Smoke and CO detector: which to fit and where

Part of the Costa del Sol security guide

The short answer

For most homes, the most practical option is one detector that combines smoke, heat and CO on the ceiling in the bedroom area, and another near the boiler or fireplace if you have one. In a kitchen or a garage, smoke sensing causes false alarms: there you want a heat-only model or a heat and CO one. If you have an Ajax alarm, the detectors alert your phone and all sound, even in an empty house.

What each one detects

Detector	Detects	Where it fits
FireProtect 2 with smoke, heat and CO	Smoke, CO, rapid temperature rise and +64 °C	Bedroom hallway, living room, boiler room
FireProtect 2 with heat and CO	CO, a rise of more than 10 °C in a minute and +64 °C	Garage, workshop, boiler room
FireProtect Plus	Smoke, CO, rapid temperature rise and +59 °C	Homes with a boiler or fireplace
FireProtect	Smoke, rapid temperature rise and +59 °C. Does not detect CO	Hallways and areas with no combustion

Carbon monoxide has no smell and cannot be seen. You only need to monitor it if something burns in the home: a gas boiler, a fireplace, a stove or a garage connected to the house.

Where it goes

On the ceiling, in the middle of the room. At least about 30 cm from lamps and anything else that blocks the airflow.
One per room you want to protect. Ajax works on each detector covering 50 to 60 m², depending on the type of room.
Close to where people sleep. If you only fit one, put it in the bedroom hallway: that is where hearing the siren matters most.
Somewhere you can reach. You need to be able to get to the test and silence button.

Where not to put it

In the bathroom or next to the shower, or near radiators: rapid temperature changes cause false alarms.
Next to fans, air vents, or open windows and doors: the draught carries smoke away from the detector.
In corners, or in places with a lot of dust or insects.
In a garage with car fumes, if the model senses smoke: Ajax recommends the heat and CO model there.
Outdoors. They are all indoor detectors.

Batteries, sealed battery or mains

The letters in the FireProtect 2 model name tell you how it is powered. According to the Ajax manual, under normal conditions:

Letters	What it means	Lifespan
RB	Replaceable batteries	Up to 5 years per set of batteries
SB	Sealed battery: at the end you replace the whole detector	About 10 years
AC	Mains power, with a non-replaceable backup battery	Needs mains wiring

Whatever the power source, the manufacturer recommends replacing the detector after 10 years, because the sensors lose sensitivity.

With or without an Ajax alarm

These detectors can work on their own: if they are not paired to a hub, they still sound their 85 dB siren, but they do not alert anyone. In an empty house that is not much use, because nobody hears it. Paired to an Ajax alarm, the alert reaches your phone and, if you have several, they all sound: within about 20 seconds on FireProtect 2, and within 60 seconds at most on FireProtect and FireProtect Plus.

What this detector does not replace

A home detector warns you; it does not put anything out. And in business premises, any fire protection system required by law must be installed by a company authorised to do so: these detectors can complement it, but they do not replace it.

Frequently asked questions

How many smoke detectors does a home need?

One for each room you want to protect. Ajax works on 50 to 60 m² per detector. If you are starting with one, put it in the bedroom hallway.

Where should a carbon monoxide detector go?

In the room where something burns, or close to it: boiler, fireplace or stove, and in the bedroom hallway if the bedrooms are nearby.

Can you put a smoke detector in the kitchen?

It is not ideal, because cooking smoke causes false alarms. Keep it away from the hob or use a heat model.

How long does an Ajax smoke detector last?

The manufacturer states a service life of 10 years. The batteries in RB models last up to 5 years and the sealed battery in SB models about 10 years, under normal conditions.

Does the detector sound if the alarm hub fails?

Yes. It sounds with its own siren, but without the hub no alert reaches your phone.

How do you silence a false alarm?

One press on the centre of the detector silences it for 5 minutes. If the smoke level is critical, it cannot be silenced.

Which detectors does your home need?

Send us the floor plan or a few photos and we will tell you how many you need, which type and where, with an itemised quote. Site visit and quote with no obligation. We speak English, Spanish, Russian and Ukrainian.

Ask for a quote on WhatsApp See the Ajax catalogue

Sources: FireProtect 2 manuals (ajax.systems/support/manuals/fireprotect-2-heat-smoke-co and ajax.systems/support/manuals/fireprotect-2-ac-heat-smoke) and dartelsolutions.com product pages (FireProtect 2 RB, SB and AC, FireProtect Plus, FireProtect). Checked in October 2026.

Smoke and carbon monoxide detectors: which one to fit at home and where it goes