A smoke detector in our house is about 4 years old. It is 120V hard wired and started beeping intermittently recently. We replaced its battery backup with a new 9V one, but the alarm still beeps every few minutes. Why might this be happening and do I need to get a new smoke detector?
thanks much..

Sometimes dust and spiderwebs and do this.

Try blowing them out.

If that does not work, then replace themm.