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Recently domestic wood pellet boilers are installed in rural and forestry houses. The fuel price per
lower heating value of wood pellet is about 20 % lower than that of heating oil on July 2010. In
spite of lower price of wood pellet, a few user of wood pellet boiler complain expensive fuel cost.
One of this reason is inaccurate or improper air-fuel ratio setting of wood pellet boiler. O2
concentration of flue gas of domestic wood pellet boiler is about 9.7 % and there are few domestic
wood pellet boiler which can control air-fuel ratio automatically.
We tested a domestic wood pellet boiler in changing boiler thermal output. The nominal boiler
thermal output is 25 kW (21 500 kcal/h). We measured thermal efficiency at 100 % boiler thrmal
load and in the same air supply condition of 100 % boiler load we measured thermal efficiency at 40,
60, 80 % boiler thermal load. The results show that at boiler load 60 % the thermal efficiency is
about 4 % point lower than that of boiler load 100 %.