Air quality issues related to the presence and emission levels of particulate matter have been recently addressed towards ultrafine and nanoparticles size fractions, with typical dimensions included in the range from 0,1 µm down to few nm. Particular attention has been paid to their emissions from combustion activities, although the large majority of investigations in this field are dedicated to traffic sources, with rather limited studies for stationary energy production systems. Following a general review of available data on ultrafine particle emissions from various stationary combustion systems and different fuels, present work describes a dedicated particulate matter stack sampling line which combines a dilution system and an electrical low pressure impactor for evaluating number concentrations and size distributions of the condensible fraction arising from dilution and cooling of flue gases with atmospheric air. Preliminary results obtained with the system for residential heating boilers fed with different fuels (wood pellets, fuel oil, natural gas) are reported and analyzed in terms of their dependence with fuel type, boiler operating regime and sampling conditions (dilution ratios, hot and cold sampling).