/ Field knowledge · IMC 606
Over 2,000 cfm, the return needs a detector.
IMC 606: return systems over 2,000 cfm need listed duct smoke detection. Combined supply/return capacity counts. A detector in the wrong place, or a shutdown that never got wired, fails commissioning.
Educational 2024 IMC 606. Confirm adopted edition and NFPA 72. This is not a fire-alarm design.
- Duct detectors: UL 268A. Other smoke detectors: UL 268.
- 606.2.1: return air systems over 2,000 cfm. Detector in the return duct or plenum upstream of filters, exhaust air, outdoor air, decontamination equipment, and appliances.
- 606.2.2: shared supply or return — combined design capacity. If combined exceeds 2,000 cfm, return smoke detection is required. Fan-powered terminals 2,000 cfm or less do not each need detectors if they shut down by an approved method.
- 606.2.3: return risers serving two or more stories: detectors at each story where any portion of the return system exceeds 15,000 cfm, upstream of the riser connection.
- Install per NFPA 72. Monitor the entire airflow. Access for inspection. Activation shuts down operational capabilities of the air system — or switches to smoke-control mode if part of 512. Where FA is required, duct detectors generally report as supervisory, not fire-alarm, signals.
Wrong velocity / no interlock
DCT: detectors in ducts with velocities outside the listed range (typically 300–4000 fpm) will not detect reliably. The most common commissioning failure is detector activation with the AHU still running.
Source: DCT 606 · install: 606.3 · tool: 2,000 cfm check · 512: smoke control.
What to send
AHU/RTU schedule cfm, whether returns are combined, and number of stories on the riser.
If the RTU is over 2,000 cfm and there is no detector
Send the schedule cfm. Masterbuild will say whether 606.2.1 is even on the path — or the combined returns already crossed 2,000.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval.