Detectors can also be hard-wired into the electrical system. But never hard-wire a detector to a circuit that can be turned off at a wall switch. Because smoke rises, each detector should be mounted high on a wall or on the ceiling to detect the first traces of smoke. For a wall-mounted unit, the top of the detector should be 4 to 12 inches from the ceiling. In a room with a high pitched ceiling, mount the detector on or near the ceiling's highest point.
Most home fires start in living areas - the den, family room or living room.
On a floor with no bedrooms, install the required detector in or near the living area. If a stairway leads to an upper story, install the detector in the path where smoke would travel up the stairs. Don't install a detector near a window, door or air register where drafts could reduce its sensitivity. Locate a basement smoke detector close to the stairway leading to the floor above. It's extremely important to test and clean all detectors regularly.