In occupied buildings, carbon dioxide, human odors and other contaminants such as volatile organic compounds (VOC) or odors and particles from machinery and the process function need to be continuously removed or unhealthy conditions will result.

Ventilation is the process of supplying “fresh” outside air to occupied buildings in the proper amount to offset the contaminants produced by people and equipment.

In many instances, local building codes, association guidelines, or government or company protocols stipulate the amount of ventilation required for buildings and work environments. Ventilation systems have been around for a long time.

In 1490, Leonardo da Vinci designed a water driven fan to ventilate a suite of rooms. In 1660, a gravity exhaust ventilating system was used in the British House of Parliament. Then, almost two hundred years later, in 1836, the supply air and exhaust air ventilation system in the British House of Parliament used fans driven by steam engines.

Today, ventilation guidelines are approximately 15 to 25 cfm (cubic feet per minute) of air volume per person of outside air (OA) for non-smoking areas, 50 cfm for smoking areas. Ventilation air may also be required as additional or “make-up” air (MUA) for kitchen exhausts, fume hood exhaust systems, and restroom and other exhaust systems.

Maintaining room or conditioned space pressurization (typically +0.03 to +0.05 inches of water gage) in commercial and institutional buildings is part of proper ventilation.


The figure above shows 20% of the total supply air is ventilation outside air (OA) and 80% is return air (RA). The outside air is brought (or forced) into the mixed air plenum by the action of the supply air fan. The outside air coming through the outside damper is mixed with the return air from the conditioned space.

The return air dampers control the amount of return air. If the room pressure is too high, the exhaust air (EA) dampers open to let some of the return air escape to the outside, which relieves some of the pressure in the conditioned space. Exhaust air dampers are also called relief air dampers.

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