Abstract One multi-fuel domestic pellet boiler (40 kW) was tested under standard laboratory conditions while fired with eight different biomass pellets and two boilers (35 kW) were tested in real life conditions while fired with DIN plus certified wood pellets. The agro-pellets include apple pomace ( Malus domestica), reed canary grass ( Phalaris arundinacea), pectin waste from citrus shells ( Citrus reticulata), sunflower husk ( Helianthus annuus), peat, wood and two types of wheat straw pellets ( Triticum aestivum). The measurements comprised of carbon monoxide (CO), nitrogen oxides (NO x ), sulphur oxides (SO x ), particle mass concentration (by DIN plus sampling methods) and combustion efficiency. Under standard laboratory conditions, wood pellets yielded the highest combustion efficiency (92.4%) followed by apple pellets (91.3%). The CO and dust emissions were the highest with peat and sunflower husk pellet, respectively. Straw pellets emitted the highest NO x and SO x . For agro-pellets, statistical differences in ash contents were significant. High ash contents and low ash melting temperature made straw pellets less suitable for this type of boilers. Reed canary grass, citrus pectin and apple pellets have advantage over others with reasonable less ash contents and less emissions. In the real life conditions, the boilers had around 90% combustion efficiencies. Comparison with the standard laboratory performance showed that the NO x emission was higher in real life condition than standard laboratory conditions; however, lowed CO and particle emissions were reported in real life conditions. In the real life condition, dust emissions were 5.0 and 5.8 mg Nm −3 with RL-1 and RL-2 boilers, respectively. The FE–SEM examination of the particles revealed that most of the dust particle had a tendency to form small clusters of sub-micron size ranging between 300 ηm and 1.2 μm.