Variations in the characteristics of residences can lead to surprisingly complex load calculations. Time-varying heat flows combine to produce a time-varying load. The relative magnitude and pattern of the heat flows depends on the building characteristics and exposure, resulting in a building-specific load profile. In general, an hour-by hour analysis is required to determine that profile and find its peak.

In contrast, the cooling procedures in the tool are extensively revised, based on the results of ASHRAE research project RP-1199,. Although the complexity of residential cooling load calculations has been understood for decades, prior methods have used a Cooling Load Temperature Difference/ Cooling Load Factor (CLTD/CLF) form requiring only hand tractable arithmetic. Without such simplification, the procedures would not have been used—an approximate calculation being preferable to none at all. The simplified approaches were developed using detailed computer models and/or empirical data, but only the simplifications were published. Now that computing power is routinely available, it is appropriate to promulgate 24 h, equation-based procedures.

General Design Guidelines for Software tool

Designed for Typical Building Use. The tool has been designed to meet representative maximum-load conditions, not extreme conditions. Normal occupancy is assumed, not the maximum that might occur during an occasional social function. Intermittently operated ventilation fans are assumed to be off.

Uncertainty and Safety Allowances. Residential load calculations are inherently approximate. Many building characteristics are estimated during design and ultimately determined by construction quality and occupant behavior. These uncertainties apply to all calculation methods. Safety allowances have been applied to the final calculated loads In addition, temperature swing has been incorporated which provides a built-in safety factor for sensible cooling: a 20% capacity shortfall typically results in a temperature excursion of at most about one or two degrees.