Diurnal variations in outdoor air temperature, wind velocity and intensity of solar radiation cause a diurnal variation in the rate of heat flow into or out from a building. This variation is strongly influenced by the heat capacity of the structure and the ventilation rate. Thus it is difficult to estimate accurately the loads on heating and cooling systems or to predict the indoor climate of buildings not provided with such systems. This paper shows how the complex thermal system of building and outdoor environment can be represented by a thermal circuit. Analysis of such circuits by mathematical or analog methods provides a practical rational means of predicting the thermal performance of buildings.

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* Presented at the 131st National Meeting of the American Meteorological Society, Columbus, Ohio, September 8–10, 1954.