Expert systems are computer programs that seek to mimic human reasoning. Currently, expert systems are being used for the design of heating, ventilation, and air conditioning (HVAC) systems. The present work involves developing of several smaller expert systems known as knowledge bases, and integrating them in one simple package. The aim of the research is to develop a such computer code for HVAC system designers which will considerably reduce man-hours during the whole design process, improve the productivity, increase the design quality, and give the customers more options to choose the best and optimum design. This thesis describes the development of a computer code, which has the ability to give all the design requirements for HVAC systems. This work which can be considered as a step towards HVAC Expert Systems, which outlines step by step calculation procedure to determine essential elements of heating and cooling loads such as U-value, air infiltration, solar heat gain, heat storage, psychrometric charts and the sunlit area of the exterior surfaces. The code (HVACSYS) consists of a main menu program and several auxiliary programs for gathering data, completing calculations, and printing project reports. The developed code is also connected with the AutoCAD package to give the final design of the HVAC systems. In the AutoCAD package, a special menu for HVAC systems design has been added (HVACCAD). This menu is developed for customizing the AutoCAD package in order to make the code interactive. Finally, a case study has been considered in which solutions were obtained using an existing package and also the developed package. Comparison of the solutions illustrates the usefulness of the new package adequately.