Book - School of Science and Technology

Period of use 66720Temperature °C15105Septembe rMayAprilJanuary00 6 12 18 24British standard timeFigure 23.2 Diurnal variations in outside temperature for Manchester: September and May curvesdisplaced to suit time base.however, the greater will be the no-load losses or, where plant operation is to be intermittent,the greater the requirement for preheating.Hence, if energy supply is to be by oil or gas firing or by some form of direct electricalheating, on-peak, no night-time operation of the system will be necessary except perhapsin the most severe weather when heat might be needed to prevent condensation. The plantwill be required nevertheless to provide preheating prior to occupation. On the otherhand, with solid fuel firing by an automatic means, some level of intermittent operationwill usually be necessary in order to keep the fire alive overnight and a no-load loss willthus arise with little heat input to the building. This loss will not be absolute of course andsome minor reduction in preheat time may be possible simply because the water contentof the boiler will not be completely cold.Equivalent full load hoursThe period during which the heating system is in use, as distinct from the period ofoccupancy of the building, may be expressed in terms of equivalent hours of full loadoperation. A number of different methods leading to evaluation of the quantity by manualcalculation has been devised and an outline of one of these is presented in the GuideSection B18. The underlying theory takes account of variations in the level of adventitiousheat gains within a building, the period of occupation, the thermal capacity of thestructure and the ability of the heating system to respond to demand. Allowance is madealso for the fact that the degree-day data compiled by the Meteorological Office relate tothe maintenance of an internal temperature of 18.3 C, as has already been explained: forother internal temperatures, an adjustment is necessary.As far as the internal heat gains are concerned, it is suggested that buildings may beclassified as shown in Table 23.5 and that the thermal capacity of the structure be