Duct smoke detectors are a critical but often overlooked component of a building’s fire protection system. Required by NFPA 90A in most commercial HVAC installations, duct detectors monitor the air moving through supply and return ducts — and shut down air handling units when smoke is detected, preventing the HVAC system from distributing smoke throughout the building. The Simplex 4098-9756 TrueAlarm duct sensor housing is the industry-standard housing for Simplex addressable duct detection installations.
What Is the 4098-9756?
The Simplex 4098-9756 is a duct smoke detector housing designed for installation directly into HVAC ductwork. Unlike ceiling-mounted spot detectors, duct detector housings use sampling tubes that extend into the duct airstream to draw air samples across the detection chamber. The 4098-9756 housing accepts Simplex TrueAlarm analog addressable sensor heads, integrating seamlessly with Simplex 4100ES, 4010ES, and compatible addressable panels.
Key features of the 4098-9756:
- Designed for horizontal duct mounting
- Includes sampling inlet tube and exhaust tube
- Compatible with Simplex TrueAlarm addressable sensors
- Relay outputs for HVAC shutdown and remote alarm annunciation
- Remote test capability via panel or remote test station
- Listed per UL 268A (Duct Smoke Detectors)
NFPA 90A Requirements for Duct Detectors
NFPA 90A — Standard for the Installation of Air-Conditioning and Ventilating Systems — mandates smoke detection in HVAC ductwork under specific conditions. Key requirements include:
- Duct detectors required in supply air systems serving areas over 2,000 CFM and in certain return air configurations
- Detector must shut down the associated air handling unit upon alarm
- Remote alarm and trouble indication required at a constantly attended location or via the fire alarm panel
- Detectors must be accessible for inspection, testing, and maintenance
- Testing required per NFPA 72 at least annually
The AHJ (Authority Having Jurisdiction) interprets and enforces NFPA 90A locally, so specific requirements may vary. However, duct detector installation in new commercial construction is nearly universal, and retrofit installations are often required during significant HVAC upgrades.
Compatible Sensors for the 4098-9756 Housing
The 4098-9756 housing is designed to accept Simplex TrueAlarm sensor heads. The most common compatible sensors are:
- 4098-9714 TrueAlarm Photoelectric Sensor: Standard photoelectric sensor head for duct applications. The Simplex 4098-9714 TrueAlarm photoelectric sensor is the most widely used sensor with this housing and provides reliable smoke detection for most HVAC duct environments.
- 4098-9757: Ionization sensor variant, used in some applications where ionization technology is specified or preferred.
The sensor head is the “intelligence” of the duct detector — the housing provides mounting, airflow sampling, and relay functions, while the sensor performs the actual detection. Replacing a sensor head is straightforward and does not require rewiring the housing.
Installation Overview
Duct detector installation must be performed by a licensed fire alarm contractor and inspected by the AHJ. Key installation considerations for the 4098-9756:
- Location: Install downstream of filters and ahead of branch duct takeoffs to sample the full duct airflow. Avoid placement near supply registers, dampers, or areas of turbulent flow.
- Sampling tube length: The inlet sampling tube must extend approximately one-third to two-thirds into the duct width for representative sampling. Tube lengths are available for different duct sizes.
- Wiring: Connect to the Simplex addressable loop per the panel wiring documentation. Relay outputs for HVAC shutdown connect to the AHU control circuit.
- Access: Ensure the detector housing is accessible through an access panel in the duct or ceiling for periodic testing and maintenance.
Testing and Maintenance
NFPA 72 requires annual testing of duct smoke detectors, including functional response testing using listed aerosol or an actual smoke source. Remote test capability on the 4098-9756 allows testing without physically accessing the duct — an important feature in hard-to-reach installations. After each test, verify HVAC shutdown occurred and that the alarm was properly annunciated at the panel.
Order the Simplex 4098-9756 TrueAlarm duct sensor housing and compatible sensors from Life Safety Consultants. We stock duct housings, sensor heads, and accessories for new installations and replacements alike.
