A boiler especially a shell boiler having an auxiliary heat exchanger 42 which is an economiser or a superheater and which consists in a tube or tubes connected in series with the main boiler heat exchanger and arranged in a bed of fluidisable particulate material. The bed is fluidised by gaseous fluid from the combustion chamber of the boiler, typically using the boiler forced- draught fan or blower to provide the necessary pressure drop through the fluidised bed. The bed of particles is typically relatively shallow and fins are provided on the tube or tubes. The second pass of firetubes in the case of a shell boiler contains relatively few tubes of relatively large diameter.