A circulating fluidized bed boiler including a furnace, a separator which is connected to the furnace to separate fluidized bed material from a flow leaving the furnace, as well as a return duct between the separator and the furnace to return the separated fluidized bed material into the furnace. The return duct includes a loop seal equipped with a heat exchanger and a supply of fluidizing medium and having an inlet and an outlet, which open into a chamber including the heat exchanger, i.e. a heat exchanger chamber, and are situated at different heights, the outlet being connected through a return conduit to the furnace. The flow cross-section area of the outlet is at an angle to the flow cross-section area of the inlet in such a way that the fluidized bed material is transferred in the heat exchanger chamber in lateral direction with respect to the inlet direction of its inlet flow. The heat exchanger chamber includes individually controllable fluidizing means at different locations in the direction of the horizontal dimension of the flow cross-section area of the outlet.