Book - School of Science and Technology

56 The building in winterFrom earlier calculations (p. 40)(AU)/A ˆ 240/419 ˆ 0:57 W/m 2 Kand (p. 50)(0:33 NV)/A ˆ 348/419 ˆ 0:83 W/m 2 KBy reference to Table 2.13, and interpolating,F 1 ˆ 1:025 and F 2 ˆ 0:93thusQ u ˆ 1:025 240 22 ˆ 5412 WQ v ˆ 0:93 348 22 ˆ 7120 Wand henceQ t ˆ 5412 ‡ 7120 ˆ 12:5kWThe various internal temperatures may now be established:t ei ˆ (1:025 22)t ai ˆ (0:93 22)1 ˆ 21:6 C1 ˆ 19:5 CThe last value above has particular significance since, for the building used as anexample when heated by panel radiators, an inside air temperature of only 19.5 Cisnecessary to maintain a dry resultant temperature of 21 C. Had a warm air system beenplanned, reference to Table 2.13 shows that F 2 would be 1.1, with the result that an insideair temperature of (1:1 22) 1 ˆ 23:2 C would be required to maintain the same dryresultant temperature of 21 C.Miscellaneous allowancesThe methods outlined for the calculation of heat requirements, as set out in the routinesdescribed earlier in this chapter, do not take account of certain elusive factors which mayaffect the steady state load for a single space or for a whole building. These, in most part,cannot be evaluated other than empirically.Allowance for heightIt appears reasonable to make allowance for the height of a heated space, bearing in mindthat warm air rises towards the ceiling, creating a temperature gradient actually within thespace. Thus, when heat is provided to maintain a chosen temperature in the lower1 1 22 m of height, it follows that a higher temperature must exist near the ceiling orroof. In consequence, the conduction loss there will be greater, inevitably, through thesurfaces of ceiling, roof, upper parts of walls and windows, etc.