When to Inspect and Clean
Your furnace should be the subject of a yearly inspection by a professional. However, do not mistake an inspection with maintenance. Many inspectors will check and even tune the active components of your home furnace but they will not clean the furnace's chimney. Chimney cleaning should occur before your annual inspection and after cold weather has passed. Spring is the ideal time for cleaning your furnace, since the heating period will have largely passed and the furnace will be as dirty as it is likely to be for a few months.
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Why Cleaning is Necessary
Oil furnaces are remarkably efficient devices, but still produce byproducts that require periodic cleaning. The most common byproducts in the burning of oil product are carbon and sulfur, both of which are prone to producing nasty build-up on the inner surface of your furnace's out-take, or chimney. These two byproducts combine to form chimney soot. The long periods of use that your chimney is likely to face in the winter can produce large amounts of soot build-up, occasionally heavy enough to fall of the inner walls of the chimney in clumps. These clumps can clog the regular flow of waste fumes from the chimney, potentially backing up nitrogen, carbon monoxide, and water vapor, which will then recirculate into your home.
While you could hire Dick Van Dyke, cleaning the chimney yourself is a relatively straightforward procedure. Purchase a set of chimney rods and brushes at your local hardware store. Shut down your furnace, then remove those components between you and the chimney. In most oil furnaces this will consist of removing a grate, the stack control and the damper immediately inside the chimney's base. Using a flexible rod, rub your chimney brush in and out of the chimney, using short strokes. Be sure to wear a dust mask and gloves. Use progressively stiffer brushes and repeat until no more soot falls into the chimney's base, then simply vacuum it up. Give any remaining dust in your furnace chimney a full hour to settle before reigniting your furnace.