Established in 1967
Building Preservation Specialists
PRESERVATION
Passive Ventilation Stack System

Passive stack ventilation operates on simple natural principles.
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· Stack effect - ducts run from 'wet' rooms - kitchens, bathrooms etc - up to roof terminals.  Warm moisture-laden air is extracted and rises up the ducts by the stack effect, requiring no electric fan or power source.
· Fresh air - wall or window inlets provide a flow of replacement fresh air into the building as required.
· Planned air flow - the position of inlets and extracts ensures that air always moves from 'dry' rooms to 'wet' rooms, where it is extracted.
· Demand control - each 'intelligent' inlet and extract vent is automatically controlled by room humidity.
· The ventilation rate thereby responds to the ventilation need in individual rooms.  No electrical connection or external power source is required.
Fan Assisted Ventilation

Fan assisted ventilation is based in similar principle to PSV, but it is designed for larger more complex installations.
· Extract - extract vents are installed in 'wet' rooms, and connected by ducting to a low power continuously running fan in a concealed position (e.g. the roof space).
· Fresh air - wall or window inlets provide a flow of replacement fresh air into the building as required.
· Planned air flow - the position of inlets and extracts ensures that air always moves from 'dry' rooms to 'wet' rooms, where it is extracted.
· Demand control - each 'intelligent' inlet and extract vent is automatically controlled by room humidity.
How does it work?

Demand-controlled ventilation
· Passive Stack System provides automatic humidity control of ventilation without using electrical power.
· Inlets and extracts open and close to a trickle automatically to allow greater or lesser airflow depending on humidity levels in each room.
· The inlet incorporates a flap control, the extract a louver control, which are operated by humidity sensitive nylon strips.  These strips respond to humidity in the air by contracting or elongating; this is the physical basis for control of the vents without the need for electrical power.
· Operating range between minimum and maximum opening of the vents is 40% to 70% (relative humidity).
· The inlet design directs incoming air upwards to avoid draughts and promote good mixing in the ceiling area, pre-warming the incoming air.
· At low relative humidity each unit closes down to a trickle setting, allowing a small controlled amount of air to pass at all times.
Ventilation effect
· Passive Stack System ventilation is driven primarily by the natural stack or convection effect by which warm air rises, entering vertical (or near vertical) duct, to reach the colder outside air.
· The air inside a house is almost always warmer than that outside.  So the warm air inside rises up the ventilation duct by a natural convection effect, carrying moisture with it.
· There is an additional contribution from wind blowing across the roof which creates a suction effect at the terminal (the venture effect).  However, this wind effect is not essential to the operation of the system and experience shows that systems continue to function fully effectively even if there is no wind.  The convection effect and venture effect interact according to the prevailing weather conditions.
· Control of airflow is maintained by responce to humidity levels inside the building.  When the humidity is low, the ventilation rate is minimised irrespective of external weather conditions.
View a Case History for Ventilation.   View the many benefits of Passive Ventilation.