Furnace Combustion Air Requirements


It is important for a furnace to have an adequate supply of air for combustion purposes. Homes should be properly ventilated to provide that air. Older homes were not designed with ventilation requirements in mind since builders of the time assumed there would be adequate ventilation provided by their cracks and crevices but modern ventilation considerations are a part of the home design process.

Need for Air

  • Furnaces tend to use up a lot of air in the combustion process and should have enough air supply for safety purposes. Furnaces produce hot gases and these rise up the chimney naturally. This outflow of gases creates a vacuum inside the house and air from outside is drawn in. Problems arise when furnaces use up more air than comes inside the house. This could lead to a production of carbon monoxide and other dangerous gases.


  • The dangerous gases are produced as a result of a process called backdrafting. This happens when a fire is burning and a house’s windows and doors are shut. When the furnace comes on, this interferes with the tendency of the combustion gases to rise out through the chimney. The suction that the fireplace draft creates tends to suck the gases into the furnace flue. This causes combustion gases to carryover from the flue and get into the house. This is the backdrafting process.

Air Requirements

  • There are some factors to take into account in determining the amount of combustion air a home requires. One is the presence of fuel-burning equipment in the home, including furnaces and other appliances such as water heaters. Another factor is the process these equipments adopt to exhaust their gases. Appliances that rely on natural ventilation for exhaust have a greater need for air. The airflow of the exhaust equipment and the tightness of the home are other factors.

Check Combustion Needs

  • A good time to check the need for additional combustion and ventilation is when homeowners make changes that might impact the ventilation capabilities of their home, for example when they are installing insulation or new windows. In a house built after 1999 the installation or replacement of an exhaust system also presents an opportunity to check combustion needs. When a homeowner is engaged in installing or adding a new furnace is also a good time to look into this.

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