The calculation of various driving forces affecting cooling and heating loads of a zone or building along with steady-state and transient heat flow models has been treated in previous chapters. The focus of this chapter is to cover methods that combine all these heat flows and interactions in order to determine design loads on the peak day(s) of the year; annual loads and energy consumption are addressed in the next chapter. The procedure begins with a detailed description of the design climatic conditions. The peak heating load is relatively simple to determine requiring only a few heat transfer phenomena and a steady-state approach. We discuss various issues pertinent to cooling load calculations that require transient heat calculations and involve many more interactive terms to be considered.

We present three different methods to determine design cooling load. The first is the transfer function method (TFM) (implemented in the well-known DOE-2 and eQuest simulation programs) that involves three distinct sequential phases: calculation of individual heat gains of three types (conduction, solar, and internal), determination of cooling loads that depend on room and other properties, and the heat extraction rate (applicable when the zone thermostat set point is not kept constant). With the advent of greater computing power, many of the simplifications of TFM can be avoided by going back to the basic heat transfer equations and solving them in a more accurate and less arbitrary manner. We describe the basic principles and assumptions of the heat balance method (HBM), and how it is implemented in the EnergyPlus simulation program is discussed. Finally, a third method, namely the radiant time series (RTS), is described and illustrated by a solved example. This method, a simplified implementation version of HBM, is well suited for spreadsheet calculations since it is a step-by-step procedure that avoids having to perform iterative calculations like HBM. The RTS calculation procedure is in some ways similar to TFM but is a noniterative modification of the latter and treats internal thermal mass effects differently. Peak or design load calculations have to be done carefully since they directly impact the proper selection and sizing of the heating and cooling equipment.