Time constants for understanding building dynamics

Abstract

Building dynamics are key to summer overheating, plant sizing and ventilation strategies. Building designers need to consider the thermal mass of a building explicitly to understand fully a building’s response. Simulation programs readily show the dynamic responses but they are deduced from the solution of embedded intrinsic differential equations without simple understanding for initial design and appreciation. The admittance method gives such a simple understanding but the thermal mass effects are not obvious. Added to which the admittance is limited to a regular 24 h cycles of heat gains and losses. The Chartered Institution of Building Services Engineers (CIBSE) response factor is an approximation to the response of a building but it is limited to defining just slow or fast responses. Frequently, designers refer to heavyweight and lightweight buildings or even long and short time constants. The latter are simple and understandable. This paper explores the time constants and derives a simple multiple time constant equation to indicate thermal storage in a building. It is suggested that the simple time constant for building components is added to the data in the next edition of the CIBSE Guide A. This will aid designers in understanding a building’s winter heating and potential summer overheating and plant size implications.