Book - School of Science and Technology

Continuous versus intermittent operation 61Saving in fuel compared withcontinuous heating (%)755025Hours of occupancy(7 days x 8 hours)Heavy structuresLight to mediumstructuresBuildings with nothermal capacity(very light structure)Buildings with infinitethermal capacity050 60 70 80 90 100 110Total hours of heating / weekFigure 2.15 Comparative fuel savings for different weekly heating periods, indicating the effect of thermalcapacityThe first building would cool quickly overnight and, without heating, the internaltemperature next morning would approximate to that outside. Furthermore, during aclear cloudless night, there might be a risk of condensation as the temperature of parts ofthe structure fell below the air temperature. Continuous heating would obviously beuneconomic in this case but, equally, a fully intermittent re gime would only be adequatein the depth of winter if a lengthy pre-occupancy boost period were provided. Theoptimum solution might be a compromise with reduced temperature night-time heating,and a minimal pre-occupancy boost.Conversely, the air temperature within the heavy building would fall only marginallyduring a winter night without a heat supply but there would be a considerable time delaythe following morning before the daytime temperature was fully recovered. Intermittentheating should, however, provide a reasonably satisfactory performance, particularly ifthere were an insulating lining added to the walls. Continuous heating, at a marginallylower but constant level over 24 hours, would also meet requirements. Economically,there might be little to choose between the alternative methods in this case.Between these two extremes lie all the buildings likely to be met in practice, be they oflight, medium or heavy construction. Unfortunately, the choice between continuous orintermittent operation does not depend wholly upon the thermal response rate of thebuilding and other factors must be taken into account. These include the proportion ofthe 24 hours (and perhaps the week also) during which the building is occupied, whichmay vary as a result of shift-work or flexible working hours in offices: further, the typeand density of occupancy and, of course, the characteristics of the heating system willhave an influence. Some of these effects have been the subject of a report by BSRIA** Billington, N. S., Colthorpe, K. J. and Shorter, D. N., Intermittent Heating HVRA (now BSRIA) Report26, 1964.