Book - School of Science and Technology

126 Electrical storage heatingAs will be appreciated, calculations based upon this relationship have to be solved by trialand error since neither the heated floor area nor the temperature of the floor surface areknown initially other than by good practice in the latter case.The environmental temperature must be calculated with some care, taking particularaccount of situations in multi-storey buildings where the space above may be providedwith floor warming also. In such circumstances, the ceiling surface over the floor areabeing considered will be above room temperature to an extent determined by the finishapplied to the floor of the space over. An approximation often adopted to cover thissituation is to make a notional adjustment to the unit emission by use of a factor D, thus:E 2 ˆ 1000(DZQ /A)whereD ˆ 0:92 for a bare floor overˆ 0:85 for carpet tiles overˆ 0:78 for underlay and carpet overOnce the upward emission has been determined, the loading of the heating element maybe established by adding an allowance for the edge loss around the perimeter and also fordownward loss to earth in the case of a solid ground floor or downward emission to thespace below in other circumstances. Each of these allowances may be calculated from firstprinciples but, again, they are commonly dealt with by means of approximations; that forthe edge loss being taken at 5 W/m run of perimeter and that for the downward loss oremission by use of a factor F selected from Table 5.5. Thus:R s ˆ 24(FE 2 A ‡ 5p)/1000 nwhereR s ˆ connected load (kW)p ˆ length of perimeter (m)The design process is completed by calculations, first to ensure that the temperature atthe plane of the heating element is not excessive bearing in mind the properties of thechosen element and second to determine the diurnal swing in room temperature in orderto establish that this does not exceed 3.5 K, as discussed previously.Table 5.5 Correction factors `F ' for floors having 50 mm insulation underCorrection factors `F'Position of floor Bare floor Carpet tiles Carpet and underlayGround floors, downward losssmall area 1.19 1.25 1.30large area 1.16 1.20 1.25Intermediate floors, downward lossto heated room below 1.09 1.17 1.27to unheated void below 1.25 1.35 1.40