Every time we change the clocks we remind you to change the batteries in your smoke detector and carbon monoxide detector.  But that may not be good enough.  Did you know that smoke detectors have a life span of about 10 years, and then lose their effectiveness?  For carbon monoxide detectors it's about half that.  Experts say if your smoke alarm is 8 to 10 years old (or older) you should replace it, and 5 to 7 years for carbon monoxide detectors.  You manufacturer can tell you the recommended time to replace various units.  The unit we showed this morning is a combination smoke/carbon monoxide detector, like the one shown here, from First Alert.