Which of the following must be present for combustion to occur?

For combustion to occur, three essential elements need to be present: air, fuel, and ignition (heat). This combination is often referred to as the fire triangle.

Air, in this context, primarily provides the necessary oxygen to support the combustion reaction. Fuel serves as the energy source that gets burned, while ignition (heat) is what initiates the reaction, allowing the fuel to combine with oxygen and release energy in the form of heat and light.

In scenarios where any one of these elements is absent, combustion cannot take place. This fundamental principle applies not only to gas furnaces but also to all combustion processes, which is why understanding it is critical for safe and effective operation of heating systems.