PermitsPros is a California and nationwide permit expediting firm specializing in building permits and fire protection. Since 2008. The D4120 is the duct detector our desk sees most; this page pairs its specs with its listing path and its permit path.
The device, from the manufacturer's published data sheet
The D4120 is System Sensor's InnovairFlex 4-wire photoelectric duct smoke detector — a pivoting housing that fits square and rectangular footprints and mounts to round or rectangular ductwork, with a plug-in sensor head. Sources: System Sensor data sheets A05-0419-000 and HVDS00502.
- Configuration: 4-wire conventional housing (the family's 2-wire housing is the separate D2); the unit is composed of the D4S sensor component and the D4P120 power board, and the sheet's ordering information names the 2D51 conventional photoelectric sensor head
- Power: 20–29 VDC, 24 VAC 50–60 Hz, or 120 VAC 50–60 Hz
- Outputs: two DPDT Form-C relay contacts — alarm initiation contacts rated 2.0 A at 30 VDC, auxiliary contacts 10 A at 30 VDC / 250 VAC, supervisory contacts 2.0 A
- Sampling tubes: DST series — DST1 (to 1 ft), DST1.5 (1–2 ft), DST3 (2–4 ft), DST5 (4–8 ft), DST10 (8–12 ft); the patented tube installs from the front or the back of the housing with no tools
- Operating envelope (per sheet): air velocity 100–4,000 ft/min; up to 50 units interconnectable for a common (multi-fan) shutdown
- Standard for the application: the sheet's architectural specifications state the housing is UL listed per UL 268A, specifically for use in air handling systems
The California listing path
The D4120's California listing is printed on the manufacturer's sheet: CSFM 3242-1653:0207, category 3242 (duct smoke detectors), covering the assembly — housing, D4S sensor, D4P120 power board. The discipline that matters in the field: the sensor head also carries its own listing under a different number, and it is a different document. File the assembly's current BML sheet and the head's evidence together, and check both for currency at submittal — BML listings expire and are revised, and "current listing" is the exact phrase reviewers bounce packages with.
What the submittal package needs for this device
- The housing data sheet and the sensor-head data sheet
- The current CSFM listing sheet for the assembly (3242-1653:0207) plus the head's listing evidence
- The DST sampling tube model and length per location, matched to the duct widths on the mechanical drawings
- Fire alarm drawings showing the detector, its circuit, the fan-shutdown relay, and the air handler it controls — the full picture is on our duct detector permit page
- Battery and voltage-drop calculations for the system
The permit path
New or relocated duct detectors are fire alarm permit work with plan review; the reviewing authority is the fire prevention division or district, coordinating with mechanical review. Exact like-for-like head replacement may be minor work in some jurisdictions — it varies by jurisdiction. PermitsPros assembles the package, files it, carries corrections, and delivers the issued permit: $250 filing service or $495 complete package; city and district fees separate. Start at portal.permitspros.com or call (888) 907-9511.
Spec facts on this page are drawn from the manufacturer's own published data sheet, cited by document number. Listing numbers are checked at submittal time — California listings expire and are revised, and reviewers ask for the current sheet.
Questions and answers
Q: Is the System Sensor D4120 a 2-wire or 4-wire duct detector?
A: The D4120 is the 4-wire conventional InnovairFlex housing — its published power inputs are 20–29 VDC, 24 VAC, or 120 VAC. (The family's 2-wire housing is the separate D2 model.) Confirming which housing is on the device schedule matters: the 2-wire and 4-wire housings wire, and file, differently.
Q: What sampling tube does a D4120 use?
A: System Sensor's DST series, sized to the duct width at each location: DST1 up to 1 ft, DST1.5 for 1–2 ft, DST3 for 2–4 ft, DST5 for 4–8 ft, and DST10 for 8–12 ft, with an ETX exhaust tube. The tube model and length belong on the submittal: the reviewer checks the tube on the cutsheet against the duct width on the mechanical drawings, and a missing tube line is a routine duct-detector correction.
Q: What is the D4120's California SFM listing?
A: CSFM 3242-1653:0207 — category 3242, duct smoke detectors — printed in the agency listings on the manufacturer's sheet (issue HVDS00502) and covering the D4120 assembly: the housing, the D4S sensor component, and the D4P120 power board. One warning from the listing design itself: the sensor head carries its own separate listing under a different number, so the assembly's sheet and the head's sheet are both part of a complete package — and both are checked for currency, because listings expire and are revised.
Q: Why does a D4120 submittal need more than one cutsheet?
A: Because the product is an assembly: the InnovairFlex housing, the D4S sensor component, and the D4P120 power board, with the sheet's ordering information also naming the 2D51 conventional photoelectric head. Each element has its own data sheet and listing evidence. A package with only the housing sheet reads as a partial device — it is the most common incomplete-cutsheet bounce in the duct category.