AHU – Outside Air Requirements

common complaint regarding buildings is the lack of outside air. This problem is especially a concern in VAV systems where outside air quantities are established for peak loads and are then reduced in proportion to the air supplied during periods of reduced load. A simple control added to the outside air damper can eliminate this problem and keep the amount of outside air constant, regardless of the operation of the VAV system. However, the need to preheat the outside air must be considered if this control is added.

Another problem is that some codes require as little as 2.4 L/s per person [about 0.25 L/(s·m2)] of outside air. This amount is far too low for a building in which modern construction materials are used. Higher outside air quantities may be required to reduce odors, VOC’s, and potentially dangerous pollutants. ASHRAE Standard 62 provides information on ventilation for acceptable indoor air quality. Air quality (i.e., the control or reduction of contaminants such as volatile organic compounds, formaldehyde from furnishings, and dust) must be reviewed by the engineer.

Heat recovery devices are becoming more popular as the requirements for outside air increase. They are used extensively in research and development facilities and in hospitals and laboratories where HVAC systems supply 100% outside air. Many types are available, and the type of facility usually determines which is most suitable. Many countries with extreme climates provide heat exchangers on outside/relief air, even for private homes. This trend is now appearing in larger commercial buildings worldwide. Heat recovery devices such as air-to-air plate heat exchangers can save energy and reduce the required capacity of primary cooling and heating plants by 20% and more under certain circumstances.

The location of intake and exhaust louvers should be carefully considered; in some jurisdictions, location is governed by codes. Louvers must be separated enough to avoid short-circuiting of air. Furthermore, intake louvers should not be near a potential source of contaminated air such as a boiler stack or hood exhaust. Relief air should also not interfere with other systems. If heat recovery devices are used, intake and exhaust airstreams may need to run together, such as through air-to-air plate heat exchangers.

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