experience, we will be significantly over-sizing the plant based on a very small risk of occurrence. Designing for 38C would result in a cooling system nearly twice the capacity of one sized for the design temperature. This situation of over-design is often exacerbated by the value engineering that inevitably results from the selection of over-sized plant in the first place. During value engineering the third chiller that the engineer selected to provide better part-load efficiency will be eliminated in favour of two machines and a cost saving. This now means that each machine will effectively be sized to 100% of the normal design condition and therefore the operation will be at part-load for the vast majority of the time, with the other machine largely redundant. Although the CIBSE Guides now recommend a
more sophisticated analysis to determine suitable heating and cooling design conditions, it is still vital that we understand the significance of the statistical probabilities revealed by this first approximation. As climate change is beginning to impact on our weather it becomes increasingly difficult to resist adopting
Operational flexibility beyond a provision that can be reasonably expected leads to over-design
design temperatures closer to the extreme events. Even with these increasing extremes, the probability of such events is still small. CIBSE, the Met Office and a consortium of
universities are presently working on the UKCIP probabilistic future weather data to develop sample weather years that can be used in analysis software and to determine new design temperatures. However, until then, we can still determine useful design information from the historic weather records, provided they are up to the present date, but we should not allow our judgment to be driven by the occasional bad weather experience. The purpose of defining diversity factors and design
temperatures is to avoid expensive over-design of the systems while balancing this with an acceptable risk of exceedance. It is essential to apply these kinds of analyses at the briefing stage in order to discuss risk and cost with the client. This allows informed decisions about system design criteria, which will result in a more robust and cost effective design. This approach can also often avoid the round of value
engineering that occurs when the cost of over-designed systems exceeds the client’s expectations, as you can demonstrate that the design is the most cost effective for the actual anticipated need with an acceptable level of non-conformity. l © Doug King
Doug King is principal of King Shaw Associates and visiting professor of Building Physics at Bath University
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www.colourfan.co.uk www.cibsejournal.com February 2011 CIBSE Journal 53
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