Duct smoke detectors are required by NFPA 90A in air handling systems to prevent smoke from being circulated through a building via the HVAC system. For facilities running Simplex fire alarm systems, two part numbers come up most often: the 4098-9687 and the 4098-9756. These are not interchangeable — they serve different roles in the duct smoke detection system. This guide explains which one you need and why.
NFPA 90A Requirements for Duct Smoke Detection
NFPA 90A, the Standard for the Installation of Air-Conditioning and Ventilating Systems, requires duct smoke detectors on:
- Return air systems with airflow exceeding 2,000 CFM
- Supply air systems serving more than one floor
- Large-capacity systems (15,000 CFM or greater in some configurations)
When a duct smoke detector activates, it must shut down the associated air handling unit to prevent smoke circulation. The control station (discussed below) typically provides the wiring interface between the detector and the HVAC shutdown circuitry.
Simplex 4098-9687: The Standalone Duct Detector
The Simplex 4098-9687 is a complete standalone duct smoke detector — the detector head and housing are integrated into a single unit. It is designed for installations where a self-contained detector is sufficient, without the need for a separate plug-in sensor or external housing.
Key characteristics of the 4098-9687:
- Integrated photoelectric sensing chamber and housing
- Designed for direct duct mounting without a separate housing kit
- Suitable for use with Simplex addressable fire alarm panels
- Includes sampling tube ports for duct air flow testing
- Typically used for replacement of existing standalone duct detector installations
If you are replacing a duct detector and the existing installation is a standalone unit (not a separate housing and plug-in sensor), the 4098-9687 is likely your replacement part.
Simplex 4098-9756: The TrueAlarm Duct Sensor Housing
The Simplex 4098-9756 is a duct sensor housing designed to accept a plug-in TrueAlarm sensor head. The housing itself does not contain the sensing element — it provides the duct mounting hardware, sampling tubes, and wiring connections, while the actual smoke detection is performed by a separate plug-in sensor installed into the housing.
Key characteristics of the 4098-9756:
- Accepts plug-in TrueAlarm sensor heads (such as the Simplex 4098-9714 photoelectric sensor)
- Housing provides duct mounting flange, inlet and exhaust sampling tubes, and wiring terminals
- Allows for sensor head replacement without disturbing the duct penetration
- Preferred in applications where ease of maintenance is a priority
- Modular design means only the sensor head needs to be replaced when the detector reaches end of life or requires calibration
If your existing installation uses a separate housing with a removable sensor head, the 4098-9756 is the housing component. You would also need the appropriate plug-in sensor to complete the installation.
When to Use Which Model
The choice between the 4098-9687 and 4098-9756 generally comes down to the existing installation type:
- Replacing a standalone unit: Use the 4098-9687. It is a complete replacement that installs directly without additional components.
- Replacing a housing with plug-in sensor: Use the 4098-9756 housing plus a compatible TrueAlarm plug-in sensor. If the housing is undamaged and only the sensor has failed, you may only need the sensor head.
- New installation: The 4098-9756 system is often preferred for new installations because it simplifies future maintenance — sensor heads can be swapped without cutting into the duct.
Simplex 4098-9842: The Duct Detector Control Station
The Simplex 4098-9842 control station is a remote test/reset/alarm station used in conjunction with Simplex duct smoke detectors. It provides:
- Remote alarm indicator (LED)
- Remote test capability (allows testing the duct detector from an accessible location without accessing the duct detector directly)
- Remote reset capability
The control station is particularly valuable when the duct detector is installed in an inaccessible location — above a ceiling, inside a mechanical room, or in a plenum space. NFPA 72 requires that duct detectors be accessible for testing and maintenance, and remote control stations are one acceptable means of providing that access.
Sampling Tube Accessories
Both the 4098-9687 and 4098-9756 use sampling tubes to draw duct air across the sensing chamber. Sampling tubes are sized based on duct width and must extend at least to the centerline of the duct. Simplex offers sampling tube kits in various lengths — verify the correct tube length for your duct dimensions when ordering replacement detectors.
Where to Order Simplex Duct Smoke Detector Parts
Life Safety Consultants stocks Simplex duct smoke detector components including the 4098-9687, 4098-9756, 4098-9842 control station, and TrueAlarm sensor heads. Browse our Simplex parts catalog for current availability.
Summary
The Simplex 4098-9687 is a standalone duct smoke detector for direct replacement of existing standalone units. The 4098-9756 is a duct sensor housing designed for use with a plug-in TrueAlarm sensor head. The 4098-9842 control station provides remote test and reset capability. Choosing the right model depends on your existing installation type — when in doubt, check the model number on the existing unit before ordering.
