The crucial point in inorganic particle formation from biomass combustion is the temperature-dependent release of inorganic compounds, especially potassium (K). Currently, common wood fuels comprise of a comparatively low amount of K, but the increased usage of wood energy requires new feedstocks in the future. Potentially new feedstocks, such as short rotation coppice (SRC), fuels from agriculture (e.g., straw), or wood from broad-leafed trees of low rotation, contain usually high ash contents and/or high K concentrations. Apparently, these feedstocks will cause increased inorganic particle emissions from biomass combustion processes. The principle of a decreased firebed temperature as a primary measure aiming at a retention of K in the ashes of the firebed is a common approach for particle emission reduction and was investigated in several previous studies. The present study describes the usage of an ash-rich fuel from SRC pellets made from willow in a residential pellet boiler modified with an unique prototype of direct water-based firebed cooling. This test setup enables the study of the isolated impact of decreased firebed temperatures and its influence on the combustion process and emissions as well. A statistically significant effect of the firebed cooling on temperatures below the burner plate as on gaseous HCl and SO2 was found. The high ash content of the used fuel limited the effectiveness of the applied direct firebed cooling in residential biomass combustion. The accumulation of a thick and thermal insulating ash layer above the burner plate decreased the heat transfer, limited the cooling efficiency, and revealed deviations from the expected particle formation process.