A device for varying the height of a bed in a fluidized combustion chamber inside a pressurized housing. Bed material is supplied to the chamber from hoppers or returned back to the hoppers from the chamber through a supply line. The supply line connects the chamber and the hoppers. The hoppers have means of increasing or decreasing the pressure. To improve the means of both increasing and decreasing pressure in the known device and of cleaning the gas that enters into contact with the material in the hopper, each hopper contains a hot-gas filter that gas can flow through in two directions and each hopper communicates above the filter with a gas connection that can be maintained at either a higher or a lower pressure than the fluidized combustion chamber.