Duct smoke detector

What it is actually for

Ducts do not usually burn, and no Canadian code is written around the case where one does. The purpose runs the other way. An air handling system is a distribution network, and left running during a fire it takes smoke from the floor of origin and delivers it efficiently to floors and suites that are not on fire yet. The National Research Council states the intent plainly: the HVAC system is to be shut down during a fire to minimise the circulation of smoke through the building.

So you are not trying to catch smoke early. You are trying to catch smoke that is already in the air stream, before the fan hands it to the rest of the building.

When the Ontario Building Code requires one

Article 3.2.4.12, Prevention of Smoke Circulation, is the governing provision in O. Reg. 163/24. Where a fire alarm system is installed, the air handling system has to be designed to prevent the circulation of smoke on a signal from a duct type smoke detector if it serves more than one storey, serves more than one suite in a storey, serves more than one of the fire compartments the article cross references, or has no fire dampers where the Code would otherwise permit them to be left out.

Part 9 carries its own version at Article 9.10.18.5, Smoke Detectors in Recirculating Air Handling Systems, which exempts a system serving not more than one dwelling unit. Part 6, where the mechanical designer looks, points straight back at 3.2.4.12 through Article 6.9.2.2.

Under the revoked O. Reg. 332/12 this was Article 3.2.4.13. A specification quoting 3.2.4.13 is citing a code that no longer exists. The longer treatment, including the American capacity threshold that is not in any Ontario provision, is in duct smoke detectors in Ontario.

Which standards apply

The head is listed to the Canadian smoke detector product standard, CAN/ULC-S529, and installed to CAN/ULC-S524. Neither Ontario code names S529 directly: it reaches your project through S524, which requires listed devices. What the Code names is S524 for installation and S537 for verification.

How it behaves in the field

A sampling tube duct detector does not sit in the air stream. It sits outside the duct in a housing, and air is driven through that housing by a pressure difference between two tubes. The inlet tube faces into the airflow and the exhaust tube faces downstream, into the lower pressure behind it. That differential is the only thing pulling a sample across the sensing chamber. Reverse the tubes and the housing sees a negative differential: the unit powers up, the LED is green, the panel reports normal, and no useful quantity of duct air ever crosses the chamber.

Every duct detector also carries a listed air velocity range, and that range is a hard edge, not advice. Below it there is not enough differential to move a sample. Above it the sample is too fast or too dilute to register. NRC's field survey of Canadian HVAC systems found duct velocities from roughly 2 to 40 m/s, which is well past the top of the range typical detectors are listed for, so measure with an anemometer at the intended location with the air handler running before you cut the hole.

What goes wrong

• Sampling tube reversed, because the air flows the other way to the direction assumed on the shop drawing.
• Tested with the fan off, which proves the sensing chamber works and nothing else. This is the failure that passes.
• Programmed as supervisory. A detector required by 3.2.4.12 is an initiating device: it produces an alarm signal on a single stage system or an alert signal on a two stage system. Put it on supervisory and the building will not evacuate when that detector sees smoke first.
• Alarm proven, shutdown not proven. The relay or damper actuator that carries out the shutdown is an ancillary device, not supervised like a detection circuit, so the detector can be healthy while the fan interlock has quietly failed.

Where it sits next to the other devices

An ordinary smoke detector protects a space. A duct detector protects the rest of the building from a fan.

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