A circulating fluidized bed boiler, comprising a furnace, a loopseal, and a loopseal heat exchanger arranged in the loopseal. The loopseal heat exchanger comprises at least an inlet chamber, a bypass chamber, and a first heat exchange chamber, heat exchanger pipes arranged in the first heat exchange chamber, and a primary particle outlet for letting out bed material from the first heat exchange chamber. The primary particle outlet has at least a first part and a second part separated from each other by a barrier element in such a way that the first part of the primary particle outlet has a first height and a first width, wherein a ratio of the first height to the first width is less than 0.5 or more than 2. Use of the circulating fluidized bed boiler such that fluidizing gas and bed material are let out from the first heat exchange chamber via the primary particle outlet.