Fire Angel

We have four of these installed, two with a 7 year battery life, two with around a 12 month life. The last two nights at 4.30 am onward ONE went into a tweeting fit every two or so minutes...no fire, just annoying enough to keep me awake and not loud enough to give me a clue which of the four it was...I finally decided it wasn't any of them but the Carbon Monoxide warning box in the lounge, so day before yesterday, I replaced THAT battery and had a full night's sleep...fate chucked in it's hand during Friday night and decided to repeat the tweeting and I definitely isolated the sound, to the alarm in the hall, this time. Good job I keep some batteries in reserve:)