A new bio fuel based boiler concept for small district heating systems has been developed. The boiler enables heat load variations from 100% down to 10% of nominal thermal output 500 kW fulfilling the most rigorous environmental restrictions. To obtain as low fuel costs as possible, an unrefined fuel in the form of wood-chips with moisture content in the range of 35-58% is used. The results are very satisfactory concerning both performance and emissions. Typical emissions of CO and NOx during the experiments are in the range 10 to 50 mg Nm-3 (5 to 25 mg MJ-1) and 130 to 175 mg Nm-3 (60 to 90 mg MJ-1), respectively.Godkänd; 2002; 20070224 (ysko