Smoke Alarm problems

bluesa1

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I have two mains wired smoke alarms (they also have PP3 batteries fitted). One is at the bottom of the stairs and one is at the top. Both of the alarms look absolutely identical but, one of them is an Optical Smoke Alarm and the other is an Ionisation Smoke Alarm. This doesn't mean anything to me I'm afraid. However, I would be most appreciative if someone could explain the difference please.

A week ago both of these alarms went off at the same time, 3.45am (middle of the bloomin night!) and I could not find a cause. They both went off again today but this time we were up and about at 6.45am. Once again there is no obvious cause. Almost a full week has passed by without a further incident.

I know that if the batteries are going then they tend to beep about once a minute but, both of these occasions it was a full blown alarm.

Just wondering if anyone on here has any inkling as to what might be causing them to go off without any obvious cause.

Many thanks for reading
 
Could be a build up of dust on the sensors. Try vacuuming the grills to clean it out. It's recommended that you vacuum the grills at least twice a year, change the batteries once a year (unless fitted with an unremovable 10 year battery) and completely replace the detectors every ten years.
 
Thanks for your input Steroc, I will vacuum them today, I'll also give them a blast with the airgun to clean them out. I have been involved with replacing sensors, are the easy to get at and are they readily available and will I know what to order?

Still don't know the difference between an optical alarm and an Ionisation alarm. Any thoughts.

Thanks again.
 
Optical detectors measure the presence of smoke particles via a light source and detector, and are generally stated to be most sensitive to smouldering fires.

Ionisation use a small amount if radioactive isotope to ionise the air and measure it's properties and are considered better at detecting flaming fires.

You can get dual detection units which combine both detection methods for the best sensitivity and reliability from false alarms.

If you have a potentially smoky/dusty area such as a garage, kitchen, or workshop then you would fit a heat detector instead which operates when a preset temperature is exceeded.
 
How old are the units? like @Steroc said smoke detectors should be replaced at 10 year intervals (should have an expiry label on the side if a newer one) if you are to replace go for the ones with rechargeable batteries that never need replacing (apart from after the 10 year life of the unit) the units will be interlinked within the mains cable so that if one activates so do any others in the house.
 
Thank you for this blickey_1, very useful information. I will check the dates on the labels shortly. I now know why they were all going off at the same time. If I need to replace, I wll do as you suggest and go for the rechargeable one.
Just out of interest as I know nothing about these things, I mentioned in my original post that one of the alarms is optical and the other is an Ionisation one. The difference has been explained but, I'm just wondering if there is a 'best siting' for either of them....i.e. is one type better at the bottom of the stairs or doesn't it matter.
Thanks for your help.
 

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