Book - School of Science and Technology

Continuous versus intermittent operation 63Some excess capacity in the heating system is desirable in order to reduce the length ofthe preheating period and, with the improved standards of insulation and lower rates ofair infiltration common in modern buildings, larger plant margins are required than wasthe case hitherto. During the preheating period:. The conduction loss is reduced by the use of curtains or shutters and closed windowsand doors minimise the loss due to air infiltration.. The effective output from the system is increased above the design capacity as a result of ahigher output from heat emitting equipment due to the lower temperature of the inside air.. The excess plant capacity allows operation at an elevated temperature.The combined effect of these items is illustrated in Figure 2.16, which represents a normalheating and cooling curve for a building heated intermittently. The effect of the changesnoted in the first two groups can be quite significant, as illustrated by an example related,once again, to the simple space shown in Figure 2.3.If shutters were used overnight at the windows and the air change reduced by half, thenthe heat loss might be reduced from 700 W/K to about 450 W/K. Taking an overnightinside temperature of, say, 10 C at the start of preheating, an average during that periodmight be 14 C; further, if the mean water temperature in the radiators were raised by aslittle as 5 C, then the dual effect would be to increase their output by about 20%. This is,of course, a considerable simplification of a complexproblem but it serves to show thatthe overall result might be that a preheat capacity of more than twice normal systemoutput could be available.Preheat and plant sizeIt will be obvious from what has been said that the thermal characteristics of bothbuilding and heating system, the time lag of the former and the response rate of the latter,have a profound effect upon the preheat time required to restore conditions of comfort ina space after a period of shut-down. The 1986 Guide Section A9 presented data whichallowed preheat time to be established relative to values of the other variables: a roundedup summary is presented in Table 2.17. The 1999 Guide Section A5 provides a more-indepthexploration of this topic.Table 2.17 Variations in preheat timesPlant size ratio aHeavyweight structureLightweight structurePreheat (h)Quickresponse systemSlowresponse systemQuickresponse systemSlowresponse system1 3.0 ± 2.1 ±2 2.5 ± 1.6 2.33 2.1 2.6 1.5 1.74 1.9 2.3 1.4 1.55 1.7 1.9 1.2 1.46 1.5 1.6 1.2 1.2Notea Ratio of maximum plant output to design load at 20 C inside/outside. (Maximum plant output includes inherent capacity ofsystem.)