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’s not quite 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 duct and fire rated duct assemblies.

This article focuses on providing detail and guidance to differentiate the types of fire rated duct, and explains how to find the appropriate fire rated duct solution. Even among the spectrum of UL certified fire rated duct products, there are varieties that are suitable for various fire rated remedies. In this article, we will focus exclusively on the two UL categories of fire rated ventilation duct products: , Fire Resistive Rated Duct Assemblies (HNLJ), and Fire Protective Rated Duct Assemblies (HNLN) which are uninsulated ventilation ducts assemblies.

Let’s examine the similarities and differences in these products, and the potential level of fire protection they afford. While 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, fire resistive rated duct assemblies and fire protective rated duct assemblies are not identical. This test method evaluates ability of duct assemblies to maintain stability, integrity and insulation performance. Per ISO 6944, the definitions for these properties are as follows:


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.


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 of time beyond the rating period.


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

Fire resistive rated ventilation duct assemblies (HNLJ) are evaluated for the duration of the published fire rating period, and meet the requirements for all stability, integrity and insulation. Fire protective rated ventilation duct assemblies (HNLN) Ventilation Duct Assemblies) 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. This information is shown in each certification for the respective products and assemblies. Below is an example of the certification information available from UL for each assembly type:

HNLJ, Ventilation Duct Assemblies
Fire-resistance rating* – Duct A 2 Hour
Fire-resistance rating* – Duct B 1 Hour
Ratings applicable for ventilation ducts installed with or without branches.
* Fire-resistance-rated duct assemblies have equal Integrity, Insulation and Stability values.
* Fire-resistance-rated duct assemblies are permitted for use in environments where the clearance to combustible materials and combustible assemblies is zero.


HNLN, Uninsulated Ventilation Duct Assemblies:
Duct A Duct B
Integrity and Stability Rating 2 Hour 2 Hour
Insulation Rating 0 Hour 0 Hour
Ratings applicable for ventilation ducts installed with or without branches.
Uninsulated ventilation duct assemblies are intended for use in environments where the clearance to combustible materials and combustible assemblies is at least 18 in.
The fire protective rated duct ventilation 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.It is important to 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 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 combustibles 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’s view is that the 18” minimum clearance requirement 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 very similar, the performance may have differences that could result in different levels of fire protection.

Content reprinted from UL’s Fire and Security Authority© newsletter, Issue 1, 2017 and may not reflect changes that have occurred since its original publication