Book - School of Science and Technology

Table 2.15 Approximate heat input per annum to a dwelling from incidental sourcesMiscellaneous allowances 59SourceHeat input(GJ/annum)Two adults, one child (body heat) 5.9Radiation from sun (15 m 2 glazing) 13.0Cookinggas 4.3electric 3.4Electrical appliances (lighting; TV; washing machine, etc.) 3.4±8.2Losses from water-heating pipes and appliances, etc. 10±30Internal heat gainsPassing reference has already been made to fortuitous heat gains and their potential tomake a contribution towards keeping a building warm in winter and thus to reduce thecapacity of any heating system which may be provided. Reports* by the BuildingResearch Establishment provide a variety of interesting facts regarding the probableannual total of such input, as summarised in Table 2.15. The total of these itemsapproximates to something over a third of the theoretical annual heat requirement of adwelling so occupied.In summer, of course, such gains are a penalty to be countered rather than a contributionto be welcomed and they will consequently be dealt with in more detail in thefollowing chapter. For application to a commercial building, the level of reliance whichmay be placed upon availability may be used to separate winter heat gains into categories:Reliable. Industrial or office machines operating permanently; electric lighting runningpermanently; occupied for 24 hours at a constant level.Relatively reliable. Industrial and office machines that will operate continuously duringworking hours; electric lighting related to occupancy; permanent staff working to anestablished attendance routine.Unreliable. Solar radiation and heat gains from industrial or office machines operatedintermittently; electric task lighting; random occupation.In the case of electric off-peak storage systems using room heaters, it has been suggestedthat full account should be taken of the 24 hour mean contribution from most ofthese sources on a `design day', as related to the thermal capacity of the structure, and thisproposition will be discussed further in Chapter 5. However, for conventional heatingsystems, total dependence upon other than the items listed in the first of these categorieswould be unwise. As to the second category, where a contribution may be absent overcold weekends and winter holidays, no allowance can be made if comfort temperature inthe space is to be recovered in a reasonable time period prior to occupation. This is not todeny that use may be made of all such heat sources, but rather to suggest that a welldesignedsystem will have sufficient capacity to cater for the full design load whileretaining the ability to respond to any fortuitous contribution when this may be available.*BRE Domestic Energy Model, Background, Philosophy and Description. BRE Digest 94: 1985.