Smoke alarm replacements - mains powered (+batt backup) vs batt-only

27 Nov 2015
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United Kingdom
We have two smoke alarms - one in the downstairs hallway, one on the upstairs landing.
They were builder-installed when the house was new in 1999. They are on a linked system. Behind each alarm is a plasterboard-type wall box with a terminal strip. There are four connections to this:
Neutral (blue)
Live (red)
Control (yellow)

The Control serves to sound both alarms, if one goes off.

After about 10 years these were due for replacement (being the ionising type). I researched and then replaced both alarms (using Which's Best Buy guide). At the time, there were no mains-powered Best Buys, so I chose battery-only models, obviously unable to make use of the linked control function.

It's time to replace them again. This time, there are well reviewed models with sealed 10 year batteries.
One common model (that gets good reviews) is the Fire Angel ST-622Q Thermoptek Smoke Alarm. This is battery only.

I read somewhere that "new build" with control wiring should *only* be replaced with mains-powered units (at least, in Scotland?)

Harder to find, not directly reviewed by Which?, but with what looks like similarly good sensor technology is the FireAngel Pro ST-230 Thermoptek Smoke Alarm. At £30 each, three times the price. This has mains, control, and 10-year (backup) lithium battery.

This seems to be the best solution. After all price is a secondary consideration for safety kit.

Thoughts/comments appreciated. The retailers don't make it easy to understand current requirements or choose items!
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One obvious thing: the wiring is old-colour-scheme and the wiring is in good condition. I really want to reuse it, not change wiring colours for the sake of it (to the new, stupid brown/black/grey nonsense).

Also: forgive my rant: the f*@king builders in 1999 originally mis-installed it by swapping the neutral and control at one end. As control carries "live" as a signal, that meant one alarm would be inoperative when when the other was sounding. Idiots. I tested, corrected, and re-tested that not long after we moved in.

At least the alarms are fed by a dedicated circuit from the consumer unit, so they did something right.
Last edited:
At least the alarms are fed by a dedicated circuit from the consumer unit, so they did something right.
There is actually an argument that it is better to feed alarms from, say, a lighting circuit -sincxe one is then much more likley to realise if, for some reason, the alarms have lost their power.

Kind Regards, John
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Aico all the way. I’ve fitted hundreds and never had any problems with them at all.

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