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Understanding Ventilation Terms

By HHI Staff

Words such as infiltration, air exchange, combustion air, and ventilation are often used incorrectly, leading to confusion. Thus, the following brief discussion is offered to give you an understanding of what various ventilation-related terms mean.


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For air to move from one place to another, there must be an air-pressure difference to push it or pull it. Air-pressure differences can be placed in three categories:

  1. Pressures resulting from natural phenomena such as the wind
  2. Pressures caused by mechanical devices such as clothes dryers that move air into or out of a house for a purpose other than improving air quality;
  3. And controlled pressures caused by ventilating fans.

When the air pressure indoors is greater than that outdoors, a house is said to be pressurized, or experiencing a positive pressure. When the air pressure indoors is less than the air pressure outdoors a house is depressurized, or experiencing a negative pressure. Sometimes, part of a house is pressurized and part of it depressurized. Pressurization and depressurization are neither good nor bad, but they can occasionally result in adverse effects. For example, radon can get sucked into a depressurized house.

Exhaust air is air that is leaving a house. It is often called stale air because it has been contaminated by people, activities, or materials inside the house. The outdoor air that enters a house is either called make-up air (because it makes up for what was exhausted), or intake air. It is also often called fresh air even though it may be contaminated with outdoor air pollutants.

Air that enters a house for the purpose of improving the air quality is called ventilation air or supply air. A general ventilation system is designed to improve the air in the whole house. A local ventilation system is designed to improve the air in one part of a house (e.g., a bathroom). There are three basic ventilation strategies:

  1. Exhaust ventilation blows stale air outdoors, causing a house to be depressurized.
  2. Supply ventilation blows fresh air into a house, causing the house to be pressurized.
  3. Balanced ventilation uses two fans to blow fresh air indoors and stale air outdoors simultaneously, so the house experiences a neutral pressure. A Heat Recovery Ventilator (HRV) provides a special form of balanced ventilation that is more energy-conserving. 
When air moves between the indoors and the outdoors (or between the outdoors and the indoors) an exchange of air results. The speed at which the exchange takes place is called the exchange rate. When air moves within a room, or from one room to another room, it is not being exchanged - it is being circulated. Forced-air furnaces and air conditioners are primarily designed to circulate (or recirculate) air in a house, as well as heat or cool it. (If a forced-air furnace or air conditioner has leaky ducts, an exchange of air might result, but this isn't usually a very desirable way to exchange air.) Ventilation systems are specifically designed to exchange the air in a house - as well as circulate it.


Exchanging the air in a house is important to dilute the concentration of pollutants found in the indoor air. If indoor pollutant concentrations are too high, they can negatively affect the health of occupants. Pollutant concentrations can also be reduced through source control, separation, and filtration. These strategies can be used effectively in addition to ventilation, but they are not a substitute for it because they cannot supply oxygen to persons in the living space.

When a fan causes air to move directly, it is called active air movement. When air moves indirectly because of a fan, or because of something else causing an air-pressure difference, it is called passive air movement. For example, when a window fan actively blows air outdoors through a window, typically an equal volume of air passively enters through another window.

For an air-pressure difference to cause an exchange of air in a house, the house must have 'holes' in it. The holes can be deliberate (cut through a wall on purpose), or random (miscellaneous holes in the structure that are often hidden or too small to be visible). Air moves through random holes passively when a house is either pressurized or depressurized. If a fan is connected to a deliberate hole, air will pass through that hole actively when the fan is operating. If the fan is not operating, but something else applies pressure to the house, air can move through a deliberate hole passively.

Infiltration is air that passively enters a house through the random gaps and holes in the building. Exfiltration is air that passively leaves a house through the random gaps and holes in the building. Infiltration and exfiltration can be caused by anything that results in an air-pressure difference between the indoors and the outdoors.

In a house with combustion appliances (e.g., gas or oil furnaces, water heaters, etc.), combustion air passes into the combustion chamber of the appliance where it mixes with the fuel and burns. Combustion air may come directly from the outdoors or it may come from the living space. If a combustion appliance is connected to a conventional chimney, the combustion by-products rise up through the chimney because warm air rises. When this happens, a negative pressure is created inside the chimney called a draft. Besides the combustion by-products, a certain amount of dilution air also leaves the house through a conventional chimney. Even though a fan isn't used, conventional chimneys are considered active exhausts. If a conventional chimney is not in use, it is inactive, and air can move through it passively in either direction. If a negative pressure in a house is stronger than the draft in an active chimney, the chimney may not function correctly. Some combustion appliances use a fan to expel combustion by-products outdoors, rather than relying on chimney draft.


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HHI is committed to accuracy of content and correcting information that is incomplete or inaccurate. With our broad scope of coverage of healthful indoor environments, and desire to rapidly publish info to benefit the community, mistakes are inevitable. HHI has established an error correction policy to welcome corrections or enhancements to our information. Please help us improve the quality of our content by contacting with corrections or suggestions for improvement. Each contact will receive a respectful reply.

The Healthy House Institute (HHI), a for-profit educational LLC, provides the information on as a free service to the public. The intent is to disseminate accurate, verified and science-based information on creating healthy home environments.


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Understanding Ventilation Terms:  Created on December 14th, 2007.  Last Modified on February 28th, 2011


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