Sensing an interconnected smoke alarm system

We are building a new house. Using information on these forums, I was planning to attach a relay and a Z-Wave window sensor to each smoke alarm, so I could implement various activities if any of the alarms go off. I’m using First Alert hardwired smoke alarms. They have an interconnect wire, which is what I planned to connect the relays to, ending up with one sensor per alarm.

I just found out today from my electrician that I am required to interconnect all the alarms in the house. This raises the following questions, which I’m hoping project here might have the answers to:

  1. Since the alarms all have to be interconnected, when one goes off, the interconnect wire will get a voltage for ALL the alarms. So does this basically mean it only makes sense to have one relay/sensor combination for the whole house, rather than one for each alarm?

  2. Is there any way I can still set things up with multiple sensors do I can know which alarm triggered all the others? It seems odd to have all the alarms in the house go off, without having any good way to know which alarm started it all. But with just a single interconnect wire, I’m not sure there’s any way to pull that off?


  1. yes - add another relay for CO if you have a detector that will pick that up.
  2. The only way I know to do this directly is by switching to Nest alarms but I assume you would’ve chosen that to start with if you wanted to spend that kind of money. One indirect alternative is to scatter temp sensors and monitor for above-normal values…
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A number of folks have done this. My alarms are Kidde and I used a variation of what’s described in the following link.

No doubt a similar relay is available for your First Alert alarm. I found mine on Amazon, YMMV, of course!

You might also find the following list of ideas & projects useful in your exploration…

Lots of ideas there!

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Thanks, MarkTr. That’s what I thought. Oh well, better than nothing, I guess. :-). So now I have a bunch of extra relays and sensors, but I’m sure I’ll find things to use them on! I have First Alert combo smoke/carbon detectors, but as far as I can tell there is just one interconnect wire. There must be some way to distinguish between a smoke alarm and a carbon alarm, since they send different numbers of beeps, but I haven’t figured out yet if there’s an easy way to get the relay to make the distinction.

Thanks for that link, John C. I actually was already using that to make my plans, and I used it as my guide for what to purchase. I still plan to use that to guide my installation.

The part I missed was that I had been planning to install my alarms “not interconnected” so I could put a relay and sensor on EACH alarm, so I could know which room was triggering the alarm. Only after I made my purchase did I discover that local code requires my alarms to be interconnected. So that’s the variation I was asking about. Thanks again!

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I believe there are separate model relays that open/close on the appropriate signals.

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Correct at least for Kidde


Okay, I do have the Kidde relays for smoke and CO, but I have First Alert alarms. I’ll try them and see if they work together. Otherwise, I’ll just live with one type of detection. Thanks again to everybody!