*Ionization smoke alarms* require replacement every ten years. As
mentioned by Fred, the radioactive particle in them deteriorates to the
point where the alarm function becomes very slow to activate. Follow the
manufacturers advice. They're not that costly to begin with if you
spread it out over the ten year life. Cheap insurance.
Whatever it is that is the basis for a 10-yr replacement period for
ionization smoke detectors, it doesn't seem likely to be owing to the
decay of the isotope. Am-241 has a half-life of 432.2 yrs.
has only lost 2% of initial activity. That wouldn't seem enough of a
sensitivity loss to me.
I looked at a NFPA study that referenced a Dallas project that tracked
new installations for a period of 10 years that claimed only 27% were
still operational after that time.
However, it did not provide comprehensive data on the causes of the
failures other than most were simply not replacing batteries or taken
down when renters moved or otherwise destroyed. An apparent other
failure mode of significance had to do with not cleaning -- I suppose if
were in kitchen or the like grease buildup and all could do it, but
again it didn't explain what needed cleaning or what the source of not
being clean was.
But source decay wasn't listed at all and if it were the problem, none
would be operational as all would decay at the same rate and have same
relative source strength (presuming the initial population all of same
Yes, you should have at least one of each kind and get new ones
according to the recommended schedule.
But I wouldn 't necessarily get rid of the old ones. The one that came
with my house, is AC powered and mounted to the upstairs hall ceiling,
is 41 years old and still works, works well I think.
In addition to keeping it this long, I did another no-no. I opened it
up and across the buzzer I put a relay that closes the circuit to a
wire-pair that goes to the Fire input of my burglar alarm. So when the
smoke alarm goes off it notifies the monitoring company.
This was really bad for a while when I made hamburgers in a skillet at
the highest temperature and if I didn't turn off the burner when I
removed the food, it set off the 18 volt siren outside my house (a
couple times at 11PM) and the monitoring company called to see if there
was a fire. (No fire engines yet.)
But by cooking the food two big "notches" less than the hottest, at
about 80%, it no longer sets off the smoke alarm and all is good again.
Very interesting. Not surprised.