Older furnaces may contain rubber parts that would be affected by biodiesel's solvent properties, but can otherwise burn biodiesel without any conversion required. Care must be taken, given that varnishes left behind by petrodiesel will be released and can clog pipes—fuel filtering and prompt filter replacement is required. Another approach is to start using biodiesel as a blend, and decreasing the petroleum proportion over time can allow the varnishes to come off more gradually and be less likely to clog. Due to biodiesel's strong solvent properties, the furnace is cleaned out and generally becomes more efficient.