OVERHEATING CIBSE GUIDANCE


is the running mean of the outdoor temperature and Tmax


the formula Tmax = 0.33*Trm Trm


of the indoor operative temperature as a function of Trm


. The likelihood of


discomfort is related to ∆T, the difference between Tmax


and, which is the actual


operative temperature in the space, Top


C. TM52 defi nes discomfort


in NV buildings using the upper temperature limits introduced by EN 15251. The use of the day-to-day changes in the running mean outdoor temperature enables the dynamic relationship between the indoor environment and the


experience of the building occupants to be described. An increased risk of overheating is then related to the number of summer hours in which the Tmax


is exceeded,


according to three overheating criteria. These – taken together – provide a


robust, yet balanced, assessment of the risk of overheating of buildings in the UK and Europe. A room or building that fails any two of the three is classed as overheating: Criterion one sets a limit of 3%


on the number of occupied hours that the operative temperature can exceed the threshold comfort


37 35 33 31 29 27 25 23 21


8 10 12 14 16 18 20 22 24 26 28 30 Running mean outdoor temperature ˚C


Figure 1. Limiting values (Tmax ) of the operative temperature to avoid overheating in buildings operating in the ‘free


running’ mode – that is, without heating or cooling – as a function of the running mean of the external temperature. The yellow line is Tupp


, the absolute upper limit of the temperature as defi ned in criterion three of TM5. August 2013 CIBSE Journal 23


. The value of ∆T is rounded to the nearest 100


+ 21.8, where is the upper limit


Probably the best way to fi nd out whether a building overheats is to ask the people who occupy it


Tupp Cat III


Cat II Cat I


Comfort temperature Tcomf


(˚C)


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