03
Mar

Code-Compliant Air Sampling Tubes for Duct Smoke Detection

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When installing duct smoke detectors, it’s critical to ensure the air sampling tube monitors the entire airstream sample. The Codes will direct you to follow the manufacturers installation instructions, where you’ll most likely find that the sampling tube must extend 2/3 and 80% of the duct width. Using tubes that are too short can result in failed inspections, liability exposure, and unsafe conditions.

Why Tube Length Matters

Air sampling tubes are available for duct widths between one and ten feet, but a 12" sample tube will not pass code if the duct is greater than 18" wide. In fact, if you do choose to use the shorter tube you could be breaking the law. The Codes are not just guidelines, they are law once adopted. Your Inspector only needs to see one incorrect installation before asking you to prove that every location is to code before sign‑off. Installing short tubes to “sneak past” the Building Inspector simply isn’t worth the risk; your installation may fail to detect deadly smoke if the sample tube doesn’t reach the required distance into the duct. More people are killed by smoke inhalation than by fire, and proper sampling tube length helps ensure the system triggers shutdowns or other connected life‑safety functions in time. Codes are enforced to save lives.

The sampling tube MUST extend at least 2/3 into the width of the duct, 80% in the case of some manufacturers.

**Make sure you purchase the correct size tubes for your particular application.

Tube support is required for all tubes greater than 3 ft.
Tubes can always be cut down to size, and some manufacturers allow splicing metal tubes together to meet the distance requirement. Always check the installation instructions.

 

Tube Length Reference Chart

Minimum tube lengths required to achieve 2/3–80% duct coverage:

Duct Width Required Tube Length
3 ft 24″ – 29″
5 ft 40″ – 48″
7 ft
56″ – 67″
9 ft
72″ – 86″

 

Tubes Proper Installation Required by Code

In all cases the Codes state that installations, tests and inspections shall be made in accordance with the manufacturer’s published instructions. The illustration above shows two sample air duct smoke detector installations. Both are “clean” installations, however only the detector to the left has been set up to properly monitor the airstream sample as required by the IMC (Chapter 606.3) and NFPA 72® (Chapter 5.16.5.2). NFPA90A Chapter 6 requires that smoke detectors be installed, tested and maintained in accordance with NFPA 72, National Fire Alarm Code®.

Designed to Save Lives
Many people fail to remember that more people are killed by smoke inhalation than by fire. If the installations you are responsible for fail because the deadly smoke stream by-passed the sample tube and did not trigger sensor head alarm and system shutdown (or other functions), you may face liability. In addition to monetary losses, think about the lives that may be lost or in jeopardy. It’s simply not worth it to install short tubes and “sneak it past” the Building Inspector to save your time and cost, nor is it legal to do so.

**Always follow the provided manufacturer’s installation instructions. If the instruction booklet is lost or missing, most can be downloaded from the manufacturer’s website.

Assembly of Fast Tubes