The fan didn't stop. Detector in alarm, panel in alarm, horn going, and the supply fan on the roof just kept pushing air down the corridor like nothing was wrong. I had landed the shutdown wires on the wrong side of the relay contact, so the circuit did the opposite of what I drew. Ten minutes of embarrassment, and I have checked that contact with a meter ever since.
That's the part of a duct detector job that bites people. Not the smoke sensing. The wiring around it.
One device, four separate circuits
A duct detector is usually a housing with a sampling tube and a smoke sensor inside, and it is really four little circuits sharing a box.
- The initiating connection. This is the part that talks to the panel. Depending on the product it lands on an addressable loop or on a conventional zone, same as any other smoke detector.
- Power. A lot of duct housings want their own 24V supply for the sensor and the relay coil, separate from the signaling line. Some pull everything off the loop. Which one you have changes the whole job.
- The relay contacts. Dry contacts that open or close the fan control circuit when the detector alarms.
- The remote indicator or test station. A lamp and a test switch mounted where somebody can actually reach them, because the housing is usually up in a ceiling or on a roof.
If you can name which of those four you're looking at on a given pair of wires, you can read almost any duct detector diagram.
What I won't print here
No terminal numbers. I'm not going to tell you what lands on terminal 5, because it changes by model and by revision, and one wrong terminal from a blog post is how somebody burns a board. The installation sheet that shipped in the box is the only source I'd trust for the exact map. If the box is gone, pull the current one from the manufacturer's site and check the revision on it.
What I can tell you is the logic.
The initiating side
Treat it like any other detector on that circuit. On a conventional zone that means the end of line resistor still sits at the last device, and the detector goes in the middle of the daisy chain, not on a T tap. If that part is fuzzy, the end of line article covers it: /blog/end-of-line-resistor-explained. On an addressable loop it is two conductors in and out, plus an address you set before you close the lid.
Class A or Class B is set by the panel and the plans, not by the detector. Check what the design calls for.
The power side
If the housing needs separate 24V, that supply has to be supervised and it has to be on the same secondary power plan as everything else. Usually that means taking it from the panel's auxiliary output or a listed booster supply, not from a wall wart in the electrical room (yes, I've seen it). It also means the current counts in your battery calc. Standby and alarm, both. A relay coil pulling in costs more than the sleeping sensor, so do not just copy the standby number into the alarm column.
Voltage drop matters here more than people think, because it is a long run to a roof and the sensor and relay both have a minimum voltage. Here is an illustrative example. The 0.15 A is a made up alarm current for the exercise, so use the number off your own data sheet. Resistances are the usual solid copper values.
| One way run | Wire | Round trip resistance | Drop at 0.15 A |
|---|---|---|---|
| 150 ft | 18 AWG | 1.92 ohms | 0.29 V |
| 300 ft | 18 AWG | 3.83 ohms | 0.57 V |
| 500 ft | 18 AWG | 6.39 ohms | 0.96 V |
| 500 ft | 14 AWG | 2.57 ohms | 0.39 V |
| 500 ft | 12 AWG | 1.62 ohms | 0.24 V |
Nothing dramatic at 150 feet. At 500 feet on 18 gauge you are already giving away a full volt before the panel's own sag and the other devices on that circuit. Do the real calc with your real worst case start voltage, and the voltage drop article walks through it: /blog/fire-alarm-voltage-drop-calculation.
The relay and the fan
This is where my fan story lives. The relay contacts are dry, meaning they carry nothing from the detector. They just open or close a path in somebody else's circuit, and that somebody is usually the mechanical contractor's starter or the BAS.
Three things I now do every time.
Know which state is normal. Most of these relays have a common, a normally open and a normally closed contact. Which one drops the fan out depends on how the HVAC side was built, so ask the mechanical guy how their circuit works, then confirm with a meter on the bench state and the alarmed state.
Don't power their circuit. If their control voltage is 120 or 24 AC, it stays theirs. Check the contact ratings on the sheet.
Test it live. Smoke or magnet test the detector, watch the fan actually stop. A relay that clicks is not a fan that stopped.
The remote station
Short section. Run it, label it, mount it where a person can reach it without a ladder if the AHJ allows. Some inspectors want the lamp visible from the floor. Others don't care. Check your AHJ. The wire type and distance limit for the remote lamp are on the install sheet, so look there instead of guessing.
Before you close the lid
Airflow direction on the sampling tube, tube length matched to the duct width, housing sealed and gasketed, and a label on the ceiling or the duct so the next tech finds it. The placement side, meaning where these are even required, is in /blog/duct-smoke-detector-requirements.
I built FireDeck because retyping the same wire lengths and device currents into three different forms started to feel like my real job, so if that's your week too, it's free while I'm still testing it.