A load profile shows the variation of space, zone, floor, or building load in a certain time period, such as a 24-hours day-and-night cycle. Peak load is the maximum cooling load in a load profile. Block load is the sum of the zone loads and floor loads at a specific time. Using a rigorous approach to find the space cooling load requires numerous computer calculations. The rigorous approach is impractical and is suitable for research work only. The finite difference method is simple and clear in concept as well as more direct in computation than the transfer function method. Refer to Wang's Handbook of Air Conditioning and Refrigeration for details. Infiltration is the uncontrolled inward flow of unconditioned outdoor air through cracks and openings on the building envelope because of the pressure difference across the envelope. The pressure difference is probably caused by wind pressure, stack effect due to outdoor-indoor temperature difference and the operation of an air system.