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DESIGN MASTERCLASS 7 OPERATIVE TEMPERATURE


temperature, significantly lower than the air temperature alone. If the building’s cooling system were


operated under air temperature control, then the refrigeration plant would be started early in the day. However, recognising the contribution of radiant temperature in thermal comfort would delay the point at which refrigeration was necessary for several hours, or even allow passive cooling to meet the demand entirely. Similarly, using large surface areas


of floor or ceiling for radiant heating and cooling can influence the operative temperature in the room, even with air temperatures that are outside the traditionally accepted range. Since large


If we introduce thermal mass or any form of surface heating or cooling system, it becomes essential that we consider the operative temperature


surface area systems can be effective at much lower temperature differentials, these systems can be used to great effect with low temperature heating and high temperature cooling sources, such as heat pumps or even when using ambient sources, such as groundwater. Humans sense operative temperature,


The importance of mean radiant temperature is well illustrated by the case of the Royal Albert Hall. This thermograph shows that, due to the unusual arrangement of the space, the internal surfaces are almost entirely lined with audience, with the same radiant temperature as each other. Thus, despite the hall having a chilled air supply, a feeling of thermal discomfort can arise due to the high radiant temperatures. The hall differs significantly from a conventional proscenium theatre, where the audience sits facing the cooler walls of the stage house and can therefore lose heat by radiant exchange


such as underfloor heating, it becomes essential that we consider the operative temperature. A large part of the heating or cooling output of surface systems, whether active or merely passive using thermal mass, is radiant, and therefore we cannot make any judgements about the comfort of the space by considering the air temperature alone. In thermally massive buildings with


passive night cooling it is not unusual to find the surface temperatures at the start of the day reduced to just above the diurnal average, around 20oC in summer. If sufficient fresh air is introduced at the outdoor condition during the day, the internal air temperature rise due to the sensible heat gains could be limited to a few degrees above ambient. However, the operative temperature under such a scenario would be the average of the air temperature and mean radiant


54 CIBSE Journal June 2011


and most thermal modelling software outputs operative temperature results, but we don’t yet control buildings on operative temperature. This is partly due to the difficulty in measuring mean radiant temperature, which varies with position in a room, but plenty of research work has been done on instruments that can measure operative temperature. It is about time that our controls industry started producing combined air and mean radiant temperature sensors. Even an approximation of operative temperature at a single point would be a substantial improvement over convective air temperature sensors, when it comes to controlling buildings with mixed heating and cooling sources. This small step would unlock a giant leap forward in the promised energy efficiency of surface heating and cooling systems and of passive thermal mass. © Doug King


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