Since old Firex and new replacement have different wiring harness, the existing wiring harness needs to be changed.
If you don't want to rewire, you need to get a quick convert adapter Kidde KA-F and perform 3-easy-step installation:
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The Kidde i4618 is an AC/DC powered, ionization smoke alarm that operates on a 120V power source with 9V battery backup. This alarm uses ionization sensing technology. Ionization sensing alarms may detect invisible fire particles (associated with flaming fires) sooner than photoelectric alarms. Photoelectric sensing alarms may detect visible particles (associated with smoldering fires) sooner than ionization alarms.
The front-loading battery door allows user to change the battery without removing the alarm from the mounting bracket making battery replacement easy and convenient. This smoke alarm is available in a 6-piece cut case with tray for easy display as well as a 6-piece bulk pack for contractors and property owners. This unit is a UL Listed product with a 5-year limited warranty. Kidde strongly recommends that both ionization and photoelectric smoke alarms be installed to help insure maximum detection of the various types of fire that can occur within the home.
Features & Benefits