Book - School of Science and Technology

58 The building in winterTable 2.14 Height factorsPercentage addition for heightof heated spaceType of system Type and distribution of heaters 5 m 5±10 m over 10 mMainly radiant Warm floor nil nil nilWarm ceiling nil 0±5 ± aMedium and high temperaturecross radiation from intermediate level nil 0±5 5±10Ditto, downward radiation from high level nil nil 0±5Mainly convective Natural warm air convection nil 0±5 ± aForced warm air cross-flow from low level 0±5 5±15 15±30Ditto, downward from high level 0±5 5±10 10±20Notea Not applicable to this application.This situation is illustrated in Figures 2.13 and 2.14, the latter showing how a warm airsystem having a discharge at low level produces a temperature gradient which is much lessthan one discharging at high level due to better mixing with room air. Where radiatorsand natural convectors are to be used, much can be achieved by correct disposition ofthese items below large cooling surfaces such as windows. It might, in fact, be said that asystem of this type which produces too large a temperature gradient has been badlydesigned. To make allowance for the effect of room height, the Guide Section A5 proposesthe use of multiplying height factors, as listed in Table 2.14.Whole building air infiltrationThe air-change and infiltration data provided in Tables 2.9 and 2.11 relate to allowancesappropriate to individual spaces. Where such spaces exist in a building more than oneroom `thick', it follows that outside air entering rooms on the windward side is warmedthere before passing through rooms on the leeward side and leaving the building. It maybe argued, therefore, that although the listed figures should be applied to all individualrooms, the wind direction being unknown, a deduction could be made, in determining thetotal infiltration for the whole building, for this use of `second-hand' air changes. TheGuide Section A4 includes the concept of a multi-zone model but acknowledges that suchmodels are very complexwhen balancing the inlet and outlet flows in all of the zones.In a different category, however, is the parallel proposal that, when a building isunoccupied, the allowance for air infiltration might be reduced. In circumstances wherean office building will be occupied for working hours only during a five day week, plus anallowance for cleaning, etc., and furthermore where security is such that it may beassumed that both outer and inner doors will be closed out of those hours, then areduction of air infiltration rates, by some proportion such as half, seems reasonable.Whole building air-tightness can be measured using a fan system temporarily connectedto a opening in the building facade. Guide Section A4 recommends a pressure of 50 Pabeing low enough not to cause damage and high enough to overcome the influence ofmoderate wind speeds. Where problems exist the air-tightness test may be accompaniedwith smoke tests or thermography analysis as a means to identify air leakages routes.Further detailed methodology can be found in CIBSE TM23.