1 NATURAL VENTILATION OR MIXED MODE? AN INVESTIGATION USING SIMULATION Andrew Wright, EA Technology Ltd., Chester CH1 6ES, UK, Virginia Cooper, and Geoff Levermore, UMIST, Manchester, Dept. Building Engineering, M60 1QD, UK ABSTRACT This paper decribes a research project to compare, using simulation, the summertime comfort and energy use of naturally ventilated and mixed-mode (limited cooling) UK office buildings. A large number of simulations were run to investigate the effects of different design factors, on four facades. Various novel graphical methods, and regression equations, are used to present the results. It was found that without cooling, comfort could only be achieved by using several passive features to reduce internal temperatures. A wider range of mixed mode designs were shown to be low energy than natural ventilation designs, with better daylighting compensating for the cooling energy. INTRODUCTION This paper describes the results of investigations into the mixed mode operation of UK office buildings compared to natural ventilation only. It is based on work carried out by VA Cooper for her PhD thesis 1 . Until the 1960s few buildings in the UK were air conditioned. Then changes in architectural design lead to more use of deep plan, taller buildings, high levels of artificial lighting, and very high levels of glazing in lightweight structures. These factors, combined with increasing traffic noise in cities, often made it desirable to seal windows. With an increased level of solar gains, comfort could no longer be achieved with natural or mechanical ventilation. Air conditioning seemed entirely superior to natural or mechanical ventilation in a society which believed every problem could be solved with technology. The main argument against air conditioning was its cost. Randell and Mitchell 2 reflected the general view held in 1969: “No one would doubt the desirability of air conditioning, but what is generally questioned is whether the cost is justified. . . . Hardly anywhere in this country [the UK] is it likely that an entirely satisfactory environment will be achieved, in general, throughout the year, without air conditioning”. Yet even then there were dissenting voices with views remarkably similar to those of 30 years later. At the same conference Thornley 3 put forward the view that “. . . there is no need to hold the temperature constant in a building providing that the temperature lies within an acceptable range for comfort. The limits of ‘swing’ can be controlled by window size and shading devices, the mass of the structure, or by designing an air conditioning system of smaller capacity than would otherwise be the case if a close control of internal conditions was decided upon.” By the late 1980s, concern over the effects of CFCs and HCFCs on the ozone layer, a general view that Sick Building Syndrome was associated with air conditioning (though without much hard evidence), and the high energy costs of air conditioning compared to natural ventilation, were causing many to doubt the value of air conditioning in the UK climate. Atria and internal ‘streets’ suddenly became popular, allowing stack-driven natural ventilation in deep plan buildings, and environmentally-inclined designers returned to classical principles of exploiting natural ventilation and daylight. But despite this trend, air conditioning continued to grow strongly in the ‘mass market’ of new and refurbished offices, partly due to the spread of computing from a few mainframe terminals to a PC on almost every desk. It caused the strongest growth in energy use, and associated carbon dioxide emissions, of any building sector. The results of studies of real buildings were inconclusive on whether occupants preferred air conditioning or natural ventilation, showing that many other factors were involved. A big debate about whether natural ventilation was preferable to air conditioning began, with almost religious fervour on both sides. But then a ‘third way’ began to emerge, already hinted at by Thornley 3 . This was ‘mixed mode’ operation, where natural ventilation is used for most of the time without cooling, but some cooling is available to ‘peak-lop’ in hot weather so that the internal temperature is limited to, say, 25°C - higher than in a closely controlled air conditioning system, but sufficient to avoid the more severe discomfort of higher temperatures. Various definitions for ‘mixed mode’ have been proposed, but for the purposes of this work Cooper’s 1 will apply: It is a building in which occupants can open windows, and which is designed with effective passive strategies for limiting the effects of the external climate. The passively designed building is utilised to provide acceptable conditions for the majority of the year, and is supplemented by a