BRK/First Alert interconnect weirdness

Hoping someone can share some insight into the interconnect weirdness I’m experiencing with a residential remodel.

Existing home has three First Alert BRK SC7010BV smoke and CO detectors hardwired and interconnected. Each are on a different floor, first, second, and basement. Testing alerts and sounds all three interconnected units.

Finished he basement and instead of unfinished space it is now half finished with an unfinished half. The original detector is located in the finished space and extended the wiring to the new unfinished space and added a brand new SC7010BV detector interconnected.

Testing all four detectors, the original three still test and sound the other original detectors but the new detector does not get set off when testing the originals.

Thought it might be the new wiring so tested with a multimeter and connectivity and voltage is fire on the AC black and white, the red DC is fine as well.

Thought the new detector might be bad, so went and bought another SC7010BV from another store. Same result, the originals work interconnected but the second new one only works by itself.

Moving one of the original detectors to the newly wired spot, it works and sets off the original interconnected alarms. If I put either of the new detectors into any of the originally wired spots they only work by themselves. So it’s not a wiring issue.

What can be incompatible with the new SC7010BV units and the three original SC7010BV units?

I even went out and bought a First Alert SMCO100V-AC smoke and CO detector listed as interconnect compatible with the SC7010BV units and it also does not work interconnected when testing.

When I put all three new detectors on the three original or new wired spots none of them alert the others when tested. Why do none of these new alarms when tested set off any other interconnected alarms?

Very strange…no idea why none of them will work properly together: as long as you use the same brand all throughout the system there should be no problems. Maybe First Alert just changed something with the SC7010BV’s design over the years & because of that newer versions won’t work with older versions? If that’s the case the only thing you could probably do to correct the problem would be to just replace all the older detectors with new ones (which is probably a good idea anyway to make sure you have detectors that aren’t out of date, seeing as how smoke detectors need to be replaced every 10 years).

I was thinking the same thing, to just replace them all. So I installed the new detectors in the three original locations and when testing they do not set off each other! What are the chances I got three bad sectors purchased from three different places and two different models? It just doesn’t make sense.

The heck!? Okay this is very strange…it seems like to me that the only reason that wouldn’t work is because something with the interconnect wiring is wrong, but you’ve said that putting an old unit where a new one would be allows the old unit to set off the others, so I don’t think it’s that.

How do you have them wired? That may seem like a silly question to ask, but sometimes you might forget to wire a leg further down the line, or maybe there’s a length of wire that’s broken or terminates early in between detectors such that there’s not a continuous length of wire from one alarm to the next.

What you might try is running a new set of red interconnect wire from one detector to the next.

It sounds like you are describing some sort of wiring disconnect between the three original and new alarms.