While visiting my mother-in-law, we wanted to get the smoke detector situation in her house up to par (this is in Massachusetts, we're from Europe, though). She has a natural gas fired forced air furnace in her basement; it blows warm air directly into various rooms instead of using radiators.

Above the furnace, we found the following fixture:

eviscerated ceiling fixture connected to two wire ducts

(Sorry for the overexposure, the lighting situation is not quite optimal there.)

To me, it looks a bit like the eviscerated remains of a smoke detector. What throws me off, though, is that it has two wire conduits leading to it: the bx conduit leads to a small box attached to the furnace, the smooth conduit leads directly to the breaker box. I'm used to smoke detectors running on an independent 9V battery, with no conduits. I'm also sure it's not just a broken light fixture, given the text on it says it's an "alarm". (The text also refers to detailed instructions on the inside of the base of the alarm, but the base is nowhere to be found.)

What is is thing and what would need to be done to repair it?

  • 2
    Some terminology used in the US: The tubing carrying wires is called conduit. Ducts or duct work carry the air from the furnace air handler to the various rooms. This type of central heating is called a natural gas fired forced air furnace. Commented May 25, 2021 at 19:42
  • 2
    Jim I thought about mentioning the conduit or wire type bx, ac or mc. This might help the OP in the future. +
    – Ed Beal
    Commented May 25, 2021 at 19:49
  • 1
    @JimStewart & EdBeal I edited my question to hopefully get the terminology right, thanks! Commented May 25, 2021 at 20:27
  • 4
    It's fairly common to have a heat detector right above the furnace that will shut the furnace down when activated. Why in heck someone removed it is beyond me. Heat rather than smoke is used to reduce false triggering in that application. If something goes horribly wrong and the temperature above the furnace shoots up, you want it shut down. The base for one can be similar to or the same as the base for a related smoke detector.
    – Ecnerwal
    Commented May 25, 2021 at 22:22
  • 3
    Great job with both the original post and the edit, BTW! And don't worry about your pic, that's definitely one of the better ones we get posted here!
    – FreeMan
    Commented May 26, 2021 at 11:52

2 Answers 2


That looks like it was part of a whole house smoke detector system.

Smoke detectors do have a limited life and when they start failing folks just pull them out.

Since the detector was close to the furnace it could have been smoke , carbon monoxide or both. I would see if the other detectors in the home work and in any case get replacements that are compatible with the others. You may find that multiple units may not be functioning.

On several jobs I have had to replace every smoke detector only the thermal detectors were still functional. In some cases I could get the same or similar model in other cases I had to replace the bases as the new models did not fit but used the same wiring.

You may be able to get more info off of the ring that is still there but getting a complete head will help you find compatible units.

  • 2
    I think Ed's on the right track. This is just the base plate for a smoke/CO detector. The wiring seems to indicate an interconnected model (two power wires and one data wire). You may be able to replace it with almost any interconnected unit that's compatible with the others in your home. Just unscrew the base plate to remove it and mount a new one.
    – isherwood
    Commented May 25, 2021 at 20:02
  • 1
    Well, if the wiring is there it makes sense to upgrade all of them to an interconnected set.
    – isherwood
    Commented May 25, 2021 at 20:42
  • 2
    @P2000 Thanks, good idea, but this is literally the only unit in the home thus wired. Therefore, I'm currently going with Ecnerwal's hypothesis that this was actually a heat detector specifically for the furnace (its location is directly above the furnace). I've searched the entire place for parts which might have belonged to it, coming up empty. Maybe the selection of heat detectors is small enough so that I can find the correct replacement. Commented May 26, 2021 at 1:24
  • 2
    I wonder if the battery units are newer, replacing some units from a wired system, and that this one wasn't tidied up when the others were, as it's in the basement out of sight.
    – Chris H
    Commented May 26, 2021 at 13:01
  • 5
    @mmathis Nonsense - if your CO detector goes off next to the furnace then something is broken and needs to be fixed. I always have a CO/combo detector in the furnace room (and everywhere else in the house) and it never goes off, nor should it. A furnace should make zero smoke and 100% of the CO should be going outside. If either of these is not true, you have a big problem.
    – J...
    Commented May 26, 2021 at 19:08

I have 4 First Alert smoke and CO detectors in my home that are all interconnected and are model SC9120B. The orange wire does the connecting. The sockets look exactly like the one in your photo. They make heat, smoke, CO, and combinations of these controls. They have a life expectancy of 10 years and are warranted for that long.

  • 5
    They often have a sticker/plate that shows the date of manufacture or date they "expire". My house is 12 years old and I just replaced them all 2 years ago. If the detector doesn't have this info, it's probably way older than that, and in need of replacement.
    – Jamie M
    Commented May 26, 2021 at 15:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.