There are several crucial steps to follow when designing an HVAC system for a commercial building. First, do a complete survey of the structure, paying careful attention to the architectural and engineering plans, and the demands they place on the heating, cooling, and ventilating system. Then, use the data you have gathered to perform a load calculation. You might use some suitable software, or you might do it the old-fashioned way, with pencil and paper. Be sure to revise the duct and distribution system design until it meets all the necessary codes and standards, and figure in just the right amount of ventilation so that the system delivers quality air, without over- or under-pressurizing the building. Throughout the process, consider the "now and later" aspect. Choose efficient controls and make the system sufficiently flexible to integrate new tech that will surely appear in our future.