Smoke detectors are wonderful things. They wake you up at night when you've left the oven on too long. They can save your life in event of a fire. But a single smoke detector by itself can only cover the area described by a circle with a fifteen foot radius (five meters). So it takes a whole lot of smoke detectors to cover a big room. I have run inspections on floors with over sixty smoke detectors. There has got to be a better way, and a duct detector might be it.

A duct detector takes advantage of the fact that as air moves through a duct any smoke in that air will move along with it. So all one need do is sample a percentage of that air and you can cover a very large area with a single smoke detector. And that's what the most basic duct detector is. You take a box and put a smoke detector inside. Two pipes run out the back. One is called the sampling tube. That pipe spans the duct and has a series of holes drilled in a line along its length. The pipe is turned so the holes face the onrushing air, a portion of which is then directed into the box with the smoke detector. The second pipe is very short and on the other side of the box from the first pipe. It simply returns the sampled air to the duct. If properly placed and the holes are aligned a steady stream of air will flow through the smoke detector to be sampled. In the field we call this a two-wire duct detector.

That's enough to create a fire alarm, and such simple detectors continue to be installed. But smoke from a fire is usually poisonous. While passing air through a duct is a good thing, passing smoke is not. So duct detectors (or a fire alarm if present) is expected to shut down the flow of air. Thus most duct detectors also contain at least one relay to shut down the fan driving the air. Damper motors may even close off the passage in event of a fire.

These detectors are more complex. The relays need to be powered, and the power that runs the smoke detector cannot be trusted to do both in most cases. So you need one set of wires to power the detector and one to activate the fire alarm. This is the more typical type of duct detector, a four wire duct detector. Really the number of wires matter less than than the fact that alarm monitoring and power have been separated.

For example consider System Sensor DH-100, a zone type of smoke detector sold in the United States and used both with and without a fire alarm. I've installed about a hundred of them. The detector has an internal transformer and can be powered by 120 or 230 volts AC, 24 volts AC, or 24 volts DC. It can generate it's own 24 DC volts to control a remote annunciator, and the power side runs the smoke detector. Monitoring is accomplished through alarm and supervisory dry contacts. There are two sets of dry contacts dedicated to controlling external devices, like said fan. That is typical for most commercial products.

So what does all that mean? If the duct detector detects smoke it must let people know about it. With a fire alarm panel, that is accomplished by the alarm and supervisory contacts together. In event of a fire, the alarm contact will close, creating a dead short, which tells zone type fire alarm panels there is an alarm, leading to flashing lights and lots of loud noise. If there is a problem, say loss of power to the detector, the supervisory contact will open, creating an open circuit. By code, any fire alarm circuit must be supervised so that the panel will know if any abnormal condition occurs. Usually that is accomplished by placing an end-of-line resistor at the extreme end of the circuit. When the panel sees a current flow and a normal level of resistance all is well. A dead short is alarm. Loss of the resistor indicates a trouble condition, which means the panel beeps to let you know it needs to be fixed. Properly wired, all that is possible with a single pair of wires. Alarm contacts do not need to operate at high voltages.

If there is no fire alarm panel, it is still possible for the duct detector to do its job. In a small building, like a Burger King, the detector relies on a remote annunciator to let people know about alarms and troubles. The annunciator will have a monitoring LED, which will flash or show a certain color if all is well. It will change if there is an alarm or trouble. Some will flash a red, or go red constantly in event of an alarm. The important thing is there will be a visible change, even if the issue is that the detector loses power. In that event the LED will go, and stay, dark.

Second the annunciator will buzz, or make some other loud noise in event of an alarm, in effect simulating the horns and strobes of a real fire alarm. That's your signal to run away. There will be a key switch so the detector can be tested, and it can be reset after an alarm.

Second, the duct detector must be able to shut off the fan before it blows fresh smoke into the building where the people are. Usually that is accomplished by interrupting the thermostat circuit, but it can be done in many ways, including passing coil voltage to a contactor. Equipment contacts then must be capable of operating safely with high voltages. I have personally metered over 280 volts on one such circuit.

If a duct detector is installed it must be installed first on the return side of a heating an air conditioning unit. That means the air coming back must be tested first, since that's where smoke will most likely be found. Detectors may be installed on the supply side, but the return comes first. In most commercial buildings the area above ceiling may be used as a plenum, a path for returning air. The rule of thumb is that if it blows air at you it's supply. Return air tends to be quiet and at low pressure.

Duct detectors do a great deal to simplify fire protection in larger buildings. One or two detectors per HVAC unit can provide smoke detection for an entire building. That makes for fewer devices to break and inspect. Duct detectors can be used without any fire alarm system in many situations. They do a lot to reduce costs when covering a larger building.

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