Plenum Space and Air Handling Space Differences

How to Distinguish Between Spaces

Decoding the confusion surrounding air handling and plenum spaces. Understanding air handling and plenum spaces is essential for designing safe building systems.

1What is the difference between plenum space and air handling space?
The difference between plenum space and air handling space lies in their function within a building's HVAC system. The NEC defines a plenum area as, “a compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.” They reference air handling space as “the space over a hung ceiling used for environmental air handling purposes,” and “areas beneath raised floors for information technology equipment,”. 
2What is a plenum space?
A plenum is a compartment or chamber that is part of the air distribution system, typically used for air return in HVAC systems. It is a designated space where air is actively moved for heating, ventilation, and air conditioning before being redistributed. Because air flows freely in these areas, any fire or smoke can spread rapidly, making fire-resistant materials essential.
3What is an air handling space?
An air handling space refers to areas that are used for environmental air circulation but are not specifically designed as plenums. For example, the space above a suspended ceiling that serves as an air return to an HVAC unit is considered an air handling space. Unlike plenums, these spaces are not enclosed chambers but still play a role in air movement.
4Why is there industry-wide confusion regarding the definition of air handling and plenum spaces?
The confusion often arises because people mistakenly refer to air handling spaces as plenums. It has become very common for the area above a drop ceiling to be mistakenly referred to as a plenum. 
5What are the potential fire safety risks in plenum spaces?
Plenum spaces pose a higher fire risk because they allow smoke and flames to travel quickly throughout a building. As a result, per the National Electrical Code, material installed in plenum spaces must be plenum-rated to meet strict fire safety standards which ensure low smoke emission, flame resistance and minimize toxic fumes. 
6What are the potential fire safety risks in air handling spaces?
Air handling spaces also require fire-resistant materials, but they are not as strictly regulated as plenums. However, the NFPA 90A standard still requires that materials used in these spaces meet low-smoke and heat-release criteria. Material installed in air handling spaces must comply with UL 2043, which tests for heat and smoke release to ensure safety.
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