There is provided a wood pellet burner assembly for a wood pellet boiler, not shown. There is a fixed and easily removable fuel apertured grate mounted above and spaced apart from a base wall forming part of a main support enclosure. A movable scraper subassembly carrying an apertured flame tube is provided which can be moved in and out of the main support enclosure to perform a scraping action on the fixed fuel apertured grate to deliver ash and clinker which have not fallen through the grate during combustion off the grate out of the main support enclosure. This cleaning operation can be carried out at suitable preset time intervals.