Book - School of Science and Technology

64 The building in winterIt has been shown that there is little advantage to be gained by providing an oversizedconventional system having a capacity much greater than that necessary to meet thecalculated heat loss, with intent to reduce the preheat period. This is due to the fact that asystem, made up of boiler plant, pipework and heat emitters, also has a time lag whichincreases with size. Margins on conventional heat emitters should be confined therefore toa nominal 10% at most in order to make allowance for the seemingly inevitable poorstandard of building maintenance.As to the influence of system type upon energy consumption with intermittent operation,it has been shown* that systems having a small thermal capacity and a rapidresponse rate, such as those using warm air, are able to achieve greater economies thanthose subject to time lag of any significance. This is an obvious conclusion and points tothe generalisation that with buildings of light construction, greatest economy in energyconsumption is achieved with intermittent operation of such systems, but only when thecontrol arrangements provided are sufficiently responsive and are thus able to preventover-run. Buildings of more solid construction are able to achieve reasonable economieswith a steady supply of heat and a less sophisticated level of control. It follows, therefore,that while warm air, convector and radiator systems are well suited to intermittentoperation, embedded panels in ceilings and floors are not so suitable.Preheat for direct and storage heatingIn the case of direct heating appliances having a fixed rate of input, some additionalcapacity is needed to provide for a rapid preheat and this may, to some extent, beavailable inherently since mass-produced equipment of this type is manufactured inmultiples of 2 or 3 kW only, and selection on a generous basis should thus suffice. Heatoutput from all storage-type systems using room heaters is continuous, even though heatinput is intermittent, and thus facilities for controlled preheat in the present sense neednot be considered.Steady state and dynamic responseThis chapter would not be complete without a note to introduce the alternative and morethorough methods of establishing not only the room-by-room heat requirements of abuilding but also preheat times and other operational routines best suited to energyconservation.Steady state calculations for winter heating, while adequate to produce results satisfactoryfor all but the most unorthodoxforms of building construction, fail to provide dataenabling full advantage to be taken of all the sophisticated control systems now available.The essence of that failure lies with the fundamental assumption that heat exchange is afunction of values of temperatures inside and outside a building taken coincidentally intime. Except in the case of token weather barriers such as glass or thin sheeting, structuralelements have both mass and thermal resistance which together introduce delay ± the timelag which has been mentioned previously.Computer based thermal modelling techniques enable transient heat flow to beanalysed and the dynamic response of the building structure to be assessed, on an hourby-hourbasis, a task which would be infinitely tedious, if possible at all, using manual*Dick, J. B., `Experimental and field studies in school heating'. JIHVE, 1955, 23, 88.