The accurate monitoring of particulate emissions from medium-scale, decentralized biomass combustion units is a major challenge for the deployment of this technology in the frame of the current energy transition. More specifically, the experimental characterization of the size of the emitted particles, i.e. using impactors, is still subject to discussions about the impact of the methodology on the measurement results. To meet with these challenges, particulate emissions from a medium-scale biomass boiler (4.5 MWth) were measured with Electrical Low Pressure Impactors (ELPI+) using two different dilution systems to examine the effect of dilution and performance of the ELPI+. For both the two stage dilution systems and one stage dilution system, no statistically significant correlation was found between the dilution ratio (DR) and particle total number concentration Ntot or between DR and mass concentration mtot. However, with both dilution system, a significant positive correlation was observed between DR and particles with diameter Dp < 0.01 µm due to measurement artefacts. With the one stage dilution system dilution systems, condensation appears to be promoted with reduced DR. When the ELPI+ impactor is not overloaded, the number concentration of fractions N< 0.01 and N0.01-0.1 reduce over time due to prematurely measuring particles. When the impactor is overloaded, N< 0.01 is overestimated and N0.01-0.1 and N0.1-1 are underestimated due to a change in cut-off diameter.