That was kind of where I was going after the OP's second comment, but I wasn't sure how to word it. This is close enough. I couldn't understand why you needed a fire alarm system and 120v smoke alarms.
A real fire alarm "system" doesn't use 120 volt stand alone/interconnectable "smoke alarms" they are two different animals. I think you may have some confusion of terminology here. When you get to the number of detection units you are talking about, cost of materials may not be as much of a difference as you may think. The true fire alarm system may still have some extra cost involved as it is likely to have stricter design and inspection requirements, and a potentially complex central controller, but can normally use smaller conductors for most of the premises wiring which saves cost from that perspective, and labor from companies that specialize in this is often a little higher - but they know what they are doing and get it right the first time which is usually significant justification in itself.
I'm sure many of us here in the Forum can figure out creative ways to link all these devices together as you would like. The short answer is they would all be in violation of not only the NEC but I'm sure local fire & building codes. You might as well get it into your head right off the bat and install a fire alarm system as Kwired pointed out. You'll come out with a better, more reliable system in the long run.
That liability factor is also what drives up the price of a true fire alarm system to some extent. It isn't that those kind of installers are expecting to have liability issues, it is that they put more time and money into education and keeping up with related codes/standards/product lines/etc. in order to be able to provide the most reliable systems they can, so that they don't have liability claims.
OK, I'll buy into that line of thinking. So, with respect to the OP, just asking that type of question and suggesting that such a Rube Goldberg might actually be feasable, automatically puts him in a category all unto his own.:happyyes:
If they are interconnected don't they all beep on a dead battery fault? Wow! 9 volt batteries ain't cheap and neither is the time to find it.
Don't they recommend changing batteries annually? If you do that you will seldom run into beeping detectors because of weak batteries - if you are replacing with good batteries. And as golddigger mentioned only the one with the weak battery beeps, if they all did that would make it even harder to find the weak one.
