the present invention relates to a steam generating apparatus comprising a boiler for receiving water to be heated , an opening in a wall of the boiler , the opening being closable by closing means , and a steam duct for conducting steam generated in the boiler to steam applying means , wherein a by - pass duct by - passing the steam duct is provided for discharging steam from the boiler to the steam applying means . the present invention further relates to a method of operating a steam generating apparatus .