failing hard-wired smoke alarms

That is probably a good idea. While you are at it, either get an alarm with battery backup or install one or two battery alarms so you have protection during a power outage. Ed

I am going to guess that you have backup batteries and one or more of them is going out. Time to replace them. I suggest also getting a can of compressed air and blowing them all out to get rid of any spider webs and dust that is sure to be there.

I would guess you have a spider or spider web in one of the units (or more than one unit). Just blowing with compressed gas may not work. I had to completely disassemble my built in unit to get rid of the web filament. I don't know about your particular brand but 5 years isn't much. My built in smoke alarm is 25 years old. I can understand interconnected units if you have a very large house, but it doesn't make any sense unless you have at least 5,000 square feet and then you might want something a bit more sophisticated.

If your built in system gives you a lot of trouble, just replace it with battery operated ones (5 units should cost less than $50). If you switch to battery operated ones, forget changing the batteries every time daylight time switches on and off or a specific date each year. I think that's promoted by the battery manufactures. My 9 volt alkaline batteries usually last about 2 1/2 years but the damn thing tell you when the batteries need to be changed.

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Well, I was afraid that might be the case someplaces, but the problem with interconnected alarms is you don't know which detector set it off, so you don't know where the smoke/fire might be and could be critical to your evacuation route. It makes my head hurt to think of the way simple things get screwed up. The obvious solution, if they decide to wire them together (which is a dumb idea), is to have a different signal (frequency or multiple blasts) key to each detector so you would know which one did the detecting.

If you have the type I have (based on light diffusion by smoke), just blowing it may not solve a spider web problem because the web (or spider) could be in the tunnel with the detector and sealed which would just blow the web/spider back into that tunnel and not out of the dectector.

You could locate the detector by just wiring around a different one for a night or two and when you find a silent night you would find which detector was having a problem. Then, just replace that unit. Good luck. Personally I would do what I want makes the most sense to be safe, regardless of laws, and then change back to whatever was required when I sold the house.