Annual Fuel Utilization Efficiency (AFUE)|An industry standard used to define furnace efficiency.
Atomize|Reduce to a fine spray
Cad cell|A light-sensitive device (photocell) in which the resistance reacts to changes in the amount of light. In an oil furnace, it acts as a flame detector.
Coping|The upper masonry course of a chimney designed to keep water out.
Downdraft|A situation in which outdoor air is forced down the chimney, preventing proper draft. Downdraft can be caused by not extending the top of the chimney above the roof line.
Flame-retention burner|A high-efficiency oil burner.
Joist|Framing member used to support floors and ceilings.
Lockout|A safety feature that prevents a furnace from automatically cycling if the flame fails.
Over-firing|A situation caused by an oversized nozzle in which excess oil is pumped into the combustion chamber.
Primary control|A combination switching and safety control that turns the oil burner on and off in response to signals from the thermostat and safety controls. 
Refractory|Materials such as ceramics designed to withstand extremely high temperatures.
Smoke spot test|A test to determine the level of smoke in the flue gases.
Soot|Fine carbon particles resulting from incomplete combustion. An accumulation of soot on heat exchangers will reduce combustion efficiency and result in increased carbon monoxide production.
Temperature differential|The difference in temperature between the flue gases and the gases above the combustion chamber.
Weatherized furnace|A furnace that has been well insulated and has weather-resistant case as specified by government standards.

Annual Fuel Utilization Efficiency
Atomize 
Cad cell
Coping
Downdraft 
Flame-retention burner
Joist
Lockout 
Over-firing 
Primary control
Refractory 
Smoke spot test
Temperature differential 
Weatherized furnace