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

Many regions specifically forbid tying garages into the building ventilation system, due to CO/fume/vapour concerns. Therefore, to legally connect it to the HVAC, it can no longer be a garage; to the point that some jurisdictions even require the garage door to be removed, or fixed in place...

No doubt there are places without that requirement... But I'm not interested in an argument for the sake or arguing... So I'm just going to drop it.

It's actually a change of occupancy.

As for air conditioning a garage, it must have it's own air handler and venting separate from the home to preclude the possible mixing of CO from within the garage, into the home.

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."

I know a lot of garages with supply registers that are not on a separate system. In fact I don't know of any that has it's own system.

We need to get back on the OP's original topic which was not heat detectors in garages

On a national level at least with residential I don't think they are required, at least not under NFPA 72 C39.

As far as I know they are (and I am paraphrasing) supplemental devices, that is to say while they may not be required by the code their use however is allowed by the code.

Of course they must be properly listed for the application.

Some common uses in a residence might be the garage or a laundry room or even a kitchen if placed carefully.

I have seen many as well, but that doesn't make it safe.

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...

Un-plug it and take it down and wait a while, does it or anything else beep? If not you have solved the problem, the detector you took down is pooched and needs to be replaced.

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.

Don't know much about them if you say something untrue like that. The entire point of code is to make stuff safer so why require a heat detector if it puts you at risk? LOL

I know this may sound like a dumb thing but did you ensure that the battery is in the right way? I had the same smokes and after changing the batteries one kept chirping. In my haste I inserted the battery the wrong way and the smoke kept chirping...

Smoke alarm reinstalled and no longer beeping. I was properly inserting battery. I took the alarm and went shopping but could not find same model and was unable to tell which new alarms had matching plug because they are all sealed in that adult proof plastic packaging. There was a Kidde that came with 3 different adaptor plugs. I bought a fresh pkg. of Duracell 9v's & when I put new battery in it stopped beeping. Expiration date on batteries I was using previously was 2013 but they tested in green. However when my stepson changed batteries in identical garage alarm this past Spring it still kept beeping. He tried a different battery and it stopped, so I am wondering if my old batteries may have been the problem even tho they tested as good. So all quiet on the Midwestern front for now. Whether it stays that way remains to be seen. I'm sure I put new battery in that alarm about two months ago. Will start gradually replacing the older alarms and will use all same brand. I assume they do not all have to have the dual smoke & co2 function.

That is correct. In fact they probably shouldn't be all the same type. What is important is that they have been tested to be compatible with each other. If you look at the user manual there should be a section discussing the interconnect feature which will list all sensors/detectors that are approved.

The sensor/detector type should be based on the location. Generally you want to pick a device that can detect as many possible threats to your life and property as possible but you also want to make sure that you are not subjected to false alarms (e.g. avoid ionizing smoke detectors near bath/shower were they could be set off by water vapor; avoid CO detectors too close to a gas stove in the kitchen or inside a garage).