The present application relates to improvements to granular fuel burning boilers such as wood pellet boilers and burners therefor. In particular, there is disclosed a wood pellet boiler having a combustion chamber, wherein an access door for said combustion chamber comprises a wood pellet burner assembly mounted on the interior side thereof. In this way, the wood pellet burner assembly is movable from and into the combustion chamber of the wood pellet boiler when the access door is opened and closed, respectively.