The main objective of the work was to develop a biomass fired furnace for small and medium scale heating systems, which has the ability to manage wet as well as unclassified biofuels with maintained low emissions of harmful substances. The system should also be able to cope with large and frequently occurring thermal output variations fulfilling rigorous environmental restrictions. The furnace consists of two combustion stages, a primary- and a secondary zone, and has a maximum thermal output of 500 kW. The studies have been focussed on the evaluation of the environmental performance of the new furnace. The main conclusion of the work is that the furnace fulfils all the pre-set requirements regarding operational- and environmental performance. The study has also shown that it is possible to use refuse in the form of horse manure mixed with wood-shavings and straw with satisfactorily environmental performance.Godkänd; 2004; 20061026 (haneit