Emissions and efficiency of three different wood pellet boiler technologies in real life conditions were compared at two different operational loads. The test consortium comprised of one 15, 20 and 32 kW boilers equipped with bottom feed burner, one 30 kW boiler equipped with top feed burner and one 35 kW boiler equipped with horizontal feed burner. The measurements comprised of carbon monoxide (CO), nitrogen oxide (NO), dust and combustion efficiency. All boilers were fuelled with DINplus certified wood pellets. Emissions and efficiency of each boiler technology varied as a function of operational loads. Magnitude of variations in the emissions and efficiency between nominal load and reduced load was narrow with bottom feed, wider with horizontal feed and was the widest with top feed boiler. At reduced load, top feed boiler had very high CO and dust emissions (5196.0 and 406.4 mg Nm(-3), respectively) which were 3.3 and 17.6 times higher, respectively, than at nominal load. Horizontal feed boiler emitted highest NO at reduced load (448.5 mg Nm(-3)), which was 1.7 times higher than at nominal load. At reduced load, combustion efficiency of all bottom and horizontal feed boilers were +/- 2% of that at nominal load; however, top feed boiler was 17% less efficient. Keeping in mind minor variations in fuel quality, different burner configurations clearly lead to important differences in emissions and efficiencies at different operational loads. In order to minimize pollutants emission and to achieve high efficiency, reduced load operations of pellet boilers should be avoided, especially in case of top feed boilers considered in the present study. (C) 2012 Elsevier Ltd. All rights reserved.