The article discusses the possibility of accelerating the startup of a steam boiler with natural circulation. During startup and load changes in thick-walled pressure elements of boilers operating at high temperatures, high thermal stresses are caused. Critical elements that limit the rate of change of working fluid units include drums, fresh steam superheater outlet chamber, injection chamber of steam coolers, tees and valve casings, rotor and turbine casing. Due to the high pressure and large diameters, the walls of these elements are much thicker compared to other elements of the boiler operating at the same pressure. Permissible changes in temperature and pressure during startup can be determined by the European Standard EN 12952-3. The article shows how the starting curves of a steam block can be determined. First, the heating rate of the component at the start of startup is determined using the EN standard. Then the heating speed of the pressure element is interpolated by a straight line between the pressures at the beginning and end of heating. The temperature of steam is determined from the solution of the first order ordinary differential equation. It was assumed that the steam is saturated. Calculation results show that the starting time of the analyzed block can be significantly shortened compared to boiler startup time in power plants. The long startup time of the boiler in power plants results from the very low heating rates of the pressure element adopted by the boiler manufacturers. It should be added that the low permissible heating rates adopted by the boiler manufacturers are too conservative and are not confirmed by three-dimensional stress analyses.