Computer assisted design studies of the dynamic behaviour of new thermoelectric units are necessary in order to verify the impact of modifications to past designs. These design checks are done with the aim of verifying the effect of the modifications on process dynamics, as well as, on control system configurations and operating procedures. This paper describes the design check of a new poly-fuel 320 MWe cylindrical-bodied boiler for an ENEL (Italian Electricity Board) power plant located in Sardinia. This boiler design differs from past designs in that it adopts a system of start-up valves in order to make start-up operations after night-time shut-down more flexible and eliminate dynamic thermal stresses, associated with start-up, on the turbine rotors. With the use of a suitable mathematical model, simulations were done of boiler and control system behaviour during start-up. This paper describes the code`s implementation and the results obtained.