By Mark – homeowner, landlord, and the guy who learned duct smoke detectors don’t come “ready-to-go” out of the box.
🏠 Introduction: Why Accessories Matter
When I first bought my Daikin D4120 duct smoke detector (which is basically the same as the System Sensor D4120), I thought I was done. I assumed it came as a complete package: mount it, wire it, and move on.
Turns out? Not even close.
The detector itself is just the brains of the system. To actually make it work—and to satisfy inspectors—you need accessories. Some are non-negotiable, like sampling tubes and relays. Others, like remote test stations, aren’t always required but make your life way easier.
In this guide, I’ll walk through each accessory I used (and some I wish I had bought sooner). I’ll break down what they do, how much they cost, and whether they’re truly essential or just nice to have.
🌬️ Sampling Tubes: The Detector’s Lifeline
Think of sampling tubes as the “nose” of the detector—they pull air from the duct into the sensing chamber. Without them, the detector can’t do its job.
🔹 How They Work
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Installed across the width of the duct.
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Perforated with holes that face into the airflow.
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Carry sampled air into the detector chamber.
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Must be capped at the far end to create suction.
🔹 Sizes & Options
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Short: 1–2 feet (small ducts).
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Medium: 3–5 feet.
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Long: 6–10 feet.
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Can be cut to fit exact duct width.
🔹 Cost
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$20–$40 each.
🔹 Mark’s Tips
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Always buy slightly longer and trim down.
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Make sure the holes are facing airflow direction.
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Don’t forget the end cap—without it, you’ll fail airflow tests.
📌 My lesson learned: On my first install, I forgot to cap the end. The detector wouldn’t trip during testing. Once I added the cap, it worked perfectly.
🔄 Exhaust Tubes: Don’t Skip Them
The partner to the sampling tube is the exhaust tube, which lets sampled air exit the detector and return to the duct.
🔹 How They Work
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Short stubby tube.
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Mounted downstream of sampling tube.
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Keeps airflow continuous through the detector.
🔹 Cost
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Usually included with the detector.
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Extra or replacement tubes: $15–$25.
📌 Mark’s reminder: Inspectors will check both tubes. Don’t assume the small one is optional—it’s part of the airflow circuit.
🎛️ Remote Test/Reset Stations: Worth Every Penny
The first time I tested my detector, I had to climb a ladder into the return plenum, spray test aerosol into the tube, and then hit reset on the detector itself. That was enough for me to order a remote test station the next day.
🔹 How They Work
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Wall-mounted unit with test/reset buttons and sometimes indicator lights.
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Connects via low-voltage wiring.
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Lets you test or reset detector without accessing duct.
🔹 Cost
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$50–$100.
🔹 Code Requirements
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Often required in commercial spaces where the detector isn’t easily accessible.
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Optional for homeowners, but highly recommended.
📌 Mark’s note: I didn’t think I needed one. After climbing that ladder once, I decided it was money well spent.
⚡ Relay Modules: The Real Power Players
If the detector is the brain, relays are the muscles—they make things happen when smoke is detected.
🔹 What They Do
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Shut down HVAC fans to stop smoke spread.
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Send signals to fire alarm panels.
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Trigger auxiliary equipment (like dampers).
🔹 Types
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Alarm relay (triggers alarms).
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Supervisory relay (sends trouble signals).
🔹 Cost
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$30–$60.
📌 Mark’s perspective: You’ll almost always need at least one relay. Without it, your detector is just a fancy noisemaker.
🛠️ Other Helpful Accessories
Beyond the big three, a few smaller items are worth budgeting for.
🔹 Mounting Hardware & Sealants
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Foil HVAC tape: $10.
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Screws/anchors: $10.
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Duct sealant for airtight fit: $10–$15.
🔹 Labels
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Inspection labels: $5.
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Required for compliance in many areas.
🔹 Test Aerosol Spray
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Cost: $15–$20 per can.
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Required for annual testing.
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One can lasts 10–12 tests.
📌 Mark’s tip: Keep a can on hand. Inspectors may ask you to demonstrate a test on the spot.
📊 Must-Haves vs. Nice-to-Haves
Not everything in the catalog is mandatory. Here’s how I break it down:
✅ Must-Haves
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Sampling tube.
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Exhaust tube.
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Relay module.
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Test aerosol spray.
👍 Nice-to-Haves
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Remote test/reset station.
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Extra mounting hardware.
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Labels for convenience.
📌 Mark’s takeaway: If you’re trying to save money, skip the nice-to-haves—but don’t cut corners on the must-haves. Your inspector won’t let you slide.
📊 Real-World Cost Scenarios
Scenario 1 – Homeowner Install
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Detector: $200.
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Sampling tube: $30.
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Relay module: $40.
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Test spray: $20.
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Total: ~$290.
Scenario 2 – Small Business
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Detector: $200.
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Sampling tube: $30.
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Relay module: $40.
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Remote test station: $75.
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Accessories: $50.
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Total: ~$395.
Scenario 3 – Commercial Building
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Detector: $200.
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Multiple sampling tubes: $60–$100.
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Relays: $100.
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Remote test station: $100.
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Accessories: $75.
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Total: ~$500–$600.
📌 Mark’s bottom line: Plan for at least $100–$200 in accessories on top of the detector.
📜 Compliance References
NFPA 90A
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Requires detectors in systems ≥ 2,000 CFM.
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Tubes and relays mandatory for functionality.
NFPA 72
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Requires test/reset functionality.
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Remote stations often mandated in inaccessible locations.
UL Standards
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Accessories must be UL-listed for compatibility.
📌 Mark’s note: If it’s not UL-listed, your inspector won’t approve it.
✅ Key Takeaways
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The D4120 doesn’t work out of the box—you need accessories.
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Sampling and exhaust tubes are non-negotiable.
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Relays make the detector actually control your HVAC.
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Remote test stations aren’t always required but save time and frustration.
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Budget $100–$200 extra for accessories.
📌 Mark’s advice: Don’t cheap out on accessories. They’re what make the detector code-compliant and functional.
🔗 Verified References
📝 Final Word from Mark
When I bought my first D4120, I thought I was saving money by skipping accessories. That decision cost me an extra trip to the hardware store, a failed airflow test, and a grumpy inspector.
Now, I budget for tubes, relays, and test stations from day one. I’d rather spend $100 more upfront than deal with surprise costs, failed inspections, or worse—an unsafe system.
Bottom line? The detector is only half the story. The accessories are what make it work.
In the next topic we will know more about: Signs It’s Time to Replace Your Old Duct Smoke Detector
