Mains smoke alarm beeping

[tonysab]

This is very strange. About 10 months ago we had some building work done and 2 new mains smoke alarms installed. At 5am today one of them started beeping, I got up and took it down, it had a 9v backup battery in it, a Duracell. Strange I thought, that it had run out already. This evening the other one did exactly the same. So I've just been out and got 2 new duracells to replace them. Fitted them and guess what, 5 minutes later one of them starts beeping again! What's going on? What is really strange is that if I take it down, obviously leaving the mains bit attached to the ceiling, and I remove the battery, it still beeps!
Any advice gratefully received.

 

[JohnD]

maybe your house is on fire

 

[Space cat]

Strange I thought, that it had run out already.

Are you sure the things are actually connected to mains power? Take the batteries out, put the alarms back on their bases and press the test buttons.

Fitted them and guess what, 5 minutes later one of them starts beeping again! What's going on?

Our alarms have a 'silence' switch to shut them up when we overdo the toast. The alarms beep every few minutes for some time afterwards, presumably to remind us that they have been silenced. If yours have these switches maybe you pressed one by accident when you put the new battery in. Or maybe it just got confused when the battery went in. It takes careful design to ensure that circuits behave correctly at power-on. Leave it a while and see if it goes quiet.

What is really strange is that if I take it down, obviously leaving the mains bit attached to the ceiling, and I remove the battery, it still beeps!

A few beeps with no apparent power source aren't so mysterious. There must be a capacitor in there with enough stored charge to run the alarm for a bit. Have you ever watched the standby light on a TV when you switch off at the wall. Chances are it won't go straight out.

If the alarm is still beeping long after you took the battery out then that's a real puzzle - or is it? I vaguely remember a thread in this forum about an alarm that kept beeping in the middle of the night despite having had its battery removed. Upon careful investigation its owner found that the noise wasn't coming from the alarm at all.

 

[tonysab]

Thanks for that Space Cat. Yes I wondered if it was connected to the mains so tried what you suggested - taking the battery out, putting it back on and pressing the test button - and it did sound ok. No more beeps for about an hour now. It's probably waiting until the middle of the night again.