I bought this 2 pack and used 1 detector. The remaining detector in the factory sealed pack, is 1st Alert's latest technology for combined CO and smoke detection. It is a 120v wired detector with adapters that allow direct wiring for new installs or plug & play with your old units wired plug. It took me 10 minutes to install...a really nice upgrade for CO alerts. You can also expand to link several detectors on multiple floors
