My early mentor had a saying that resonated with me — “Avoid Wishful Thinking.” I’ve found that advice has served me well over the course of my 30-year (and still going) career. Living by that credo can take quite a bit of discipline and resilience. There will be pressure to do “good enough” when it’s reasonably questionable as to whether what is being presented as “good enough” really is minimally acceptable. In the business of life safety, the minimum matters: tread lower with great care and discretion. Unfortunately, when it comes to smoke control, I’ve come across many who still apparently believe in “close enough” with a healthy dose of “wishful thinking” on top.

Wishful thinking equals avoidable mistakes. If only it were that easy. The matter really comes down to whether you think these life safety systems are important or whether you’re happy enough to see a “close enough” semblance of the prescribed infrastructure on-site at the end of the day.

Advancing compliance for smoke control systems within the fire duct industry requires more than a casual understanding of how to apply fire-resistive testing concepts such as symmetry, continuity, and orientation to these products. Unfortunately, the fire-rated duct industry promulgates products that might just be the most misused products in fire protection today, bar none … and I’m not just talking about ducts in smoke control systems when I say that. But, we’ve got to start somewhere.

Fire-rated duct product selection without an understanding of these basic fire-resistance fundamentals is prone to leading to avoidable mistakes. Doing so without an understanding of the test specimen jargon from the relevant fire-resistance test standards is doomed from the outset.

Fundamentals of fire resistance

 

Fundamentals of fire resistance 2

 

I understand this message may cause disbelief because what I am about to spell out is contrary to the conventional wisdom many of you have been hearing for years. Here’s the unfortunate truth — there is no such thing as basic, 2-inch pressure class SMACNA duct that will adequately address the “probable temperatures and pressures” of the design fire as set forth in Section 909 of the IBC.  Moreover, there is nothing you can just throw over or add to a basic 2-inch pressure class duct that will magically make it fire rated. Get over it. It doesn’t exist. The idea is attractive. In the real world, you need to know:

  1. There is a drastic difference in the performance capabilities of Type 1 Commercial Kitchen Exhaust Duct aka grease duct and fire-rated ventilation ducts, like those used in smoke control systems;

  2. There is a significant difference between Duct A and Duct B test specimens;

  3. How to validate which one you are getting from any given manufacturer (make no mistake, you will find many who will not be forthcoming with that answer);

  4. That test specimens are not the same as “field applications;”

  5. That an ESR from ICC, UL, Intertek, SwRI, IAPMO, etc. that asserts a fire-rated duct test standard, like ISO 6944, is equivalent to the prescriptively referenced ASTM E119 test standard does not mean the product can be used in any configuration and be considered equivalent. (The test results for each listed design configuration tell you what passed the test, and if the test results don’t establish fire-resistance symmetry performance and continuity for your project application, you indeed have a problem using that product);

  6. That an ASTM E119 listing may only cover the penetration, akin to a UL 1479/ASTM E814 test result (this kind of product design should not be applied as equivalent to a shaft or other enclosure requirement);

  7. That Duct B from the ISO 6944 test standard does not mean the same thing as Condition B from the ASTM E2816 test standard (and there actually are listing cards designed to cause that confusion);

  8. That most fire-rated duct products do not maintain the same FRR in the same configuration for both horizontal and vertical duct orientations;

  9. That more than 90 percent of the product approvals do not reach two hours FRR threshold for Duct B, which is where most of your requirements will need to minimally comply, including stairwell and elevator shaft pressurization, shaft replacement and offsets serving four or more stories, laboratory exhaust, utility stack vaults, and more;

  10. That drywall designs protecting horizontal ducts require at least one wall below the entire length of the assembly for compliant “continuity” support of the assembly (and two walls for spans larger than for a 5-foot duct width); and

  11. Most importantly, that putting a note on a drawing pointing to a duct saying, “Two hours FRR” is not actually a design.

 

All fire-rated ducts are not created equally

A UL or other label merely means some test specimen passed some test not that the product is good for all purposes. For example, grease duct is tested to only 500ºF for hours, then only briefly tested along the standard time-temperature curve in a simulated fire for only a half hour. If a duct passes the test, it gets to bear the UL label. Do you want to use that duct for your stairwell pressurization system? Please don’t. Those product configurations must be tested to the same fire exposure as ASTM E119, which is going to be more than 500ºF. Smoke control ducts are not grease ducts. You need the correct listing for the product configuration you want to use. Since you cannot trust the mere presence of a UL or other label alone, you need to get in the habit of asking for the listing card.

Assembly No. V-2

 

Assembly No. V-34

 

What’s “B” got to do with it?

Because most fire-rated ducts are not tested to, or at least tested completely to, ASTM E119, you’ll most likely need to understand the difference between Duct A and Duct B test specimens from a fire-rated duct test standard, like ISO 6944 or Conditions A, B, C, or D from the parallel test standard ASTM E2816. So, which test approvals should I see on the product listing card? For an ISO 6944 test, the listing card must minimally show Duct B for the minimum hourly fire-resistance rating. For ASTM E2816, you’ll want to see Conditions C and D to cover both vertical and horizontal duct orientations. The performance capabilities between the strong and weak side test specimens can be significant. The code sets forth the lower of the two results as the allowable hourly fire-resistive rating for the assembly.

 

The answer is on the listing card … and nowhere else

You will be offered product data sheets, ESRs, and just about anything other than the product listing card. If you are being given the runaround on getting the actual listing card, you probably won’t like what you see if you do get it. Ask yourself, if this critical life safety infrastructure requires a minimum level of fire-resistance performance, why would a supplier be going to such great lengths to hide the answer on what their product is actually approved for? Trusted voices don’t obfuscate when presented with simple questions.

 

Test specimens are not field applications

Duct A and Duct B are test specimens that answer the question of conformance with the code’s requirement to provide fire-resistive performance from fire exposure from either side. Don’t fall prey to inferior duct assemblies that do not meet this requirement. Duct A tells you the strong side of the duct was tested. Duct B tells you the weak side of the duct was tested. The code tells you the assembly’s allowable use is the lower of the two results. Period. Duct A or Duct B do not tell you whether a duct is supply air, return air, exhaust air, or otherwise. All of these ducts have openings, and all may or may not be “on” during any given automatic or required manual over-ride condition.

Smoke control systems with multiple zones and sequences will have scenarios where supply air ducts are not under positive pressure when exposed to fire under particular automatic sequences. Manual over-ride panels and their associated functionality are not “optional,” which means the duct infrastructure must be functional, if called upon manually well into the event.  The minimum fire resistance prescribed for the infrastructure already accounts for occupancies mandated to be fully sprinklered. Fire-resistance deductions from the minimum prescribed by code are not empowered by a rational analysis. The rational analysis must demonstrate minimum compliance plus any additional performance justified by good engineering practice. The applicable code language does not allow minimum compliance or sound engineering practice. And, since the kind of duct air service does not equate to test specimen designations in any North American model code or standard, you’ll always need Duct B to satisfy the minimum required fire-resistance performance.

 

Are any ducts tested to ASTM E119?

Yes, there are assemblies including ducts that are tested to the prescriptively referenced ASTM E119 fire-resistance test standard. These listing cards, too, can be confusing though. Most duct assemblies with ASTM E119 listings have been tested only as a penetration, as would normally be validated through testing to ASTM E814 or UL 1479. This is an allowable use of the ASTM E119 test standard, but these penetration test results do not equate to fire resistance the same as an actual enclosure — a fact easily lost without careful attention to the actual listing.

There are duct assemblies that are fully tested to ASTM E119, but those listing cards will demonstrate such performance by fully incorporating the duct as an element of the entire assembly by fully spanning from floor to floor or wall to wall. Be wary of applying ASTM E119 approvals, which are applicable only for a penetration firestop as if they are the same as an enclosure. A vertical duct listing for ASTM E119, which was tested only as a penetration firestop, is not a shaft alternative. Once again, you will find all ASTM E119 listings are not the same when it comes to fire-rated duct assemblies.

Prescriptive Fire Resistance

 

ISO 6944 vs ASTM E2816

The ISO 6944 fire-resistive ventilation duct test standard is the most commonly found listing card. Since the finalization of the ASTM E2816 test standard, more assemblies will be available with the different jargon used to describe the four test specimens needed by either test standard to fully demonstrate fire-rated duct assembly in the horizontal or vertical orientation under both strong and weak side fire exposures. Knowing the difference in jargon between the two test standards is essential because it is far too easy to mistake Condition A and Condition B on an ASTM E2816 listing card for Duct A and Duct B on an ISO listing card. Condition A and Condition B equate to Duct A, while Condition C and Condition D equate to Duct B.

 

Orientation Matters

Many products are only tested to the needed hourly fire resistance in one orientation, for example only vertical or only horizontal ducts. These approvals do not extend to all duct installations any more than the Gypsum Association design guides would allow a gypsum wall assembly to be turned horizontally beneath a duct offset. Horizontal drywall assemblies exist for the protection of horizontal ducts. These assemblies do not look anything like wall assemblies because it is much more difficult to achieve any fire-resistive rating in the horizontal orientation whether the metal framework of the assembly is a stud, channel, or duct. If your smoke control system involves horizontal and vertical duct runs, check the listing card to validate both orientations were approved for the hourly rating applicable to your application. You will be surprised by how many product configurations are limited by orientation.

 

Why is two hours FRR an important threshold?

Smoke control systems serving many applications are left for the rational analysis to determine the “probable temperatures and pressures” to which the ductwork will be exposed. But there are many, such as stairwell and elevator shaft pressurization applications, where the hourly rating will be minimally two hours FRR, which extends to enclosures for fans, power, and control wiring and associated devices, e.g. VFD, BMS, and FA modules, all of which can only be compliantly served by Duct B.

Equivalent Fire Resistance

 

Other systems, where ductwork serves four or more stories, may avoid shaft enclosures or offsets where suitably rated for two hours FRR Duct B. Also take care that much of the equipment, e.g. fans, VFD, and modules, are environment-dependent for their allowable operation specifications. Even a high-temperature-rated power ventilator cannot be enclosed or wrapped in a fashion that does not allow the motor and electronics to “breathe.” You would not install any of this equipment in an unventilated mechanical or electrical room. Any enclosures provided for fire-resistive-rated protection of any of this infrastructure must afford the same accessibility for service and maintenance while providing a proper operation environment.

The fire-rated enclosure must also be “dedicated,” which means one stairwell pressurization fan, VFD, or control module must be independently protected … even from each other. Cramming all of the smoke control infrastructure into one two-hours FRR mechanical/electrical room does not provide the dedicated system that is minimally required by code. This two hours FRR threshold is unfortunately where most fire-rated duct products fall out of their listed approvals. Most are approved for two hours FRR only for their strong side test specimens, which is insufficient to establish the minimum code requirement that the duct be protected from fire exposure on either side, regardless of the air service of the system served by that ductwork.

 

Why not just always use drywall enclosures?

If it all seems too confusing to get properly listed/approved fire-rated duct specified for and actually installed on your project, you are not alone. And, it might be understandable that defaulting to drywall enclosures could seem like the solution to all of your problems. But, there are limitations that come even with drywall enclosures. These limitations often make the option impractical for a given project application. While you can open the UL Fire Resistance Directory and find dozens and dozens of shaft wall options for fire-rated protection of vertical duct risers, the number of horizontal solutions is remarkably sparse.  The Gypsum Association’s published designs are equally sparse. And, the performance-based designs and tables from the code are limited to vertical wall and partition configurations. You will find the only approved drywall designs in the horizontal orientation such that a duct offset that is properly protected will demand at least one wall below the enclosure for its continuity support.

Further, if the duct to be protected by this horizontal drywall assembly is more than 5-feet wide, the continuity will demand a FRR wall on both sides of the entire duct path to provide the necessary continuity support. In the modern space plan for most occupancies, not even one FRR wall runs along the path from the louver on the exterior wall to the shaft served by this horizontal duct offset. And even if this wall did exist in the space plan, the 10- to 12-inch-deep drywall assembly required below the duct to satisfy its horizontal orientation will very often fall outside the minimum allowable ceiling height along the duct path. There are many reasons to use fire-rated duct assemblies, one of the most common being that the conventional enclosures are simply impractical within their own approval limitations.

 

Conclusion

The design professional can calculate reasonable design fire scenarios, pressure differentials, airflow, and exhaust rates in order to achieve solutions to complex smoke control challenges. However, for the smoke control system to work properly during an actual fire event, the infrastructure must be robust. There are minimum requirements prescribed for this infrastructure, including the ductwork. It may be more difficult to get this infrastructure correct than you might want to believe — it matters. 

Back To Top