During the winter, oil-fired appliances are subject to long running cycles. These long run times produce oil soot that may adhere to the sides of the chimney and eventually fall to the base of a masonry chimney or directly into the top of the oil-fired appliance. This fallen soot build-up may restrict the flow of flue gases which consist mostly of nitrogen, carbon dioxide and water vapor.
The combustion process also produces carbon monoxide (a tasteless, odorless, poisonous gas). Carbon monoxide may spill back into the house instead of going up the chimney if appliances are not adequately vented or when the chimney base is not properly swept.
A common misconception is that your oil service technician takes care of the chimney. The reality is that the oil burner company may shovel out the base of a brick chimney and they may brush out the connector pipes, but likely will not clean the chimney system.
Additionally, some oil burner technicians may tell the homeowner that the system is okay without even inspecting the entire chimney. This includes going to the roof and inspecting the interior as well as the exterior masonry (if applicable), flashing, chimney cap, etc.