Oil furnaces burn waste or heating oil sprayed into the combustion chamber. When too much or too little fuel enters the chamber, a misfire occurs. Misfires cause sputtering or explosions in the burning process. While small and occasional misfiring may not seem like a problem, they indicate serious problems with the furnace.
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Heating oil often contains impurities that create soot or other residue that gathers in the combustion chamber. Dust also accumulates in the chamber. When this material builds up, it catches fire and causes the furnace to misfire, according to HeatingOil.com. Keeping the combustion chamber clean with periodic maintenance prevents this problem. Fires in the chamber can ignite buildup in the vent pipes if they grow large enough, causing a dangerous and damaging chimney fire.
Anything that interrupts the flow of heating oil to the nozzle that sprays it into the chamber causes a misfire so the furnace can't properly ignite. The pump that sends oil to the nozzle eventually wears out after years of use, says the Miura Company. It may give out intermittently, causing random misfires that are hard to pinpoint. Filters that keep debris in the oil from clogging the nozzle need replacement or cleaning at least once a year. The oil nozzle itself also needs to be routinely cleaned and replaced to prevent misfires.
When some of the many sensors or valves in your oil furnace break, fuel oil begins to leak into the combustion chamber. In the periods when there's no combustion in the chamber, this fuel builds up. An explosion known as a puff back occurs when it does ignite again as the extra fuel all burns up at once, according to Inspectapedia. Puff backs are serious misfires that send ash and smoke throughout the home. Once you experience one of these explosions, shut down the furnace until it can be repaired.
Oil furnaces use a variety of ignitors that produce a spark or a hot element to ignite the spray of fuel. These ignitors eventually need replacement. As an ignitor becomes worn or dirty from soot, it begins to malfunction randomly and cause intermittent misfiring. When your furnace randomly fails to start correctly, examine the ignitor for soot and check for cracks or signs of wear. Your furnace manufacturer can tell you how long the ignitor for your unit is expected to last and where to find replacements.