Fire Response of HVAC Systems in Multistorey Buildings: An Examination of the NZBC Acceptable Solutions

Abstract

It is recognised that smoke is the major killer in most fires. In buildings with mechanical heating, ventilation or air conditioning (HVAC) systems the traditional reaction to a fire was to shut the HVAC system down, although in recent years some buildings have included a smoke management mode as part of their HVAC system, and/or have dedicated smoke management equipment (eg stair pressurisation). The current Building Code Approved Documents give little guidance on the appropriate actions for HVAC systems to take in the event of a fire, and some requirements of the Acceptable Solutions are unclear. The objective stated in the Approved Documents is to avoid allowing smoke to spread to other firecells via the air conditioning system. HVAC systems can be utilised to actively manage smoke movement and can achieve this in a variety of ways. This report attempts to provide some improvements to the Approved Documents and to give general guidelines to assist non-mechanical fire engineers and non-fire mechanical engineers in designing or specifying appropriate responses to a fire in a typical multistorey building. The report does not examine smoke control in atria or other large spaces. The various generic classes of ventilation or air conditioning systems are described and the appropriate behaviour of each under fire conditions is discussed. Results of some computer modelling of air (and cold smoke) flows around typical buildings are presented. The modelling indicates that the current levels at which active smoke control is invoked in the Acceptable Solutions are appropriate. It also suggests that the frequent practice of shutting off the ventilation system on a fire alarm may not be the best solution to managing smoke flows within the building. Particular sections of the Acceptable Solutions relating to mechanical ventilation which are unclear or confusing are also discussed with suggested amendments proposed.