Home Improvement → Re: Smoke/heat alarm. Can't get to battery compartment.

The First Alert that is beeping is Model SC9120B with a 2008 date on the back. I will try the breaker/disconnect procedure mentioned in this thread and see if that works. This is what First Alert says about the chirping in their FAQ's:
"It is likely that the reason your smoke detector keeps chirping and beeping is that the battery is low. Whenever your smoke detector keeps chirping, replace the battery immediately.
• A different device or appliance such as a security system, monitor, carbon monoxide alarm, or other device which has a similar low battery or alert signal.
• Some of the same factors that cause unwanted alarms can cause intermittent alarms: dust and insects in the detector or power interruptions in hardwired detectors.
• Improper wiring on AC or AC/DC smoke alarms. AC alarms will chirp every 5 seconds if the interconnect wire is grounded. The orange interconnect wire should NEVER be grounded; it should only be used to interconnect other smoke alarms or compatible devices."

Because you have such a mix of detectors interconnected; it'll be tough to nail down the cause... It could be the detector that's chirping; but it could be a different one causing it, too.

Whatever brand you have the most of, or want to go with, get one, and replace the chirping unit...

That particular model has a malfunction indicator. It will chirp 3 times every minute concurrent with the green led flashing.Does yours beep/chirp once or 3 times every minute ?

Edit: reading product reviews on amazon for this model includes several reports of false "low battery" indication with fresh batteries. There is a 7 year limited warranty, so you may want to contact customer support. You might be able to get a free replacement.