This work deals with the transient behaviour of a circulating fluidised bed (CFB) boiler fired with biofuels. An effort is made to describe the transient processes by using concepts from the theory of automatic control. In this way the time constants of the furnace processes are formulated. The computed parameters from an analysis of non-stationary heat balances are compared with experimental data from the 12 MW CFB boiler at Chalmers University of Technology (CTH). It is shown that the transient processes can be well described.