When the smoke detector first starts signaling low battery with short widely spaced high pitched chirps, or if when you press the test button the alarm does not trigger.
After replacing the batteries, press the test button. If the alarm does not trigger with fresh batteries, replace the smoke detector.
If your home smoke detector is tied into your electricity and is equipped with a battery backup you should not need an additional smoke detector. You should insure that the batteries are charged and that the smoke detector is functioning properly at least once a month if not more often.
Very often, the beeping coming from a smoke detector is a sign that the batteries may be going dead. If you want the beeping to stop, change the batteries.
It's best to replace them once a year so that your smoke detector is up-to-date and warns you of any fires. Here's is a website that gives you more information on why you should replace the batteries http://www.cpsc.gov/cpscpub/pubs/5077.html
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Rechargable batteries should last for over a year. Often they only come with a non-defective warranty.
According to my dealer, it should be changed at 150,000 km.
Passwords should be changed every month.
Check the laws for your specific country. Quite often, there are specific regulations about that. It may also depend on the type of batteries.
It is recommended to change the batteries in your thermostat once a year to ensure it functions properly.
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When its old
When it is dirty.