My smoke detector (Firex 120-1123C) started chirping. When I removed them I saw it was out of date so I assumed that was the cause. After a lot of research I found a compatible replacement (Kidde 4870). Having replaced the old ones with new ones I thought I solved the problem but the new ones chirp as well. They seem to know when you are just falling asleep, then they start chirping. As they are brand new what could the problem be?  I've checked in the loft for a fourth alarm but there isn't one.

Hi Steve

Here two initial checks:

1) Check that the green LED is on to indicate healthy mains power supply. If not, has the fuse been switched back on?

2) Is there any other alarm in the room (eg a CO alarm)? Sometimes the low battery beep of a CO alarm can be perceived as a low battery beep of a smoke alarm.

Failing this you might need to ask an electrician to check the correct wiring of your alarm. Maybe the power supply is not always on live and instead switched by your light switch).

 

Harry

 

 

 

 

I'm owning one of the  Firex 120-1123C as well, when it start chirping. It means that the 9V battery need to be replaced. Hope this help. :)