Remove one (or reduce one sufficiently) and the triangle of combustion can collapse.
In a common NG gas furnace the heat is the igniter, the fuel is Natural Gas and the oxygen is provided by combustion air.
Combustion air is literally just the air needed to provide a continuous supply of air for proper combustion (burning). In the case of burning fuels like natural gas our goal is to achieve complete combustion where the end products being vented are CO2 and H2O and this requires the right mix of air and fuel.
For perfect combustion you need about a 10:1 ratio of air to fuel with safe levels of extra air or “excess air” putting us more into the 13.5:1 to 15:1 range.
All gas fired appliances must have both a flue / chimney to exhaust the leftover products of combustion (outlet) as well as combustion air to provide the oxygen for burning (inlet).
In high efficiency furnaces the combustion air is generally piped in, directly from the outside straight into the combustion chamber. This creates a dedicated source of oxygen and also a cleaner install as no other provisions need to be make for combustion air.
In 80% furnaces the burners usually have “open” combustion and they rely on air being drawn into louvers on the furnace cabinet. In this design the space on which the furnace resides must have open communication to the outdoors or other “uncontained” space.
The International Fuel Gas Code requires the following combustion air openings for a room containing combustion appliances:
Vertical opening – One-inch free area for each 4,000 Btu/hr. input of gas burning appliances in the room.
Horizontal duct opening – One-inch free area for each 2,000 Btu/hr. input of gas burning appliances in the room.
Mechanical fan – One CFM of air for each 2,400 Btu/hr. input of gas burning appliances in the room.
Indoor air – 50 cu. ft. of area for each 1,000 Btu/hr. of the appliances.
Not to get into the specifics of code becasue there are lots of specifics that you need to pursue beyond a tip like this, but you must have a dedicated method to get significant air to the furnace to ensure safe and complete combustion.
If you do not, the real possibility exists that the furnace could begin burning improperly creating an unsafe condition for the occupants due to Carbon-monoxide.
Different parts of the country provide combustion air in different ways, but you MUST have some method of providing unlimited fresh air to a furnace or to the room in which the furnace is located. This means when a furnace is in a tight space, ensure you have some sort of significant combustion air.