The effects of temperature and gas composition on the mechanical properties of sintered a-SiC heating elements were studied during the pre-melting stage of Si-rich ashes resulting from the co-combustion of biomass and coal in a high temperature, (1600oC), furnace. The use of elevated temperatures, approximately 1600oC, for the pre-melting of the ashes in combination with the composition of the tested samples often causes ruptures of the SiC heating elements resulting to reduced availability of the furnace as well as to high economic costs for the replacement of the destroyed elements. Depending on the temperature and the composition of the ash sample, failure of the SiC heating elements happens even after 12-15h of operation. The working temperature is shown to be the leading factor for this problematic behaviour, while the presence of carbon impurities in the ash samples in combination with alkali chlorides, carbonates and sulphates shown to significantly accelerate the process.