For most of the EU, final energy is consumed in buildings as low-temperature heat for space heating and hot water. This is mostly produced by burning fuels that are linked to adequate environmental load environment, regardless of whether it is a fossil or bio-fuel. It is therefore high efficiency fuel which is a natural requirement in designing, implementation and operation of heat sources for the supply of buildings. For this reason, before commonly used conventional mono-production of heat a more efficient technology should be preferred, combining heat and power. This technology is, however, much more difficult as far as investments are concerned. Accordingly, its share in total production of heat needs to be set so that the operation of the heat source was also economy effective. This paper presents a simple method to solve this problem.