Understanding Fire Rated Ducts

We all know the old saying: if it looks like a duck, walks like a duck and quacks like a duck, it must be a duck. While that may hold true most of time, it is not so clear when fire-rated duct products are involved. In today’s competitive marketplace, many products have similar and comparable counterparts. This is also the case with fire-rated ducts and fire-rated duct assemblies.

This article focuses on the different types of fire-rated ductwork and explains how to find the right fire-rated duct solution. Even among UL Certified fire-rated duct products, there are types that are suitable for different fire-rated remedies:

• Fire-resistive Ventilation Duct Assemblies, UL product category (HNLJ).
• Fire-protective Uninsulated Ventilation Duct Assemblies, UL product category (HNLN).

Both products and assemblies are evaluated to the test method ISO 6944, Fire Resistance Tests — Ventilation Ducts, which is similar to ASTM E2816, Standard Test Methods for Fire Resistive Metallic HVAC Duct Systems. This test method looks at the ability of duct assemblies to maintain stability, integrity and insulation performance. Per ISO 6944, the definitions for these properties are as follows:

• Stability – The time beyond the rating period at which the suspension or fixing device can no longer retain a duct in its intended position or when sections of the duct collapse.
• Integrity – The ability of a duct or ductwork to remain free of cracks, holes or openings outside the compartment in which a fire is present for a specified period beyond the rating period.
• Insulation – The ability of a duct or ductwork to maintain its integrity without developing temperatures on its external surface, outside the compartment in which the fire is present, that exceed 250°F (140°C) as an average value above ambient and/or 325°F (180°C) as a maximum value above ambient at any one point.

What is the difference between fire-resistive rated duct assemblies and fire-protective rated duct assemblies?

Fire-resistive rated Ventilation Duct Assemblies (HNLJ) are evaluated for the published fire rating period and meet the requirements for stability, integrity and insulation. Fire-protective rated Uninsulated Ventilation Duct Assemblies (HNLN) have been evaluated for the duration of the published fire rating period for stability and integrity, but in some cases, these ducts may not have satisfied the standard definition for insulation.

The following table is an example of the certification information available from UL Solutions for each assembly type.

The fire-protective rated Uninsulated Ventilation Duct Assemblies (HNLN) and corresponding certification program were created for ventilation duct assemblies evaluated to the requirements of ISO 6944 that may not achieve an insulation rating and are for use in applications where an insulation rating (thermal protection) is not required.

Note that there is a required minimum 18-inch clearance to combustibles when utilizing these fire protective solutions. This clearance qualifier exists because the duct assembly may be incapable of providing an insulation rating that prevents nearby combustible items from igniting either from a fire inside the duct (B Duct), or from a fire outside the duct (A Duct) passing heat to the other side of a fire separation.

It should be noted that the minimum clearance to combustibles requirement applies to all combustible materials, including those categorized as limited combustible materials or those suitable for use in plenum locations. It is also important to understand that, per the International Building Code, the minimum clearance requirement is not to be waived even in locations that do not require a T-Rating. UL Solutions’ view is that the 18-inch minimum clearance requirements must be interpreted as a declaration of the assemblies’ capabilities and limitations.

In summary, when it comes to these types of ventilation ducts, it is important to know what fire rating requirements exist for the intended building, and care should be taken to understand which UL Certified products and assemblies satisfy those requirements. Care should be taken to determine that a ventilation duct assembly is installed in accordance with the UL certification and manufacturer’s instructions, and that the supporting construction, e.g., walls or floors, is prepared to accommodate the duct assembly. While the name and appearance of the duct may be similar, the performance may have differences that could result in different levels of fire protection.