Industry experts agree – a proper oil furnace/boiler clean-out should take 1½ to 2 hours. Anything less will not be enough to keep the equipment running at peak efficiency, and will end up wasting heating fuel. Consequently, (although you might not even realize it) your fuel fills will be higher than they need to be. The best solution is to always insist on a proper, in-depth clean-out. See our Service Department section for details.