50Pa.
- An as-built air permeability higher than 3m3 /(h·m2) at 50Pa.
For less airtight houses, they have the option of natural background ventilation and can use a combination of trickle vents on windows and passive stack ventilation.
For airtight houses, a continuous mechanical supply is required using a system such as continuous mechanical extract ventilation (MEV) or Mechanical Ventilation with Heat Recovery (MVHR).
Passive Stack Ventilation (PSV)
Passive stack ventilation works on the same principles as how older houses were ventilated. By using natural air flow manipulated by natural pressure.
For example, old sash windows could open a gap at the top, which allows warm air to escape from the room. By also opening a gap at the bottom of the window, the pressure caused by air being sucked out of the top means that cool air can be sucked in through the bottom (based on the principle of warm air rising). Old chimneys also worked, in the same way, to allow the heat from the fire to move up through the walls of the house creating pressure and this would suck cool air into the room through air vents.
Imagine the same principle, but a skylight or vent at the top of a house which allows heat to escape and in turn the pressure created allows cool air to be drawn in through vents on the ground floor at a lower level.
Wind moving across a roof vent creates pressure that sucks air up and out of the vent. In turn, this creates airflow upwards through the house.
Using cross ventilation also creates pressure to draw air through rooms so all parts of a house are ventilated.
Passive Stack is designed to use natural air pressure through a system of vents and ducts for natural extraction and air flow ventilation.
However, it can be draughty and requires manual intervention unless you install an intelligent system that opens and closes the vents automatically.
Mechanical Extract Ventilation (MEV)
The mechanical extract ventilator (MEV) is a multi-room integrated system that has a central extraction unit usually fitted in the loft space. It has ducts to the bathrooms, wet rooms and kitchens that continuously extract moisture from the air.
As a progression of the natural passive stack system, the MEV system uses mechanical ventilation. The extraction ducts create pressure that draws in air through external background vents and trickle vents in the windows. This generates an air flow and cross ventilation through the rooms to ensure whole house ventilation.
The system also has a boost facility for accelerated extraction on demand to remove excessive moisture and fumes.
A mechanical extract ventilator can usually only be fitted to new build properties as the ducting is integrated into the fabric of the building. Retrofitting can be difficult and expensive.
Mechanical Ventilation with Heat Recovery (MVHR)
Mechanical ventilation with heat recovery (MVHR) is a multi-room system that uses centralised extraction the same as an MEV system. The difference to an MEV is that the MVHR extracts and supplies the air from the same system.
Ducts to bathrooms and kitchens extract the humid air and then pass it through a heat exchanger to warm it before recirculating it into other living spaces.
MVHR systems are highly efficient and economical when used in airtight houses and are a central part of the design standards of Passivhaus.
