Return air ducts, plenums and concealed spaces may be of combustible construction when serving a dwelling unit.

Return air ducts, plenums and concealed spaces may be of combustible construction when serving a dwelling unit.

• A. serving a dwelling unit
• B. approved by the architect
• C. in an office building
• D. treated with fire resistant material

Answer: (A)

The idea being tested is that in residential construction there is an exception that allows certain HVAC ductwork to be combustible. Specifically, return air ducts, plenums, and concealed spaces may be of combustible construction when they serve a dwelling unit. This means that for single homes or the individual dwelling units within a residential building, the code permits using materials that can burn for these components, rather than requiring noncombustible materials everywhere. The reason this is the correct understanding is that the fire safety rules for dwelling units are more permissive for these specific parts of the air distribution system, reflecting typical residential construction practices and cost considerations. However, if the ducts or plenums serve more than one dwelling unit or pass through shared or non-residential spaces, stricter noncombustible construction requirements generally apply. Why the other ideas don’t fit: code approval by an architect doesn’t override material requirements, and in office or commercial spaces the rule does not apply—the materials would normally need to be noncombustible. Simply treating combustible materials with fire-resistant coatings doesn’t change the fundamental combustibility of the material itself; the allowance is about the material class, not surface treatment.

The idea being tested is that in residential construction there is an exception that allows certain HVAC ductwork to be combustible. Specifically, return air ducts, plenums, and concealed spaces may be of combustible construction when they serve a dwelling unit. This means that for single homes or the individual dwelling units within a residential building, the code permits using materials that can burn for these components, rather than requiring noncombustible materials everywhere.

The reason this is the correct understanding is that the fire safety rules for dwelling units are more permissive for these specific parts of the air distribution system, reflecting typical residential construction practices and cost considerations. However, if the ducts or plenums serve more than one dwelling unit or pass through shared or non-residential spaces, stricter noncombustible construction requirements generally apply.

Why the other ideas don’t fit: code approval by an architect doesn’t override material requirements, and in office or commercial spaces the rule does not apply—the materials would normally need to be noncombustible. Simply treating combustible materials with fire-resistant coatings doesn’t change the fundamental combustibility of the material itself; the allowance is about the material class, not surface treatment.