A method for heating a heat exchange medium in a fluidized bed boiler (100), the method comprising burning first fuel (165) in a first furnace (162) of the fluidized bed boiler (100) to produce first flue gas (163); recovering heat from the first flue gas (163) to a heat exchange medium using a first heat exchanger (310); conveying the heat exchange medium from the first heat exchanger (310) to a second heat exchanger (320), of which at least a part is arranged in contact with a fluidized bed of the fluidized bed boiler (100); burning second fuel (175) in a second furnace (172) of the fluidized bed boiler (100) to produce second flue gas (173); conveying the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recovering heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330). A fluidized bed boiler (100) for performing the method. A loopseal heat exchanger (400) that is, when installed in a loopseal of a circulating fluidized bed boiler, configured to burn second fuel (175) in a second furnace (172) of the loopseal heat exchanger (400) to produce second flue gas (173); convey the heat exchange medium from the second heat exchanger (320) to a third heat exchanger (330); and recover heat from the second flue gas (173) to the heat exchange medium using the third heat exchanger (330).