In this short podcast, Bryan answers a listener-submitted question about duct smoke detector facts and wiring. He focuses on Honeywell duct smoke detectors (namely the D4120W), but there are some general best practices that can be applied to most duct smoke detectors.
The power supply may be 24v or 120v AC or DC and may have multiple inputs. You should not break that constant power source. There are also fan shutdown contacts, which may be normally open (alarm condition) or normally closed (non-alarm condition). Supervisory contacts open to indicate a trouble condition to the panel when something in the circuit isn't working properly (NOT an alarm condition). There is often a resistor at the end of the loop, and the alarm panel recognizes a specific resistance. Alarm initiation contacts close and create a shorted condition when there is an alarm condition (the supervisory contacts open if there is a trouble condition).
Smoke detectors in the ducts are different from the ones on ceilings; they usually shut down blowers to prevent the distribution of smoke through the ductwork (and possibly exhaust it to the outside). They are required in duct systems designed for more than (or at least) 2000 CFM per NFPA90 and IMC 606.2.1, though those codes conflict. NFPA90 requires installation in the supply duct, but IMC 606.2.1 requires installation in the return; the AHJ may interpret the code either way. (Smoke detectors in systems >15000 CFM must be in both the return and the supply.)
Overall, remember that different detectors are suitable for different velocities. You will want to keep them downstream of bends for at least 18 inches, test them according to the AHJ, and ensure that they are mounted in the right configuration as recommended by the manufacturer.
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