Combustion air is the air required to provide oxygen for rapid oxidation, or combustion of an inflammable material. In the design of boilers and internal combustion engines, providing combustion air in the proper quantity and at the correct temperature is a major consideration.
The source of combustion air for fire places and wood heaters in homes is frequently overlooked, which leads to inefficient heating at best and possible asphyxiation due to carbon monoxide poisoning. All burning requires oxygen. In order to get oxygen, a wood fire uses oxygen from the air around it and heats it, causing more air to be exhausted up the flue or chimney. This creates a low pressure zone around the fire, pulling more air through the path of least resistance. Unless a special air inlet is provided, this air will be pulled in through the inevitable leaks around doors, windows and other openings in the house. This cold air will mix with the warm air in the house and cool it, while creating drafts. If a central heating system is being used at the same time the fire is burning, it is likely to actually run more often to compensate for the cold air flowing by the thermostat. Conversely, if the house is very tight and the damper is closed so as not to allow cumbustion gases to escape, inefficient combustion will cause the generation of carbon monoxide to build up in the house, which can potentially be fatal.
Control of combustion air is also the main method of temperature control in wood or charcoal fired cooking appliances. These are always used outdoors to allow dangerous gasses to dissipate to the atmosphere.