The passenger cabin heating and cooling has a considerable impact on the fuel economy for buses, especially during the waiting period. This problem becomes more significant for the hybrid buses for which the impact of the auxiliary load on the fuel economy is almost twice that on the conventional buses. A second-law analysis conducted in this study indicates that a heat-driven AC system has higher energy utilization efficiency than the conventional AC system. On the basis of this analysis, a concept waste-heat-driven absorptive aqua-ammonia heat pump system is proposed and analyzed. Results of the analysis show that the heat-driven system can reduce the engine auxiliary load significantly because it eliminates the conventional AC compressor. In the AC mode, its energy utilization efficiency can be up to 50%. In the heating mode, the effective efficiency for heating can be up to 100%. In cases where the engine waste heat is insufficient or unavailable (e.g., when the engine is shut down), the required heat for operating the system can be provided by a directly-fired backup fuel burner. This heat-driven system can separate the passenger cabin heating and cooling from the engine propulsion system. This system is very appropriate for hybrid buses. Although the absorptive heat pump system proposed in this study targets on buses, it can also be applied to any type vehicles for which the system can be packaged.