A new early stage design tool aims to provide educational and practical benefits to professionals
data for any location in the world;
l Choose between normal or warm summer conditions;
l Calculate and display U-values and other thermal properties information; and
l Provide help and guidance within the tool.
The admittance method The admittance method was used in the PDA as it provides a good balance between speed and accuracy, appropriate to the sketch design stage when ideas are being explored. It was originally created in the 1960s by Danter and Loudon1,2
and,
although developed as a manual calculation procedure, it remains a useful and widely used technique that takes account of the dynamic effects of heat storage in the building fabric. A more detailed explanation of the admittance method can be found in CIBSE Guide A3
, but a brief overview is
offered here. The underlying principal of the
admittance method is that the behaviour of space can be characterised as being in a steady cyclic state, in which daily variations in heat gain and loss about the mean value are assumed to be equal across 24 hours. This allows the dynamic nature of the space to be calculated in response to a sine
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wave within a 24 hour period. It also enables the calculation to be split into a constant (mean) response and the variation from the mean at any point during the day (swing). The mean response of the building fabric
is expressed by the U-value, whilst its cyclic response is described by the decrement factor and admittance value (referred to as the Y-value). Finally, a solar gain factor deals with the absorption and release of solar gain from the fabric, along with the transmission and absorption properties of the glazing. Use of the admittance method in the PDA
follows five basic steps: l The mean heat gains/losses are calculated for the chosen design day;
l The mean operative temperature is then determined using the result from step 1 and mean weather data for the design day;
l The swing (mean-to-peak) heat gain/loss is calculated for each hour of the day;
l The swing in operative temperature is calculated for each hour of the day and added to the mean value to give the actual operative temperatures across the day; and
l The absolute heating/cooling load can now be determined for each hour of the day.
March 2012 CIBSE Journal 61
The admittance procedure is capable of reproducing the results of more complex calculation methods when the same assumptions are made
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