This paper analyzed the energy efficiency of small-sized coal-fired Combined Heat and Power(CHP) at part heat-load, and presents the “low heat-load Triangle” based on heat-load duration diagram. The energy efficiency of coal-fired CHP in the “low heat-load Triangle” is much lower than separate generation because the heat-load is so little. Therefore, it is an effective way to improve the energy efficiency of CHP plant by minimizing the “low heat-load Triangle”. Separate generation instead of coal-fired cogeneration can be a better scenario in this special area. The study of the CHP district heating in 9 cities of different latitudes in China has shown that distributed gas boilers can be used in the “Triangle Areas”, which can lead to a visible energy saving. But the energy saving rate has a remarkable difference among cities of different latitudes. So the cogeneration and separate generation sources should be reasonably matched for utilization so that the energy saving rate can be considerably enhanced.