The brief
Develop a dual-mode ventilation system for a space used daily as a car park and, under a defined scenario, as a shelter. In normal mode, the system must control air exchange in response to a CO sensor; in protective mode, it must supply air for occupants through the protected paths specified in the design.
Switching between scenarios must change the air route, fan airflow and heating operation together. Clear coordination between staff's manual actions and automatic control commands was a separate part of the brief.
Solution: different air supply modes
During car park use, the mechanical airtight shut-off dampers in the normal ventilation ducts remain open. The system ventilates the space, with controls responding to the CO sensor signal. The carbon monoxide sensor is used specifically for car park requirements; it does not determine the fresh air needs of shelter occupants.
When switching to protective mode, designated staff manually close the specified airtight shut-off dampers. Following this change in configuration, air passes through protected sections. At this site, the relevant ductwork uses 2 mm thick steel. This is a feature of this particular solution: thickness, joints, supports and structural penetrations must be considered in conjunction with the design.
Blast protection devices and expansion chambers are specified in the protected air paths. Moving plates in the devices close the passage in response to an external pressure impulse, while the chambers form part of the system that attenuates its transmission indoors. These elements are not interchangeable: protection depends on the coordinated design of the entire air path.
Blast-resistant doors are also specified for the space. Ductwork and penetration assemblies are constructed in accordance with the design's fire safety provisions. Metal thickness or resistance to a pressure impulse alone cannot be taken as evidence of fire resistance.
How airflow and temperature are controlled
In shelter mode, variable frequency drives adjust fan speeds. At this site, this means reducing airflow compared with car park mode. The operating airflow is determined by the design occupancy and the resistance of the protected air path, rather than simply applying the same speed percentage to every shelter.
An integral electric heater maintains the supply air temperature. In car park mode, the system therefore provides ventilation; in shelter mode, it also maintains the temperature conditions specified in the design.
Controls are centralised on a single panel and linked to the building's fire safety systems. Automation does not, however, remove the need to close airtight shut-off dampers manually: this remains a separate staff action when preparing the space. The sequence of actions, start permissives and responses to alarm signals must be documented in the control sequence and operating instructions.
Engineering outcome
A unified ventilation architecture was developed for two uses of the space. In car park mode, air exchange is controlled according to CO readings; in shelter mode, the system switches to protected air paths with adjusted airflow and supply air treatment.
Manual operation of the airtight shut-off dampers is coordinated with automated fan and heating controls. This allows shared ventilation infrastructure to be used without conflating the functions of everyday and protective modes.
For more on the requirements of different structure types, see ‘Shelter ventilation’. For a similar site, SM Systems can develop a ventilation design, carry out building services installation and provide maintenance. To begin work, we need information on the space's intended use, occupancy, existing ventilation and available electrical supply.