A vertical burner (10) for a domestic heating apparatus (12) fed by granular biomass fuel such as pellets, olive pulp, maize and suchlike, comprises a support frame (13) inside which a combustion unit (11) is installed, having inside it a combustion chamber (25). The combustion unit (11) comprises an internal conveyor body (26) which delimits the combustion chamber (25) inside it and, outside it, a containing chamber (22) for the comburent air, wherein the internal conveyor body (26) is made of material with low heat conductivity and high resistance to temperature, able to maintain, in normal use, the internal temperature of the chamber (25) at a determinate combustion temperature. The internal conveyor body (26) is shaped so as to vertically define one or more parts (28, 30, 32) of the chamber (25), which are shaped with cross sections having predetermined and different geometry so as to determine an ascent of the combustion fumes upward at a determinate speed of outflow, so that they have adequate time to remain in the chamber (25) in order to allow a substantial complete combustion.