Developing countries such as Pakistan are experiencing severe energy crises because of an increasing difference between energy demand and energy production. Since buildings are the major consumers of energy in Pakistan, immediate steps must be taken to reduce energy consumption by devising energy efficient measures for buildings. The majority of buildings in the country use distributed heating/cooling devices, i.e:, air conditioners (A/Cs), to regulate building temperature. These A/Cs consume most of the energy in the buildings and are often subjected to problems such as heating or cooling unoccupied spaces and overheating/overcooling some areas in the building. These problems arise mainly because of independent manual controls of A/Cs and can be addressed by installing centralized heating, ventilation, and air-conditioning (HVAC) systems. However, the cost of deploying an actual HVAC system in these buildings remains high. Costs are associated with disruption to occupants as well as the dollar cost of installing the HVAC infrastructure. Moreover, the cost associated with removal or replacement of the existing A/C units is also substantial. Therefore, in this work, we abstract the functionality of the HVAC system by using the existing distributed heating/cooling infrastructure in buildings to provide a low-cost centralized command and control mechanism namely Retrofitting low-cost Heating Ventilation and Air Conditioning (RetroHVAC) system. We designed an occupancy detection system and incorporated it into the RetroHVAC system to enhance its energy-saving potential. Additionally, a desktop application was designed to enable user interaction and policy enforcement. Experimental evaluation comparing different RetroHVAC control strategies based on time scheduling, temperature threshold monitoring, and real-time occupancy information show that the RetroHVAC system can substantially reduce the building energy consumption without compromising user comfort.