Book - School of Science and Technology

Heat losses 21Heat lossesThe conventional basis for design of any heating system is the estimation of heat loss and,for the purpose of calculation, it is assumed that a steady state exists between inside andoutside temperatures although, in fact, such a condition rarely obtains. In the past, airtemperature difference has been the sole criterion although the mean radiant temperaturewithin the enclosure, if considered, may well call for a higher or a lower air temperature forequal comfort. The method of calculation recommended in the current edition of the GuideSections A5 and A9 is in terms of dry resultant temperature within the space to be heatedand this, as was explained in the preceding chapter (p. 12), takes account of the meanradiant temperature. This refinement will be discussed subsequently but, in this preliminaryintroduction, the air temperature difference will be used wherever appropriate.Each room of a building is taken in turn and an estimate is made of the amount of heatnecessary to maintain a given steady temperature within the space, assuming a steadylower air temperature outside. The calculation falls into two parts: one relating toconduction through the various surrounding structural surfaces, walls, floor and ceiling;and the other to the heat necessary to warm to room temperature that outside air which,by accident or design, has infiltrated into the space.Adjacent rooms maintained at the same temperature will have no heat transfer throughthe partitions or other surfaces between them and these may thus be ignored. Furthermore,if certain surfaces, such as the ceiling or floor, are used for heat output, then thesewill also not be taken into account so far as heat loss from the room is concerned; theywill, however, have inherent losses upward or downward to unheated areas and those willhave to be allowed for separately.The conduction element is calculable from known properties of the building materials,but the infiltration element presents problems, in that what is called the air change rate or,alternatively in energy terms, the ventilation allowance, is not easy to assess other than byexperience. This air change rate is no more than a natural ventilation effect, arising from anumber of extraneous circumstances but without which a space would quickly cease to behabitable, and although this element must be dealt with empirically, the ground rules arereasonably well established.It might be thought that, with so many assumptions and `rule of thumb' estimates, heatloss calculations are very little better than guesswork. In practice, however, they have provedto be a reliable basis for an overall assessment, partly due to the fact that all areas in thebuilding are treated in a like manner and are thus consistent in response. In addition, thebuilding structure is itself a moderator, as a result of its thermal inertia, whichremainsasasignificant factor, even in a lightly constructed building which still has floor slabs, partitions,furniture, etc., to absorb and emit heat and thus smooth out any violent fluctuations.Conduction lossesThe conduction of heat through any material depends upon the conductivity of thematerial itself and upon the temperature difference between the two surfaces. Ignoringfor the moment any heat transfer by radiation within a space, it is the air-to-air transfer ofheat through building materials which is relevant. On either side of a slab of buildingmaterial, it may be supposed that there is a film or relatively dead layer of air whichretards the flow of heat. This is illustrated, in Figure 2.1, for a thin material such asa single sheet of glass. The air within the room, being relatively still, offers a higherresistance to heat flow than that outside where wind effects, etc., have to be considered.