Combustion Air is the air that is required to provide complete combustion for a fuel source. Enough oxygen is required so that there is not an issue with carbon monoxide. This is an issue in houses and anywhere that has a furnace, water heater, boiler, etc. that isn't electric.
All of the codes and references (1, 2, 3) that I have seen use either the volume of the room that contains the equipment or an area of free-flowing vent from the outside in a ratio to the BTUs or Watts of the equipment.
Neither of these measures seem to be directly related to the chemical equation of combustion.
- Comparing the volume of the room doesn't seem to take into consideration a replenishment of the oxygen from anywhere.
- Specifying an area of vent seems to ignore the speed of air through the vent. It also doesn't seem to relate to the size of the chimney.
What is the logic behind either specifying a room volume or vent area for combustion air?