AM13: Mixed mode ventilation
About this book
CIBSE AM13, "Mixed Mode Ventilation," is an Application Manual published by the Chartered Institution of Building Services Engineers that provides guidance on the design and operation of buildings that combine both natural and mechanical ventilation systems. Mixed mode buildings represent a strategically important middle ground between fully naturally ventilated buildings and fully mechanically conditioned buildings, offering the energy efficiency advantages of natural ventilation for the majority of occupied hours while providing mechanical backup when natural ventilation is insufficient. The manual defines three principal mixed mode strategies. "Concurrent" systems operate both natural and mechanical ventilation simultaneously, with each contributing to the total ventilation requirement and the balance between them controlled by sensors and building management systems. "Changeover" systems alternate between natural and mechanical ventilation depending on weather conditions, occupancy, or air quality, typically with natural ventilation providing the primary strategy and mechanical systems only engaging when conditions require. "Contingency" systems are primarily naturally ventilated buildings that have mechanical backup systems installed as insurance against extreme weather conditions or operational failures, which may rarely or never be used in normal operation.
The manual covers the design process for each strategy, with particular attention to the integration of controls and building management systems. The challenge of coordinating natural and mechanical ventilation in real time — ensuring that opening windows do not counteract mechanical ventilation or introduce noise — requires sophisticated control logic, which AM13 addresses in detail. It provides guidance on the specification of window actuators, CO2 and temperature sensors, weather stations, and building automation system programming.
A significant portion of the manual addresses the thermal modeling and simulation approaches needed to predict mixed mode performance. Conventional building energy simulation tools handle mixed mode systems imperfectly, requiring specific modeling approaches that AM13 describes in methodological detail. The post-occupancy evaluation section helps practitioners assess whether installed systems are achieving their design intent.
Mixed mode design has become increasingly important as architects attempt to maintain the environmental quality benefits of natural ventilation in urban buildings where facade noise, security, or air quality concerns would otherwise preclude operable windows. Sources: CIBSE (cibse.org); CIBSE AM13 official publication; CIBSE Journal technical articles.