The paper presents selected results of simulations of a rapid increase in the supercritical power boiler thermal load. The simulations concerned the possibility of obtaining the required increase in the live steam mass flow rate at the boiler outlet in a set period. This condition must be met to achieve the required increase in the power unit load, of the order of a few dozen MW in a few dozen seconds. For this purpose, it is necessary to decrease the boiler pressure and increase the fired fuel mass flow rate at the same time. The pressure reduction is carried out by opening, at a constant rate, the control valves installed on the pipeline at the turbine inlet. Assuming that the rate is known, the unknown parameters are the values of the target pressure reduction and the time of the pressure decrease process duration. The values vary depending on the power unit initial load. After the pressure decrease process is stopped, the required increase in the power unit load (the increase in the live steam mass flow rate) should be maintained only by firing more fuel. The required values of the target reduction in pressure within a specific range of the power unit load can only be determined by developing a precise mathematical model of the entire boiler and performing simulation computations. The obtained results are of fundamental practical importance because they are used by the boiler manufacturer to generate the power unit modified sliding curves. The results presented in the paper relate to a supercritical power unit now being constructed in one of the Polish power plants. An in-house mathematical model was developed for the power unit boiler. The model comprises all heating surfaces of the boiler and enables simulation of the boiler operation in conditions of rapid changes in loads. (C) 2019 Elsevier Ltd. All rights reserved.