Based on the clean energy heating context in China, a novel thermo-activated PCM composite building envelope system was proposed to further reduce building energy consumption and improve indoor comfort by utilizing low-grade hot water produced by valley current. A comprehensive heat transfer model was built and validated by comparing with reference test results. Under the condition of heating season in cold area like Tianjin, the effects of pipe spacing, PCM thickness and orientation on thermal behavior and energy saving potential were numerically investigated. The results show that the thermal performance of the composite wall has been changed, and the primary energy as well as the operation cost, taking south wall for example, are reduced by 14.07% and 56.03%, respectively. The pipe spacing shows a more obvious influence than PCM thickness, thereby 100 mm for pipe spacing and 30 mm for PCM thickness seem to be the satisfactory values when taking the thermal resistance and heat charge/discharge into account simultaneously. Orientation has big influence on the performance and the improvement in north orientation is most obvious among the four orientations. The primary energy and operation cost saving ratio in north orientation show a reduction by 20.3 and 9.0 percent respectively compared with that in south. This study can provide reference for pipe-embedded PCM envelope design and its application to buildings of clean heating and low energy consumption.