You have replaced the battery for your hard wired smoke alarm, yet it keeps beeping. This happens if the battery gave low battery warning beeps prior to replacing the battery or even with scheduled replacement.
You have searched online for the manual but can’t find it. If you did, you realise this scenario is not covered.
It appears the solution lies in cancelling a persistent low battery status. Most devices are power cycled when a battery, its only power source is removed and replaced, not so for a device still connected to the mains power.
So to enact this solution you will need access to the switch for the circuit on which your smoke alarm lies in the distribution box.
Steps in the following order worked for me.
Main Power switch off
It will still keep beeping (must be a capacitor in the power train). Wait till it feebly beeps till it dies.
Wait a little longer- about 5 minutes.
Replace battery – will beep on lid going into place.
Turn on mains power
Give a good vacuum of the device while there, if you can.
If this doesn’t work, and you’ve tried many other attempts to solve it, it is important to keep in mind that:
1)If you have this specific model its approaching end of life and probably needs replacing anyway (10 years)
2)It may still be an ionisation type detector (usually has a radiation warning symbol on it) which is not the preferred type. Photoelectric detectors are preferred – another good reason to replace it.
Hope this works for you – let me know in the comments if it did.