Plain English:
606.1 Controls Required
Air distribution systems must have listed/labeled smoke detectors where required.
Duct smoke detectors must comply with UL 268A. Other smoke detectors must comply with UL 268.
606.2 Where Required
Smoke detectors are req'd where indicated in 606.2.1 through 606.2.3.
Exception: detectors are not req'd where the air system cannot spread smoke beyond the room or space where smoke starts.
606.2.1 Return Air Systems
Return air systems over 2,000 cfm require smoke detection.
Detectors must be in the return duct or plenum upstream of filters, exhaust air, outdoor air, decontamination equipment, and appliances.
Exception: return smoke detection may be omitted where served areas have area smoke detectors connected to the fire alarm and compliant with 606.4.
606.2.2 Common Supply and Return Air Systems
Shared supply or return ducts/plenums must be evaluated by combined design capacity.
If combined capacity is over 2,000 cfm, return smoke detection is req'd.
Fan powered terminal units do not each need detectors if they are 2,000 cfm or less and shut down by an approved duct, plenum, or area smoke detection method.
606.2.3 Return Air Risers
Return air risers serving two or more stories require smoke detectors at each story where any portion of the return system exceeds 15,000 cfm.
Detectors must be upstream of the riser connection to any ducts or plenums.
On Plans: Why it matters
Most IMC 606 comments come from unclear thresholds and detector locations. The plans may show a detector symbol, but not the system cfm, shared return condition, riser condition, upstream location, or FA exception basis.
Code Path: Where to show it
M-001: IMC 606 basis, thresholds, and exception criteria
M-101: detector locations in return ducts, plenums, and risers
M-501: duct smoke detector installation and access details
M-601: AHU / RTU / fan schedule with design cfm and detector requirement
M-603 / FA / E: shutdown sequence and device coordination
Check: Do
Show system cfm so the triggers are clear
Locate return detectors upstream of filters and air connections
Evaluate shared returns by combined design capacity
Coordinate fire alarm interface and shutdown sequence
Review Risk: Don't
Don't place detectors downstream of filters or outdoor air connections
Don't ignore common return plenums serving multiple systems
Don't omit riser detection where the 15,000 cfm trigger applies
Field Tip:
Start with the fan schedule. Flag systems over 2,000 cfm, shared return paths, and multi-story return risers. Then trace the return path before filters, outside air, exhaust connections, and equipment.
Masterbuild QA Lens
Fire damper placement (606.2) is determined by the fire-resistive assembly type - not by a generic rule. Confirm the fire damper schedule identifies the rated assembly for each damper location, the dynamic or static closing rating matches the application, and the UL listing number is in the schedule. Dynamic-closing applications (ducts in active systems) require dynamic-rated dampers - static-only dampers will fail.
Drawing / Submittal Check
For 606.1-606.2.3 (where smoke detection is required): the drawings must show smoke detectors on all supply ducts over 2,000 CFM and on return ducts serving more than one floor, as required by IMC 606. Confirm the locations against the duct schedule - all ducts meeting the threshold must have a detector, not just the ones that were remembered. A complete list of required detectors vs. shown detectors is the best QA check.
Common Review Risk
The most common 606.1-606.2.3 error is a duct that was added or resized during construction and now exceeds the 2,000 CFM threshold, requiring a duct smoke detector that was not in the original design. Any duct that changes to exceed the threshold during construction requires a smoke detector addition and an electrical permit amendment. Confirm the duct smoke detector list is updated after any duct resizing.
When To Escalate
Escalate when a duct smoke detector is required for a duct that was resized during construction, and the electrical permit amendment for the additional detector has not been filed. An uninspected smoke detector that is connected to the fire alarm panel without a permit is a code violation that can hold the certificate of occupancy.
Duct System Coordination
Smoke detection in air distribution systems (606.1-606.2.3) requires duct smoke detectors at specific duct locations based on system CFM. Show detector locations on the duct plan and confirm sampling tube length matches duct width.
Ventilation Coordination
For duct smoke detector locations (606.1-606.2.3): the duct smoke detector must be located in the supply duct upstream of the first branch takeoff - where the sampled air is representative of the air handling unit discharge, not of a single zone. Confirm the sampling tube location is in the main duct, not in a branch serving a single room.