Flue Gas Condensers (FGC) are used to increase the thermal output of biomass boilers. This reduces the emissions per unit of produced energy but furthermore, fine dust particles will be collected in the condenser. These condensers therefore have a double effect on the specific particulate matter emissions. In addition to the mechanisms that cause particle capture such as thermophoresis and diffusiophoresis, other agglomeration or condensation growth mechanisms also influence the size of the emitted particles. Due to the combination of these mechanisms, the capture efficiency depends on particle size. A size range presenting a lower capture efficiency, called a penetration window, is generally observed. Measurements on a 5 MWth boiler showed that this penetration window is in size range 0.07–0.49 μm where the capture efficiency is reduced by around 20%. Measurements on an 18 kWth boiler showed that the penetration window is in the size range 0.04–0.49 μm and the capture efficiency can even be negative in this window. For the medium-scale boiler, the condenser reduces the overall particulate emissions by 64% in number and 62% in mass per m3, and 70% in number and 69% in mass per MJ. For the small-scale boiler, the condenser reduces the overall particulate emissions by 4% in number and 50% in mass per m3, and 14% in number and 55% in mass per MJ.