Book - School of Science and Technology

Temperature difference 53Table 2.12 Winter external design temperature (1986 CIBSE recommendations)Location External design temperature, t ai ( C)Low thermal inertiaHigh thermal inertiaWithoverload capacityWithoutoverload capacityWithoverload capacityWithoutoverload capacityBelfast 2:5 5:0 1:5 4:0Birmingham 5:0 7:5 3:0 5:5Cardiff 3:0 5:5 1:5 4:0Edinburgh 4:5 7:0 3:5 6:0Glasgow 4:0 6:5 2:0 4:5London 3:0 5:5 2:0 4:5Manchester 3:5 6:0 2:5 5:0Plymouth 1:0 3:5 0.0 2:5Arising from an analysis of background data to international norms, Billington*proposed that the British Isles be divided into three zones for the purpose of determiningstandards for structural insulation and for selection of external design temperatures. Inplace of the earlier single datum of 1 C, the alternatives shown in Figure 2.12 wereproposed. A later study y analysed data for eight selected locations in order to establish thenumber of occasions per annum when the mean temperature falls below given levels. Theresults of this study were used in the 1986 Guide Section A2 as a basis for recommendedwinter design conditions, and are included here in Table 2.12. The 1999 Guide Section A2introduces selection based on variable risk of this occurrence with the choice being agreedbetween designer and client. Table 2.12 assumes that for buildings with low thermalinertia, on average, only one day in each heating season had a lower mean temperatureand for buildings with a high thermal inertia one two day period in each heating seasonhad a lower mean temperature. In order to take account of the `heat island' conditions instreets and around buildings in towns, adoption of a design basis one or two degreeshigher than the levels listed in that table would seem to be reasonable for systems servingbuildings in city centres.Temperature ratiosThe relationship between the various temperatures noted under the two immediatelypreceding headings is a function of the thermal characteristics of the method of heatingadopted. The inside air and mean radiant temperatures depend, for a given environmentaltemperature:. for convective heating, upon the conduction loss alone. for radiant heating, upon the infiltration loss alone.As may be appreciated, there are few methods and even fewer types of heat emittingequipment which fall wholly in one or other of these categories. If, for example, we wereto take the same building as used in previous examples with a normal infiltration rate then*Billington, N. S., `Thermal insulation of buildings', JIHVE, 1974, 42, 63.yPetherbridge, P. and Oughton, D. R., `Weather and solar data', BSER&T, 1983, 4, 4.