The Rationale for Mechanical Ventilation

Abstract

Accessible online at: www.karger.com/journals/ibe Ventilation of buildings has been a major design issue of recent years. The decision whether to ventilate naturally or mechanically, and the implications for comfort, health, productivity and energy use have been much debated. This has influenced the design of buildings, dictating form and aesthetic, with some notable examples of innovative natural ventilation, such as the Contact Theatre in Manchester and the energy-efficient mechanical ventilation in the Elizabeth Fry Building at the University of East Anglia in Norwich. The traditional concepts that naturally ventilated buildings must be narrow plan and mechanically ventilated buildings are not energy-efficient have been countered by innovative design, both architectural and engineering. It is now generally accepted that there is not a preferred solution to ventilation design, whether mechanical or natural, and that appropriate ventilation design is needed, with the emphasis on design! We need ventilation to provide fresh air for breathing, to dilute and exhaust pollutants and odours and often to exhaust heat gains. The appropriate ventilation solution should be energy-efficient and effective in relation to health and comfort. Too much ventilation incurs an energy penalty and too little results in poor internal air quality. A balance is needed, which may vary over time with occupancy, thermal conditions and pollution levels. Ventilation design will affect the form and space planning of a building. We therefore need to take an early design decision about the choice of the ventilation system, and it should not happen by default. Failures are common and this can impact on the lifetime performance of the building. Typically people spend around 90% of their time in buildings – poor ventilation can impact on their comfort, health, well-being and productivity. Traditionally, ventilation was natural, with openings located to facilitate the supply and extract under the natural forces of temperature and wind. Even so, many buildings relied on uncontrolled air leakage through cracks and gaps in the external envelope to ventilate, only resorting to opening windows during incidents of peak heat gains or pollution. As buildings became more complex in their planning, and higher and spatially deeper, mechanical ventilation offered the only means of ventilation. This often became combined with heating and cooling, humidity control and filtration, providing air-conditioned buildings with little environmental contact with the outside. However, significant amounts of fan power were needed to move the air around the building, and air-conditioned buildings became associated with the profligate use of energy. This was coupled with the growing concern over poor indoor air quality, often associated with ineffective air distribution, re-circulation of exhaust air and problems of maintaining and operating such complex systems within resources allocated to building management. Mechanical ventilation developed a bad reputation compared to natural ventilation, and was frequently blamed for sick building syndrome and other health and comfort complaints. The response was, wherever possible, to turn back to natural ventilation, with the system often strongly repre-