FIRE ALARMS

do they ever need to be completely replaced?

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The fire alarm people say yes. Apparently there is some radioactive material involved, that deteriorates over several years.
Fred
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On 7/30/2020 1:49 PM, Fred McKenzie wrote:

*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.
Gil
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On 7/30/2020 6:42 PM, Gil wrote:

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.
>> t_halfC2.2; >> lambda=log(2)/t_half; >> exp(-lambda*t_half) % make sure no typos, etc., ... ans 0.5000 >> exp(-lambda*10) % after 10 years ans 0.9841 >>
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 vintage).
--


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In alt.home.repair, on Thu, 30 Jul 2020 19:09:26 -0500, dpb

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.

Me neither.

Very interesting. Not surprised.
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