Energy conservation in heating systems has great influence on overall sustainable development strategy. Heating technology, especially combined heat and power(CHP, cogeneration), has been attracting increased interest and rapidly developing in recent years. However, the theoretical potential of heating from the perspective of thermodynamics has not been clearly illustrated. This paper presents a theoretical analysis of heating systems based on the 1st and 2nd laws of thermodynamics using the specific fuel consumption(SFC) analysis method, clarifies the theoretical potential limitation of heating systems and illustrates the process of reversible heating on the basis of the total energy system. A novel concept(green heating, GH) and a new indicator(green-heating index, GH Index) are scientifically defined to characterise different options for heating and to quantitatively evaluate their fuel consumption levels. Thereafter, the fuel consumptions of four typical space heating modes are compared and discussed to present the application of GH theory and validate the power of the proposed concept and index. Furthermore, two different CHP technologies are analyzed with an instantiation case, and a novel CHP system is recommended, which may scientifically support the development of heating technology and further improve the processes of energy-saving and CO2 reduction. Energy conservation in heating systems has great influence on overall sustainable development strategy. Heating technology, especially combined heat and power (CHP, cogeneration), has been attracting increased interest and rapidly developing in recent years. However, the theoretical potential of heating from the perspective of thermodynamics has not been clearly illustrated. This paper presents a theoretical analysis of heating systems based on the 1st and 2nd laws of thermodynamics using the specific fuel consumption (SFC) analysis method, clarifies the theoretical potential limitation of heating systems and illustrates the process of reversible heating on the basis of the total energy system. A novel concept (green heating, GH) and a new indicator (green-heating index, GH Index) are scientifically defined to characterise different options for heating and to quantitatively evaluate their fuel consumption levels . The fuel consumptions of four typical space heating modes are com pared and discussed to present the application of GH theory and validate the power of the proposed concept and index. technology and further improve the processes of energy-saving and CO2 reduction.