A heat recovery steam generator includes an exhaust-gas channel with an economizer heating surface and evaporator heating surface. The heating surfaces are connected to one another such that, on a feed water side, the economizer heating surface is upstream of the evaporator heating surface. A water/steam separator is arranged on the feed water side downstream of the evaporator heating surface. An excess pipe length system is outside the exhaust-gas channel and is designed in such a way that, after a complete filling of the economizer heating surface, feed water, in a riser of the excess pipe length system, reaches an overflow and thus passes into the evaporator heating surface via a down pipe. A vent line branches off the overflow of the excess pipe length system. A first filling line and a first valve are arranged between an economizer filling outlet and the evaporator outlet of the evaporator heating surface.

The once-through boiler includes a water supply and at least an economizer, an evaporator superheater. No valves are provided between the economizer, the evaporator and the superheater. The high-pressure turbine includes a control valve. The method for low load operation of a power plant with a once-through boiler and a high pressure turbine includes providing a parameter indicative of the stable operation of the once-through boiler in once-through operation, and on the basis of this parameter adjusting the control valve in order to regulate the pressure within the economizer and evaporator and/or adjusting the temperature of the water supplied to the economizer.