A dual fuel boiler has a granular fuel burner and a fluid fuel burner. The fluid fuel burner is movable between a retracted stored position at a side of a combustion chamber-of the boiler and an extended operative position in which it extends out over a brazier of the granular fuel burner. Thus in the operative position substantially all of the flame and hot combustion gases generated by the fluid fuel burner in use are directed away from the brazier to prevent damage to the brazier.