Do you know the difference between fire dampers and fire smoke dampers?
This is a questions that seems to come up a lot in the fire alarm install and design world. The definition of a damper is "A person or thing that damps or depresses". In other words it is a plate that is placed within the duct work of an HVAC system to regulate or in some cases stop air flow.
Now in the fire alarm industry we are more concerned with the terms "Fire Damper", "Smoke Damper" or "Combination Smoke Fire Damper". Whats is the difference? Well it is actually quite simple as we explain below.
A "Fire Damper" as defined in the CMC (California Mechanical Code) 2022 Edition Section 206.0 - "An automatic-closing metal assembly of one or more louvers, blades, slats or vanes that close upon detection of heat so as to restrict the passage of flame and is listed to the applicable recognized standard." The automatic means can be accomplished one of two ways: 1) by a fusible link that will melt upon heat thus releasing the louvers into the closed position. 2) by motorized actuators that will close upon loss or gain of power. These are typically controlled by and addressable relay module.
A "Smoke Damper" as defined in the CMC (California Mechanical Code) 2022 Edition Section 206.0 - "A damper arranged to seal off airflow automatically through a part of an air duct system so as to restrict the passage of smoke and is listed to the applicable recognized standard."
A "Combination Fire Smoke Damper" is exactly that, a combination of a "Fire Damper" and a "Smoke Damper".
When are they required and how do I know when to use what type?
Keep in mind that the fire alarm contractor will not be providing or installing these. However, it is nice to know what to look for on a bid set of plans or during a job walk. First up is the "Fire Damper". A fire damper is installed in HVAC duct work at the intersection of a rated barrier such as walls or partitions. The damper is in place to secure the integrity of the rated barrier in the event of heat or flames around 165 degrees F. Like we stated above, the fusible link will melt thus releasing the louvers on the damper. Once the louvers are closed or shut, the fire barrier is now secured from allowing flames to penetrate prior to the rating level of the barrier itself.
Smoke dampers are similar however, they close in the presence of smoke. Now since the smoke damper cannot obviously sense smoke, we need to install a smoke detector. The smoke detector can either be a system smoke (tied to a building fire alarm system) or a stand alone smoke solely in place to activate the smoke damper. Smoke Dampers are required to be installed on walls that separate smoke barriers.
What's a smoke barrier you ask...... A smoke barrier is a continuous surface such as a wall, floor, or ceiling constructed to restrict the movement of smoke. A smoke barrier can be either vertical or horizontal.
Now a "combination smoke fire damper" is located in a situation where both fire rating and smoke barriers come into play. A combination fire smoke damper also needs a smoke detector just like the smoke damper. The smoke detector can either be a duct smoke detector (System Sensor D4120 or DNR) or a pendant mounted detector within the duct work itself. Once the detector senses smoke particles it will either through programming or local relay base close the damper louvers.
If you work in the fire alarm industry, chances are you have encountered a System Sensor D4120 4-wire conventional duct smoke detector. One of the most important wiring details with these detectors is properly supervising the detector when one or more D4120s are connected to a conventional IDC (Initiating Device Circuit) or an addressable monitor module.
This guide explains System Sensor D4120 wiring, including the alarm contacts, supervisory contacts, end-of-line (EOL) resistor, resettable power, and the proper concept for connecting multiple conventional duct smoke detectors to one IDC.
Important: This article explains the wiring concept using the System Sensor D4120 as the example. Always follow the current manufacturer's installation instructions, the fire alarm control panel or monitor-module instructions, equipment listings, applicable NFPA 72 requirements, and the AHJ-approved design for the specific installation.
System Sensor D4120 Duct Smoke Detector
The System Sensor D4120 is a conventional 4-wire photoelectric duct smoke detector designed for HVAC applications.
Unlike an addressable duct detector that communicates directly with an addressable fire alarm system, the conventional D4120 typically requires separate power and uses dry contacts to communicate alarm and supervisory conditions to the fire alarm control equipment.
This distinction is important because the IDC can remain electrically intact even while a separately powered detector has developed a problem. Proper supervision of the detector therefore matters just as much as supervision of the field wiring.
System Sensor D4120 Terminal Identification
For the wiring arrangement discussed in this article, the most important D4120 terminals are:
Function
D4120 Terminals
Alarm Contacts
4 (Common) and 5 (Normally Open)
Supervisory Contacts
3 (Common) and 14 (Normally Open)
24 VAC/DC Power
9 and 10
120 VAC Power
120 VAC input terminals
Do not confuse the alarm contacts with the supervisory contacts.
The alarm initiation contacts are associated with terminals 4 and 5, while detector supervision uses terminals 3 and 14.
Correction to the original 2013 article: An earlier version of this guide accidentally referred to supervisory terminals "#3 and #4" in one paragraph. The System Sensor D4120 supervisory contacts are terminals 3 and 14. Alarm initiation uses terminals 4 and 5.
System Sensor D4120 Alarm Contacts: Terminals 4 and 5
The D4120 provides alarm initiation contacts used to report an alarm condition to compatible fire alarm control equipment.
For the D4120:
Terminal 4 = Alarm Common
Terminal 5 = Alarm Normally Open
In standby, the normally open alarm contact remains open. When the duct detector enters alarm, the alarm contacts change state and can initiate an alarm through the connected IDC or monitor module, depending on the approved system design.
System Sensor D4120 Supervisory Contacts: Terminals 3 and 14
The D4120 also provides supervisory contacts that are extremely important when the detector is connected to a supervised fire alarm initiating circuit.
For the D4120:
Terminal 3 = Supervisory Common
Terminal 14 = Supervisory Normally Open
The manufacturer's wiring arrangement uses these contacts so a detector trouble condition can interrupt the supervised circuit and produce the appropriate trouble indication at the fire alarm control equipment.
This is particularly important with a 4-wire detector because detector operating power is separate from the initiating circuit.
Why a 4-Wire Duct Detector Must Be Properly Supervised
Imagine that the IDC wiring to a duct detector is perfectly intact, but the detector itself loses operating power.
If the installation only supervises the IDC wiring and does not properly account for detector integrity, the fire alarm panel could potentially see an electrically normal initiating circuit even though the detector is no longer capable of performing its intended detection function.
That is the problem the supervisory arrangement is intended to prevent.
Proper wiring allows a qualifying detector trouble condition to interrupt the supervised circuit so the fire alarm control equipment receives a trouble indication rather than silently leaving an impaired detector in service.
Where Does the D4120 End-of-Line Resistor Go?
The end-of-line resistor must be the value specified by the fire alarm control panel or monitor-module manufacturer.
It is not simply placed wherever it is physically convenient.
With the manufacturer's conventional 4-wire D4120 arrangement, the EOL resistor is located at the end of the supervised initiating circuit and the circuit is routed through the detector supervisory contacts as shown in the approved System Sensor wiring diagram.
This allows the circuit to supervise more than just continuity of the alarm-contact wiring.
Field Rule: The EOL resistor belongs at the electrically supervised end of the circuit, not inside the panel simply because it is easier to install there.
How to Wire One System Sensor D4120 to an IDC
At a conceptual level, a conventional D4120 installation involves three separate functions:
Operating power for the detector
Alarm initiation through the alarm contacts
Detector/circuit supervision through the required supervisory arrangement
When 24-volt power is used, the D4120 power input is connected at terminals 9 and 10 in accordance with the manufacturer's instructions.
The alarm initiation portion uses terminals 4 and 5.
The supervisory portion uses terminals 3 and 14.
The EOL resistor value and IDC wiring configuration must match the connected fire alarm control panel or monitor module.
How to Wire Multiple D4120 Duct Detectors on One IDC
This is where installers can get into trouble.
When multiple conventional D4120 duct detectors share one initiating circuit, the goal is not merely to make every detector capable of initiating an alarm. The installation must also maintain the required supervision of the detectors and field wiring.
Simply connecting the alarm contacts of several detectors and placing the EOL resistor at the final detector without following the manufacturer's supervisory arrangement can leave earlier detectors inadequately supervised.
Conceptually, the installation must maintain:
The alarm initiating path through the required alarm contacts
The supervisory path through the required detector supervisory contacts
The separate operating-power circuit
The EOL device at the electrically supervised end of the initiating circuit
Example: Three D4120 Duct Detectors on One Monitor Module
Assume three conventional D4120 duct smoke detectors are connected to one compatible addressable monitor module.
All three detectors need to be capable of initiating the intended alarm condition.
But now imagine:
D4120 #1 develops a qualifying trouble condition
D4120 #2 loses operating power
D4120 #3 remains completely normal
If the wiring arrangement only provides effective detector supervision at the final detector, the fire alarm system may not receive the intended trouble indication from the first two detectors.
That is precisely why the supervisory path must be carried through the detectors in accordance with the manufacturer's approved wiring arrangement rather than treating the EOL resistor as nothing more than a device placed at the last physical box.
Do Not Loop Wire Under the D4120 Terminals
There is another small installation detail that matters tremendously for supervision.
Do not simply loop an unbroken conductor underneath a terminal screw.
System Sensor instructs installers to break the wire run at the detector terminals so the connection itself remains supervised.
If an installer loops a continuous conductor underneath a terminal and the terminal connection becomes loose, the circuit can potentially remain electrically continuous while the detector itself is disconnected from the circuit.
Breaking the conductor and terminating both ends properly makes the detector connection part of the supervised pathway.
D4120 vs. DH100ACDCLP Wiring
The D4120 replaced older System Sensor duct detector models including the DH100ACDCLP.
One helpful feature for technicians familiar with the older detector is that System Sensor retained several important terminal assignments.
Function
DH100ACDCLP
D4120
Alarm Contacts
4–5
4–5
Supervisory Contacts
3–14
3–14
That continuity made replacement work easier for technicians already familiar with the DH100 series.
However, never assume that an old detector can be replaced solely by matching terminal numbers. Verify the complete current installation instructions, voltage, accessories, relay functions, equipment compatibility, and approved system design.
Watch the Original D4120 Wiring Video
The following Fire Alarms Online video demonstrates the concept of wiring multiple conventional System Sensor D4120 duct smoke detectors while maintaining proper alarm and supervisory circuit integrity.
What Happens If Only the Last Duct Detector Is Properly Supervised?
This was the central lesson of the original Fire Alarms Online article.
Suppose several duct detectors have their alarm contacts connected to the same initiating circuit, but the supervisory arrangement is applied incorrectly so only the final detector effectively controls circuit supervision.
The detectors might still initiate alarms correctly.
That does not necessarily mean the installation is properly supervised.
A problem affecting an earlier detector could potentially go unreported if the circuit arrangement does not monitor that detector's integrity as intended.
Alarm operation and trouble supervision are two different functions. Both have to work.
Should You Meter the D4120 Contacts?
A meter can be extremely useful when troubleshooting or verifying dry-contact operation, but technicians should begin with the manufacturer's terminal diagram rather than guessing based on normally open or normally closed labels.
Relay and supervisory contact diagrams are typically shown in a defined equipment state, and contacts can change state when the detector is powered, normal, in alarm, or experiencing a trouble condition.
Before altering field wiring:
Verify the manufacturer's terminal diagram
Identify the contact common
Identify normally open and normally closed contacts
Verify the detector's current operating state
Use the meter to confirm the expected contact state
This is particularly important when troubleshooting older installations or equipment from different manufacturers.
D4120 Alarm Contacts vs. Auxiliary Contacts
The D4120 contains additional relay contacts that can be used for approved auxiliary functions such as HVAC fan shutdown.
These should not be confused with the alarm initiation contacts used to communicate with the fire alarm control equipment.
System Sensor specifically distinguishes the detector's alarm initiation contacts, supervisory contacts, and auxiliary relay contacts in its wiring documentation.
Always use the contacts intended for the required function.
Duct Detector Fan Shutdown Wiring
Duct smoke detectors are commonly associated with HVAC shutdown, but the required control arrangement depends on the mechanical system, applicable code, fire alarm sequence of operations, and approved design.
A duct detector can provide relay contacts for an approved HVAC control function, but the fire alarm initiating circuit and the fan shutdown circuit serve different purposes.
Do not treat the HVAC shutdown relay wiring as a substitute for properly reporting the detector condition to the fire alarm system where such reporting is required.
Testing the System Sensor D4120 After Wiring
After installation, the detector and associated fire alarm functions should be tested to verify the intended sequence of operation.
Depending on the installation, testing can include verifying:
Normal detector power
Alarm initiation at the FACP
Detector trouble indication
Loss-of-power supervision
Remote test/reset functions where provided
HVAC shutdown or other required control functions
Restoration after reset
Proper air sampling through the duct detector
Duct-type smoke detector testing is more than simply introducing smoke into the detector. Sampling-tube installations must also be capable of properly sampling the HVAC airstream.
Value specified by connected fire alarm control equipment
Legacy Model
DH100ACDCLP and related DH100 series applications
Frequently Asked Questions
What terminals are the alarm contacts on a System Sensor D4120?
The D4120 alarm initiation contacts are terminals 4 and 5. Terminal 4 is common and terminal 5 is normally open for the alarm initiation contact arrangement discussed here.
What terminals are the supervisory contacts on a D4120?
The System Sensor D4120 supervisory contacts used in the manufacturer's conventional 4-wire system wiring diagram are terminals 3 and 14. Terminal 3 is common and terminal 14 is normally open.
What terminals provide 24-volt power to the D4120?
The D4120 accepts 24 VAC/DC operating power at terminals 9 and 10. Follow the manufacturer's instructions for the specific power source and installation.
Can multiple D4120 duct detectors be connected to one IDC?
Multiple conventional duct detectors can be used on a compatible initiating circuit when permitted by the connected equipment and when wired according to the manufacturer's approved arrangement. Detector supervision, circuit limitations, EOL placement, power requirements, and control-panel compatibility must all be considered.
Where should the EOL resistor be installed?
The EOL resistor must be located at the electrically supervised end of the initiating circuit and be the resistance value specified by the connected fire alarm control equipment. For D4120 installations, follow the manufacturer's wiring diagram so detector supervision is maintained.
Is the D4120 a 2-wire or 4-wire duct detector?
The System Sensor D4120 discussed in this article is a 4-wire conventional duct smoke detector. Its operating power is separate from its alarm initiating contacts.
Did the D4120 replace the DH100ACDCLP?
The D4120 succeeded older System Sensor conventional duct detector products including the DH100ACDCLP. System Sensor retained alarm terminals 4–5 and supervisory terminals 3–14 between these models, which helped simplify many replacement applications.
Does a duct smoke detector replace open-area smoke detection?
No. A duct smoke detector is intended to detect smoke within an HVAC duct system for its listed application. It is not a general substitute for open-area smoke detection required elsewhere in a building.
The most important lesson when wiring conventional System Sensor D4120 duct smoke detectors is that getting an alarm at the panel is not the same thing as having a properly supervised circuit.
The D4120 alarm initiation contacts are 4 and 5, while the supervisory contacts are 3 and 14. When multiple separately powered duct detectors share an IDC or compatible monitor module, the complete wiring arrangement must maintain the required supervision of the detectors and field wiring.
Follow the System Sensor wiring diagram, use the EOL resistance specified by the connected fire alarm control equipment, break field conductors at supervised terminals rather than looping them underneath terminals, and test both alarm and trouble conditions when commissioning the system.
Technical Disclaimer: This article is provided for educational and troubleshooting purposes. Always follow the current System Sensor installation instructions, fire alarm control panel or module documentation, equipment listings, approved sequence of operations, applicable NFPA 72 requirements, adopted codes, and AHJ requirements. Disconnect and secure applicable power sources before performing wiring work.
If any packaged air conditioning units are run during the drywall installation phase of building construction to assist in the drying of joint compound, the fine dust created by sanding of those drywall joints may compromise the System Sensor smoke detector sensor heads within the duct housing. The 2D51 System Sensor Head in the InnovairFlex Duct Smoke Detector models D4120, D4120W and D4S may display a “maintenance” condition that will require replacement or cleaning of the sensing chambers on the smoke detector head. The “maintenance” condition will be indicated on the fire alarm control panel, on the sensor itself or power board of the duct smoke detector (the sensor LED will blink “red” every five seconds and the power board LED will blink “amber” every five seconds).
To avoid this condition, it is recommended that the System Sensor smoke detector heads be removed during the construction phase and replaced once construction is completed and the Certificate of Occupancy is issued. The sensor heads twist out for removal and twist in for insertion.
Below is an extracted image of the System Sensor duct smoke detector sensor head:
Cleaning the sensing chamber on the sensor head
If the System Sensor smoke detector heads are not removed during the construction phase and the sensor chamber becomes dirty causing a maintenance condition (it will not always be visible on the exterior black screen on the sensor head), the sensor head must be cleaned with compressed air. To clean the sensor head chamber, follow these step-by-step instructions see the video demonstration below:
1. Remove the System Sensor smoke detector head to be cleaned from the duct smoke detector housing by twisting it in a counterclockwise direction.
2. Remove the smoke sensor cover by pulling outward on each of the four removal tabs that hold the cover in place.
3. Vacuum the screen carefully without removing it. If further cleaning is required continue with Step 4. Otherwise, skip to Step 7.
4. Remove the chamber cover/screen assembly by pulling it straight out.
5. Use a vacuum cleaner or compressed air to remove dust and debris from the sensing chamber.
6. Reinstall the chamber cover/screen assembly by sliding the edge over the sensing chamber. Turn until it is firmly in place.
7. Replace the cover using the holes for the LEDs for alignment and then gently push it until it locks into place.
8. Reinstall the System Sensor smoke detector head in the duct smoke detector housing by aligning it in the housing and twisting it in a clockwise direction.
Note: Cleaning only the sensor head’s exterior black screen will not remove any drywall dust in the smoke sensing chamber. Note: Should the System Sensor duct smoke detector remain in a “maintenance” condition after cleaning, then it may require the complete replacement of the sensor head. The replacement sensor head is model 2D51.
Duct smoke detectors are a common device found in most fire alarm systems from the past and present. Whether the duct smoke detector is addressable or conventional, they both work the same way. A sample tube is inserted into the duct work and samples the air for smoke. If smoke is detected then the photoelectric smoke detector head within the duct smoke detectors sends an alarm signal to the fire alarm panel and shuts down the air conditioning unit.
With that said, most of these duct smoke detectors are located in areas that are not very easily accessed. These areas could include areas such as above t-bar or hard lid ceilings, attic spaces or roof tops. If the actual duct detector is located above a ceiling then access hatches or panels must be provided for obvious reasons. Also if the duct smoke detector is located in an area out of sight then you must provide a detailed description of the device location within the fire alarm control panel program as well as a remote LED plate on the ceiling below the device. This will assist the fire department, end user and technician locate the device. In fact, some jurisdictions such as L.A. County are requiring the remote LED plate to be installed any time you have a duct smoke detector above a ceiling and out of plain view.
With all of this information it is hard for me to understand how some of these local fire alarm installation companies are getting away with installing the remote LED plates on the wall directly above the thermostat controls for the particular HVAC unit. This picture was taken at a local restaurant in the Valencia, CA area. Now with these remote LED plates installed on the wall it makes it easy for fire alarm testing but it totally eliminates their true purpose. If there is a fire reported at the building with a signal coming from a duct smoke detector then the fire department is going to want to know exactly where it's location is the minute they enter. The LED on the wall does not exactly help.
NFPA 90A2, “Standard for the Installation of Air Conditioning and Ventilating Systems”, specifies that “Smoke detectors listed for use in air distribution systems shall be located as follows:
(1) Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 2,000 cfm (944L/sec).
(2) At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 15,000 cfm (7080 L/sec), and serving more than one story.”
Exception Number One:
“Return system smoke detectors are not required when the entire space served by the air distribution system is protected by a system of area smoke detectors.” (NFPA 90A, 2002, 6.4.2.2)
Exception Number Two:
“Fan units whose sole function is to remove air from inside the building to outside the building.” (NFPA 90A, 2002, 6.4.2.3)
Application Documents
There are several important documents that provide guidance concerning the performance, application and installation of System Sensor duct smoke detectors:
• U.L. Standard 268A, Standard for Smoke Detectors for Duct Applications
• NFPA Standard 90A, Installation of Air Conditioning and Ventilating Systems
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