Design Dialogue: Inside the Design Process
- leelusty
- Mar 17
- 5 min read
By James Lusty
Sub-Ducts: A Smarter Alternative to Fire and Smoke Dampers
When designing safe and efficient ventilation systems, fire protection is a top priority. Traditionally, fire and smoke dampers are required at shaft penetrations to prevent the spread of smoke and flames. However, sub-ducts offer a streamlined alternative that eliminates the need for dampers—reducing installation costs, simplifying maintenance, and improving overall system reliability. By leveraging key exceptions in IBC 2021 and IMC 2021, sub-ducts provide a compliant, low-maintenance solution for kitchen, bathroom, and dryer exhausts. In this post, we’ll explore why sub-ducts are a preferred choice, how they work, and what you need to know to stay code-compliant.
What Are Sub-Ducts?
Sub-ducts, in the context of building exhaust systems, are a specific type of duct configuration used primarily to manage exhaust from appliances or spaces like bathrooms, kitchens, or laundry areas in multi-story buildings. They’re a clever workaround to meet fire safety codes without needing fire and smoke dampers at every point where an exhaust duct penetrates a rated shaft. Here’s a breakdown of what I know about them:
Sub-ducts are smaller exhaust ducts that connect individual exhaust points (like bathroom fans, residential kitchen hoods, or dryer vents) to a larger, vertical main shaft that carries exhaust out of the building. Rather than exiting directly outside, they penetrate the shaft and rise upward inside it—per IMC 2021 Section 504.4.4 (for clothes dryers) and Section 607.5.5 Exception 1, a minimum of 22 inches. This design leverages a "trap" effect to limit smoke or fire spread back into occupied spaces, a key consideration under IBC 2021 Section 717.5.3, which governs shaft enclosure penetrations.
Why Use Sub-Ducts?
The primary driver is fire safety without the complexity of dampers. IBC 2021 Section 717.5.3 requires fire and smoke dampers at shaft penetrations unless an exception applies. Exception 1 to this section allows sub-ducts for kitchen, bathroom, or dryer exhausts in lieu of dampers, provided specific conditions are met. IMC 2021 Section 607.5.5 Exception 1 spells out these conditions, aligning with the IBC by permitting sub-ducts to skip dampers if they’re built right. This saves on installation and maintenance costs—no moving parts to fail or clog, which is a real issue with greasy kitchen exhaust or lint-heavy dryer exhaust.
Key Requirements:
To comply with IBC 2021 and IMC 2021, sub-ducts have to hit these marks:
Material and Construction: IMC 2021 Section 607.5.5 Exception 1.1 specifies steel sub-ducts, typically 26-gauge minimum, to handle heat and maintain integrity in a fire. This ties into IBC 2021 Section 717.5.3 Exception 1’s allowance for sub-ducts as a damper alternative.
Height: The 22-inch upward rise inside the shaft is codified in IMC 2021 Section 607.5.5 Exception 1.1—it’s the minimum to create that smoke-trapping geometry. IBC 2021 references this via its exception to damper rules.
Fan Operation: IMC 2021 Section 607.5.5 Exception 1.3 mandates a continuously operating exhaust fan at the top of the shaft. This keeps airflow moving upward, critical for the sub-duct’s passive fire protection to work. IBC 2021 doesn’t contradict this—it’s assumed in the exception’s reference to mechanical code compliance.
Power Supply: Here’s where it gets strict—IMC 2021 Section 607.5.5 Exception 1.4 requires the fan to be “powered continuously in accordance with Section 909.11,” which cross-references IBC 2021 Section 2702 for standby power (like a generator). This ensures the fan runs during a fire-induced outage, a requirement rooted in IBC 2021 Section 909.11’s smoke control provisions.
Application: Sub-ducts are limited to specific exhausts—IMC 2021 Section 607.5.5 Exception 1 lists bathroom, kitchen, and dryer exhausts explicitly. IBC 2021 Section 717.5.3 Exception 1 mirrors this, restricting the damper exception to these uses in Group B or R occupancies with sprinklers.
How They Work in Practice
Picture a four-story apartment building (Group R occupancy under IBC 2021 Section 310). Each unit’s bathroom exhaust ties into sub-ducts that enter a fire-rated shaft (IBC 2021 Section 713 for shaft enclosures). Per IMC 2021 Section 607.5.5 Exception 1.1, these sub-ducts rise 22 inches inside the shaft, and a rooftop fan (IMC 2021 Section 607.5.5 Exception 1.3) pulls air out continuously, backed by standby power (IBC 2021 Section 2702). If a fire hits, the shaft’s 1- or 2-hour rating (IBC 2021 Section 713.4) and the sub-duct’s design keep smoke from backtracking, assuming the fan stays on.
Benefits
Cost Savings: Skipping dampers (allowed by IBC 2021 Section 717.5.3 Exception 1) cuts upfront and upkeep costs.
Space Efficiency: Modern sub-duct risers fit tight spaces, reducing pressure drops in the shaft—IMC 2021 Section 504.4.4 nods to efficient dryer exhaust design, which applies here.
Aesthetics: One shaft exit beats multiple sidewall vents, aligning with IBC 2021’s practical approach to building design.
Challenges
Fan Placement: IMC 2021 Section 607.5.5 Exception 1.3 insists on a top-mounted fan. A bottom fan pushing air up voids the damper exception under IBC 2021 Section 717.5.3—you’d need dampers then.
Power Dependency: IBC 2021 Section 909.11 and IMC 2021 Section 607.5.5 Exception 1.4 demand standby power. If it fails, smoke could pool—IBC 2021 Section 909’s smoke control intent gets shaky here.
Code Compliance: Miss the 22-inch rise or standby power, and you’re out of compliance with both IBC 2021 Section 717.5.3 Exception 1 and IMC 2021 Section 607.5.5 Exception 1. Inspectors won’t budge.
Misunderstandings: Some think a fire-rated shaft (IBC 2021 Section 713) or duct lets you skip standby power. Wrong—IMC 2021 Section 607.5.5 Exception 1.4 is clear.
Real-World Context
Sub-ducts shine in multi-family high-rises (IBC 2021 Group R-2). Stacking exhausts into one shaft beats punching holes in exterior walls, especially as sidewall venting fades for cost and looks. Pre-fab sub-duct risers now meet IMC 2021’s steel and height rules, making installs faster.
Here is an example of a prefab sub duct riser. They even have a 2 hr rated prefab shaft. https://lfsystems.net/prefabricated-shaft-system/
A Critical Take
Sub-ducts are slick—IBC 2021 Section 717.5.3 Exception 1 and IMC 2021 Section 607.5.5 Exception 1 nail down a passive safety fix. But the fan’s standby power (IBC 2021 Section 2702) feels brittle—what if the generator dies? The 22-inch rise might not cut it in a chaotic fire with shifting pressures. Codes could evolve—maybe add relief dampers at the shaft top (not yet in IBC 2021 or IMC 2021)—but for now, sub-ducts balance safety and practicality decently.
While sub-ducts are a smart fix, they’re not flawless. The reliance on a continuously running fan with backup power feels like a weak link—what if the generator fails or maintenance slips? And in a fire, smoke behavior can be unpredictable; that 22-inch rise might not always be enough if pressures shift. Plus, codes can lag behind real-world needs—some argue for relief dampers at the shaft top to vent smoke if power cuts out, but that’s not standard yet. Still, for now, sub-ducts strike a practical balance between safety and simplicity in building design.
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