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Smoke Detectors

Duct Smoke Detectors — Frequently Asked Questions

A primer on selection, installation and testing.
Duct Smoke Detectors
Which type of duct smoke detector is preferred for HVAC applications, photoelectric or ionization?

Photoelectric detection is preferred for several reasons, including:

1. Detection Capability — Photoelectric detection responds better to the larger smoke particles found in ductwork during a fire.

2. NFPA Recommends Photoelectric — The National Fire Alarm Code Standard 72, section A. explicitly states, “In almost every fire scenario in an air-handling system, the point of detection will be some distance from the fire source, therefore, the smoke will be cooler and more visible because of the growth of sub-micron particles into larger particles due to agglomeration and recombination. For these reasons, photoelectric detection technology has advantages over ionization detection technology in air duct system applications.”

3. Environmental Immunity — High humidity and condensation can cause false alarms with ionization detectors. Photoelectric detectors operate more efficiently, generating fewer false alarms.

4. Industry Preferred — Photoelectric detection is preferred by the fire alarm industry, manufacturers of commercial packaged air conditioning units and major retailers.

5. Low-Flow — Photoelectric detectors are capable of operating in air speeds as low as 100 feet-per-minute to meet new HVAC applications and codes with variable air volume systems and fire smoke dampers.

What is the best procedure for testing duct-mounted smoke detectors?

There are four simple steps to test duct-mounted smoke detectors:

1. Verify the detector is installed per NFPA 72 guidelines and is in accordance with the manufacturer’s installation instructions.

2. Employ the detector’s built-in test feature, such as the test magnet or accessory test switch. These features are designed to meet NFPA and Underwriters Laboratories functional test requirements that ensure the detectors are operable and will respond to minimum smoke requirements.

3. Measure the pressure differential across the sampling tubes (exhaust and intake) with a manometer to ensure the detector will respond to smoke in the duct airflow. This is the manufacturer’s acceptable test.

4. Apply smoke directly to the detector head to initiate an alarm. The sampling tubes may need to be blocked off for this test and then reopened afterwards.

Why is a smoke bomb test not recommended for ionization duct smoke detectors?

Based on evidence with in-house and field-testing of ionization, duct-mounted smoke detectors, there are three notable reasons:

1. Ionization smoke detectors are most sensitive to smoke particles ranging from .01 to .3 microns. Particles produced by smoke bombs tend to become larger the farther they travel from the source, triggering a slow response.

2. Smoke bombs produce cold smoke particles, which are larger and not as easily detected by ionization smoke detectors. These particles are also dependent on relative humidity, distance traveled from the source and time of activation. This phenomenon is caused because the smoke is more of a mist than suspended solids in warm gases. In other words, the smoke doesn’t represent a true smoke composition or fire signature for smoke detector activation.

3. It is possible to pass a smoke bomb test, but to be out of the required manometer range for sampling, giving the installer a false sense of proper operation.

Although unadvisable, if you choose to conduct a smoke bomb test, use a photoelectric smoke detector, which typically responds to smoke particles between .3 and 10 microns, and, if you have a respiratory ailment, use a self-contained breathing apparatus.

Where can a duct-mounted smoke detector be installed in relation to the duct?

Duct-mounted detectors can be installed horizontally and vertically to – and on top of, within and underneath – the duct. However, we do not recommend installing underneath the duct because condensation may drain into the electronic circuitry and cause electrical damage.

To determine the best location, we recommend comparing the pressure differential between the sampling and exhaust tube. The pressure differential must be within specified limits of .0015 to 1.20 inches/water for photoelectric smoke detectors. The detector housing cover must be securely fastened to complete the airtight enclosure for proper air sampling and to restrict contaminants from entering the detector head.

In order to shut down the HVAC system, what do I connect my HVAC or RTU (Roof Top Unit) to?

To provide immediate shutdown during an alarm, connect to the controller voltage from the RTU. Connecting to the thermostat will only allow a gradual shutdown of the HVAC system. In some cases, the RTU manufacturer requires a shutdown of this type because, if stopped too fast, the bearings could become damaged. Be sure to review each situation accurately.

Can any test station and other accessories be used with any duct smoke detector manufacture?

No. Duct detectors, test stations, and other accessories manufactured by different companies are not compatible with each other due to a different electrical makeup.

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