Abstract: A system and method for designing an HVAC system using a database to manage values of properties corresponding to design elements. Elements include substantially all physical components and connections available for creating an HVAC system design. A user interface represents design elements arbitrarily selectable by a user and connectable to one another in a schematic to establish the HVAC system design. The system may provide, automatically, default values corresponding to the properties corresponding to the design elements. A user may select arbitrarily, from the design elements, an arbitrary number of selected design elements to be interconnected in the HVAC system design. A user may place and interconnect each arbitrary design element, while the system calculates, automatically, values of properties characterizing the arbitrary design elements; validating correctness of the interconnections and properties, calculating performance parameters, and providing drawings.