This paper presents results from an experimental study performed on a 13 kW(th) commercial domestic boiler using pellets as fuel. Four different types of pellets were used and, for each one, the boiler was tested as a function of its capacity and the fan regulation affecting excess air. Measurements were performed for boiler heat load, pellets consumption rate, flue-gas temperature and composition. Mass balances allowed the calculation of the flue-gas flow rate and associated heat losses. Losses from incomplete combustion have also been quantified. Under boiler steady-state conditions the flue-gas O-2 concentration changes with boiler load and ventilation due to the regulation scheme of the boiler. Flue-gas CO shows a minimum for values of O-2 in the flue-gases of about 13%. NOx emissions are independent of excess air for low values of nitrogen in the fuel whereas, for larger values, NOx emissions increase with the O-2 present in the combustion products. The fractional conversion of the pellets nitrogen into NOx is in line with literature data. The boiler start-up was characterised by the temperature evolution inside and above the bed showing the propagation of combustion in the bed during about 10 min. During boiler start-up, a maximum in CO emissions was observed which is associated with the maximum combustion intensity, as typified by the flue-gas O-2 concentration and temperature, regardless the pellets type. (C) 2004 Elsevier Ltd. All rights reserved.