Worldwide, currently biomass ranks fourth as a primary energy source, providing approximately 14% of the world’s energy needs following coal, oil and natural gas. Over the past decade, domestic biomass heating has received more governmental and public supports than ever before in many developed countries, such as the United Kingdom. Although biomass combustion releases some combustion pollutants, biomass is renewable and produces little net CO2 emissions to the atmosphere. Due to the low sulphur and low nitrogen contents of many biomass materials, substituting biomass for fossil fuels, particularly coal, can reduce SOX and NOX emissions. The present study aims to investigate flue gas emissions, particularly carbon monoxide and nitrogen oxides, of a domestic biomass boiler under various operating conditions. The biomass boiler used in the present study satisfies the current EU regulation (EN 303-05) on the emissions of domestic biomass boilers. The emissions of the boiler have been measured not only under normal combustion conditions, but also under ‘idle’ conditions. The results are analysed and presented.