It is the objective of this thesis to take into account thermal balance of circulating fluidised bed combustors (CFBC), aiming at efficiency assessment, as regards standards, CSN 07 0302, and DIN 1942. It points to differences between calculations along these two different standards, concerning the solution of problems like different comparative temperatures, to which the calculation of physical losses is related to, varying calculation of mean thermal capacity, or problem of electrical motors input. This article is based on the project GA 617 50 11 solving - "Combine combustion of coal and biomass in fluid boilers"., Zdeněk Kadlec, Bohumir Čech and Jan Matoušek., and Obsahuje bibliografii