A gas furnace vents natural emissions that are caused by the heat-generating combustion process inside the furnace. These emissions are considered harmful, as they can be made up of toxic gases and potentially can cause a fire when in contact with combustible material.
As of 2015, there are three vent options for a gas furnace: natural, direct and sidewall power. A natural vent system is also called a Type B vent design and uses air inside a house or building to produce combustion. Because gases produced during this process are of a relatively high temperature, they can be vented through a vertical pipe system with relative ease and efficiency.