As a home inspector, and the owner of Magpie Property Inspections, an important aspect of a home inspection for me is checking on the presence and condition of smoke and carbon monoxide detectors. During this home inspection in Aurora Colorado it was noted that the original smoke detectors had been removed and updated with a combination carbon monoxide and smoke detector. There are several manufacturers which offer combination units like the one pictured on top above.
Most manufacturers recommend replacing smoke detectors at between 7 and 10 years. It is always a good idea to check the smoke detectors in your home and keep the batteries changed at least once per year. Whether the units are hard wired or battery only, they contain batteries for either primary power or back up protection. Many smoke detectors become discolored as they age, like the second picture above, so if you have a home that is a few years old or more, and the smoke detectors are becoming brown or yellow, it is likely time to change them.
One other note, in many locations it is now required that homeowners have carbon monoxide detectors installed in the dwelling as well as traditional smoke detectors. In my home state of Colorado, a carbon monoxide detector must be installed and operational within at least 15 feet of all bedrooms or sleeping areas. Replacing the old units with combination units near the bedrooms is a great idea. You can also install carbon monoxide only devices that plug in the wall outlet to make it simple and inexpensive to provide this vital protection for the occupants of your home.