The design load of an HVAC system is the amount of heating or cooling required to maintain a comfortable temperature in a given space. To calculate the design load, you need to consider the size of the space, the insulation of the building, the number of windows, the climate, and the number of occupants. You can use a variety of tools, such as a heat loss calculator or a Manual J load calculation, to determine the design load. These tools will help you determine the amount of heating or cooling required to maintain a comfortable temperature in the space. Additionally, you can use a Manual S calculation to determine the size of the HVAC system needed to meet the design load.