Building ventilation is the process of removing stale air from inside of a structure and replacing it with fresh air from outdoors. This process is part of the heating, ventilation and air conditioning (HVAC) system of a building, which is typically designed and installed by a mechanical or HVAC contractor. Building ventilation is measured in terms of air changes per hour (ACH), which is the percentage of air volume that is removed and replaced from the building each hour. This can range from .5 ACH in a tightly sealed building to 1.5 ACH in a loosely-constructed home.
Proper ventilation is critical to maintaining a healthy indoor air quality within a building. As humans perform basic metabolic processes, they release carbon dioxide into the air. Household items like paint, furniture and flooring often contain chemicals that can off-gas into the air. Building ventilation helps to remove chemicals and carbon dioxide and replace them with fresh air. Ventilation systems are also used to control humidity levels and reduce odors within a building.
Natural ventilation systems do not require fans, HVAC equipment, or ductwork. Instead they rely on pressure differences between exterior and interior areas to help move air. Pressure changes are constantly occurring in a building, and may be caused by wind, temperature changes, or human activities. A natural building ventilation system uses operable windows, louvers and grilles to help air enter or exit a building. While natural systems are highly effective at producing air changes, they are not generally successful at controlling humidity levels, especially in warmer regions.
Mechanical ventilation systems use fans to draw fresh air inside, and rely on HVAC ducts to distribute the air throughout the building. The fresh air is known as “supply air,” and passes through the building's air handling units as it enters the home. As supply air is blown into a room, existing “return air” is forced into exhaust grilles, where it travels back through the duct network and is exhausted to the outside.
The type of ventilation system used in a building can have a significant impact on the energy efficiency of the structure. A natural building ventilation system does not consume energy to operate fans or air handling units, but does allow heated or cooled air to escape through open windows and air vents. Mechanical ventilation systems require energy to operate fans and equipment, but do not allow as much conditioned air to escape through vents or windows. The energy efficiency of each type of ventilation system should be carefully weighed against the ventilation needs of a building before a system is selected.