ARWadsworth wrote:

"no-one" wrote
We have mains powered (with battery backup) smoke alrms on three
floors. Today they have started going off for no apparent reason -
sometimes full aalrm, sometimes a few pips.
Is the cause likely to be the batteries need renewing?
More likely that one of the smoke detectors is faulty or full of dust

Mine had a honking BIG spider in it, gave me quite a fright when I
gingerly took it apart [well unit sort of separates from the mains feed
which was still live] and the spider ran over my hand.