I bought a house and just replaced 8 hardwired smoke alarms that were 10+ years old units like this. They had the typical high voltage red/white and red interconnect wires, with thick wires.

However there's a 9th unit in my house that looks very different and has a different wiring configuration. It is only a few inches away from one of the other units.

The wiring / size of wires / voltage is different on this one.

What is the purpose of having this separate setup? Should I replace it with a similar unit (e.g. same model number)? What specifically would I be searching for if I wanted to find an alternative unit to replace it with?

There is a wired alarm system in the house, does this unit tie into it somehow and maybe that's why the wiring is different? If so does that mean that only this detector is tied into the alarm?

unit backing


2 Answers 2


Two alarm systems don't make for twice the protection

What you have here is a smoke/heat combination detector for a central station fire alarm system. Note that it cannot alarm by itself -- it requires the "brain" of a compatible central station fire alarm control panel as well as a notification appliance such as a horn or a strobe connected to the panel in order for the system to work and sound the alarm when smoke or excessive heat is detected.

So, in your case, this smoke detector is the detector for your alarm system. The rest of the hardwired alarms will set each other off when they detect smoke, but only this detector will alert the alarm system when it detects smoke, provoking the alarm system into dialing the monitoring service as well as setting off whatever horns, strobes, or whatnot that have been connected to the alarm system.

In general, buildings only use one type of fire alarm system (multiple-station or central station), not the mix of both you see in your house. Multiple-station systems are easier and cheaper to maintain and also more familiar to home owners; however, a properly configured central station system can automatically notify the FD if there's a fire, and can also supervise the fire alarm wiring so that the alarm panel will tell you there's trouble if a wire breaks or alert the monitoring company that someone is tampering with your house's fire protection systems. A properly installed central station fire alarm system with monitoring also may give you a discount on homeowner's insurance; however, monitoring itself is a monthly expense, and central station systems can be more costly and difficult for an individual homeowner to maintain.

  • Thanks! Do you think I should just replace this unit with a new version of the same exact model?
    – philfreo
    Commented Jan 14, 2017 at 3:00
  • 1
    @philfreo -- for now, go ahead and replace it with an identical model. You'll want to figure out what you want to do long-term though. Commented Jan 14, 2017 at 16:58
  • You can replace that smoke detector with an "Interconnect" device (like this one) which will take the signal from your 8 interconnected smoke alarms and connect it to your central alarm system. You need to find an interconnect model that is the correct one to go with your systems though - the one I linked is not necessarily the correct one for you.
    – Moshe Katz
    Commented Dec 12, 2017 at 19:26

Hardwired fire detectors are different. I have them in my house and I like them. There can be upto 6 or more wires involved. There is power pair from the alarm system, a signal pair to indicate trouble to the panel, and often they are strung together so that one will trip the sounders in all of them.

ThreePhaseEll alluded to "supervision". My detectors power cycle every 24 hours as a functional check. And since they run from the panel, there is no battery to replace. (except the main one)

Unfortunately, other than 12v power and NO/NC contacts, the other signaling protocols are often proprietary and change with the times. I've only had one unit fail in my house, and when I went to replace it, BRK told me they didn't make that any more and offered a similar but different model. I swapped that into a floor that only had a single on the run.

You have to look at the requirements of the panel, and the wiring in your walls to decide how to go. Your photo shows 5 terminals on the base (one may be a common) but "Two Wires" stamped in plastic. The fact that only two wires are connected and a resistor shows that there may be some interesting signalling going on here. Vendors of these devices often have options to install in different applications. If you can find the documents on that panel, it will help your choice.

Another comment, in the modern era, these interactions can get interesting. My alarm panel has no concept of what to do for CO detectors (now required) and the detector laws are changing, so I have to replace my hw detectors every 10 years no matter what.

  • 1
    The 10 year lifetime has nothing to do with the power source (hardwired or battery), but rather the smoke- and fire-detecting components within the unit. Ionization units use a small radioactive source, which needs to be replaced, while photoelectric units may just be replaced as a precaution (not sure). Current recommendation is to use photoelectric units only and not ionization ot dual sensor units. Note that this recommendation is for smoke alarms only; CO alarms must be installed as well
    – mmathis
    Commented Jan 14, 2017 at 20:16
  • I understand, that's why it's a concern. In my experience the cheap battery powered units don't last that long, and replacement is easy. My house units have lasted 25 years and are still "working". And replacements are not that easy to find.
    – DaveM
    Commented Jan 15, 2017 at 5:27

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