Biomass heating system for the combustion of fuel in the form of pellets and/or chips, possessing: a boiler with a combustion unit , a heat exchanger with a plurality of boiler tubes , wherein said combustion unit possesses the following: a combustion chamber with a revolving grate , with a primary combustion zone and with a secondary combustion zone ; wherein the primary combustion zone is encompassed laterally by a plurality of combustion chamber bricks and from below by the revolving grate ; wherein the fuel on the revolving grate is combusted with primary air that flows through the revolving grate ; wherein a plurality of secondary air nozzles is provided in the combustion chamber bricks ; wherein the primary combustion zone and the secondary combustion zone are separated at the level of the secondary air nozzles ; wherein the secondary combustion zone of the combustion chamber is fluidically connected to an inlet of the heat exchanger characterised in that the secondary air nozzles are arranged in such a way that in the secondary combustion zone of the combustion chamber eddy currents of a flue gas-air mixture of secondary air and flue gas that results from the combustion of the fuel develop about a vertical central axis (A2), wherein the eddy currents lead to an improvement in the mixing of the flue gas/air mixture. a boiler with a combustion unit , a heat exchanger with a plurality of boiler tubes , wherein said combustion unit possesses the following: a combustion chamber with a revolving grate , with a primary combustion zone and with a secondary combustion zone ; wherein the primary combustion zone is encompassed laterally by a plurality of combustion chamber bricks and from below by the revolving grate ; wherein the fuel on the revolving grate is combusted with primary air that flows through the revolving grate ; wherein a plurality of secondary air nozzles is provided in the combustion chamber bricks ; wherein the primary combustion zone and the secondary combustion zone are separated at the level of the secondary air nozzles ; wherein the secondary combustion zone of the combustion chamber is fluidically connected to an inlet of the heat exchanger a combustion chamber with a revolving grate , with a primary combustion zone and with a secondary combustion zone ; wherein the primary combustion zone is encompassed laterally by a plurality of combustion chamber bricks and from below by the revolving grate ; wherein the fuel on the revolving grate is combusted with primary air that flows through the revolving grate ; wherein a plurality of secondary air nozzles is provided in the combustion chamber bricks ; wherein the primary combustion zone and the secondary combustion zone are separated at the level of the secondary air nozzles ; wherein the secondary combustion zone of the combustion chamber is fluidically connected to an inlet of the heat exchanger characterised in that the secondary air nozzles are arranged in such a way that in the secondary combustion zone of the combustion chamber eddy currents of a flue gas-air mixture of secondary air and flue gas that results from the combustion of the fuel develop about a vertical central axis (A2), wherein the eddy currents lead to an improvement in the mixing of the flue gas/air mixture. <EOS>