Among the factors that affect the energy efficiency of the air source heat pump system, water volume influences efficiency by determining the unit's ONOFF time under partial load, especially in the low thermal inertia heating systems in the applications of northern China. The water volume in relation to cycling losses is extremely important. In order to strip out the multiple influence factors and clarify the impact of water volume on system efficiency through the actual heating system, an analytical model, presenting of the effect of system water volume on energy efficiency is established firstly. It is identified total water volume, including water volume in the buffer tank, terminals and pipelines by means of the proposed analytical model and the actual monitoring data of 6 fixed speed air source heat pump systems. Considering buffer tank heat loss and partial load loss caused by overmatching household air source heat pump systems, and combining with the efficiency and economy optimization, a recommended allocation index of total water volume 30~40 L/kW under nominal heating capacity for rural houses in northern China is put forward.