Book - School of Science and Technology

fact it would not be an exaggeration to say that it was a death knell for this and similarapplications.It is common experience that the successful application of floor warming depends to alarge extent upon the swing in space temperature during the discharge period or, perhapsmore significantly, during the period when the heated space is occupied. This swing,which is a function of the thermal properties of the building and in particular those of theheated floor, should not exceed about 3.5 K. This means that if it is desired to maintain anaverage of 19 C, then the temperature will be up to 20.75 C in the morning and down to17.25 C in the afternoon. It goes without saying that while the former might be justacceptable, it is unlikely that the latter would be tolerated. In an office building, heat gainsfrom lighting, occupancy and other sources might be enough to `top-up' the afternoontemperature to a barely satisfactory minimum but no, or at best limited, margin wouldexist for any sub-normal condition. In domestic premises, some form of direct heatingwould certainly be required for evening use.Similarly, if occupants of the heated space are not to experience discomfort as a resultof hot and ± consequently ± tired feet, the maximum surface temperature of the floor mustbe limited to 28 C. It is good practice to take a mean value of about 25 C for designpurposes, recognising that while the surface temperature may reach the maximum tolerablelevel first thing in the morning at the end of the charging period, this is likely to bebefore occupancy begins.Finally, it is necessary to consider the construction of the floor which is to be warmed,in whole or in part, and the floor finish which is to be applied. The thickness and positionof any insulation are both critical to performance and affect directly the maximumupward emission which is tolerable. Table 5.4 lists these maximum values.The unit emission from the heated floor surface is determined from the coefficient ofupward heat transfer, a practical value for which is 10 W/m 2 K, and the differencebetween the mean surface temperature of the floor and the environmental temperatureof the space to be heated:E 1 ˆ 1000(ZQ/A) 10(t s t ei )whereE ˆ unit emission (W/m 2 )A ˆ heated floor area (m 2 )t s ˆ temperature of floor surface ( C)t ei ˆ environmental temperature of space ( C)Capacity of room stores 125Table 5.4 Maximum upward heat emission from floors charged over 7 hoursMaximum upward emission (W/m 2 )Bare floorWith carpetPosition of floor(see Figure 5.1) 50 mm screed 100 mm screed 50 mm screed 100 mm screedGround floor A 17 28 20 31Ground floor B 31 55 42 68Intermediate floor 23 39 30 48