Smoke detectors: Optical and ionisation

If you are expecting flames before thick smoke then ionisation are best. Kitchen and nearby?

Where something will smoulder and create smoke then optical are better. Furniture?

Optical are usually recommended for the circulation spaces such as the hallway and landing. Ionisation for the other rooms (apart from the garage and kitchen that should have heat detectors as you already know).

When I installed the detectors in my brothers I installed optical in the lounge (to avoid false alarms from kitchen fumes), ionisation in the hallway, optical on the landing and ionisation in the master bedroom.

If I am only installing the mimimum legal requirments for a house I usually put an ionisation detector in the hall and an optical detector in the landing, but I suppose it would work just as well the other way around.

You obviously exceeded the minimum requirements of the regs 12 years ago when you installed your detectors:-).

Now a chat with the BCO on a new build last week was interesting. This is a new build 4 bedroomed bungalow and the windows cannot be used as a fire escape. To complicate matters there was only one door between the kitchen and the bedrooms. He asked for all the bedrooms to have interlinked smoke alarms fitted and an extra alarm in the dining room (this was the day after the loft insulation went in)

Mine all run off the burglar alarm. They're bog-standard PP3 powered detectors, but they use a module instead of the battery which runs from the 12V alarm supply, and monitors the detector's current draw, to signal back to the panel when that smoke detector triggers (from increased current draw of the sounder). At least I never have to go around them all replacing batteries, but I doubt these would have ever conformed to anything ;-)

Was it more than 2 floors? I'm probably out of date on current regs.

It is a 4 bedroom bungalow:-)

When my ionisation alarm goes off the reason is always obvious, over-done toast usually, but about once a year my optical ones go off for no apparent reason at all. Doesn't seem to be connected with a change in temperature or humidity (steamed up sensor) perhaps it's insects crawling into the sensor?

Optical sensors can build up dust particles which a strong wind can put in motion all at once. The dust sets off the detectors.

While in the military just about every spring when people first propped the hallway doors open in the dormitories and a breeze blew down the hallway we usually had one or two false alarms caused by the dust buildup being cleared out of the optical detector alarms.

I wrote a FAQ for this for UK_Selfbuild ... let me know if you want a copy .. it explains what is needed where.

I think the only reason for fitting optical is that they are not radioactive and thus do not decay as those tend to over time. also of course presumably when an optical one dies you can throw it in a normal bin.

Brian

erm, surely smoke is smoke assuming the designer did their work correctly about airflow and they are placed sensibly. Does not seem to matter which you have, the slightest hint of burnt toast and they go off!

Brian

They still have a limited lifespan as they get dirty.

Domestic "radioactive" smoke detectors can be put in household waste - but only 1 per bin. There's a regulation on it somewhere.

You shouldn't put any sort of smoke detector in a kitchen really. A better idea is to use heat detectors, which sense the rate of rise of temperature in the room. Some people put smoke detectors in simply because they are a lot cheaper than heat detectors - and get false alarms.

We recommend optical for rooms with soft furnishing (living rooms, bedrooms= ) which tend to smoulder these days rather than burn with all the fire reta= rdants sprayed on the fabrics. And of course anywhere near the kitchen (usu= ally hallway). I would only recommend ionisation for very specific areas su= ch as landings, although I have had a lot of discussions about this. Howeve= r, thousands of customers later, nobody had any problems with these recomme= ndations, yet.

I had one unexplained alarm from the optical sensor. It was about 4am when the house was empty, about 6 months after that sensor was fitted. Never done it in the 2 years since then. Never had a false alarm from an ionisation alarm (and I have a lot more of them).

Ionization detectors use americium-241 which has a half-life of 432 years, so yes, they do decay over time, but I don't think you need to worry about that.

They claim (well mine do) to only have a life of 10 years. However, mine are as sensitive to problems with the toaster as ever they were at about 15YO.