Smoke, heat and carbon monoxide alarms: how each works and which room it goes in
A smoke alarm looks for particles of smoke in the air. A heat alarm waits for the room to pass a set temperature or warm up very fast. A carbon monoxide (CO) alarm measures a poisonous gas you cannot see or smell. In a typical UK home that means smoke alarms in the hallway, landings and living room, a heat alarm in the kitchen, and a CO alarm in any room with a boiler, gas fire or stove.
How does a smoke alarm know there is smoke?
An optical smoke alarm shines light across a dark chamber, away from a sensor. Smoke particles scatter that light onto the sensor and the alarm sounds.
FireProtect 2 uses what Ajax calls a dual-spectrum sensor: blue and infrared LEDs measure how much is in the air and how large the particles are. Ajax's HazeFlow 2 software compares that pattern with real smoke, so steam is less likely to set it off.
How does a heat alarm work, and why use one in the kitchen?
A heat alarm ignores smoke. Ajax builds the FireProtect 2 heat sensor to Class A1R requirements, and it alarms in two cases:
- Fixed point: the air passes 64 °C.
- Rate of rise: the temperature climbs more than 10 °C in a minute or less.
Steam and cooking fumes do not set it off, which is why kitchens get one. MHCLG fire statistics for the year ending March 2026 put cooking appliances behind 42% of accidental dwelling fires in England. Because it needs real heat, Home Office guidance for small guest accommodation puts smoke alarms, not heat alarms, in hallways, staircases, living rooms and bedrooms.
How does a carbon monoxide alarm work?
CO comes from appliances that burn gas, oil, coal or wood. A CO alarm uses a chemical cell that reacts with the gas and tracks the dose over time rather than sounding at the first trace. Ajax's limits for FireProtect 2 are:
- above 50 ppm: alarm within 90 minutes
- above 100 ppm: within 40 minutes
- above 300 ppm: within 3 minutes
The sensor is rated for 10 years. Fit CO alarms as well as smoke and heat alarms, never instead.
Which alarm goes in which room?
| Room | Alarm | FireProtect 2 option |
|---|---|---|
| Hallway and every landing | Smoke | Heat/Smoke |
| Living room (the room used most) | Smoke | Heat/Smoke, or Heat/Smoke/CO with a gas fire or stove |
| Kitchen | Heat | Heat, or Heat/CO if the boiler is there |
| Bedrooms | Smoke (recommended extra) | Heat/Smoke |
| Utility or laundry room | Heat | Heat |
| Room with a boiler, gas fire, open fire or wood burner | CO, plus smoke or heat for that room | CO, or a combined unit |
| Bathroom | None normally | None |
Open-plan layout? Scottish Government guidance lets one alarm cover the area if no point is more than 7.5 m from it, and makes it a heat alarm if the space includes the kitchen.
Where on the ceiling, and how far from the boiler?
Position matters. In the same MHCLG figures, smoke alarms failed to operate in 19% of dwelling fires. In 49% of those failures the smoke or heat never reached the detector, and in another 17% the fire was in an area with no alarm. A missing (1.8%) or defective (3.1%) battery was far rarer.
- Smoke and heat alarms: on the ceiling. The Scottish Fire and Rescue Service (SFRS) advises keeping smoke alarms about 30 cm from walls, lights, doors and vents, and not too close to the kitchen door.
- CO alarms: Approved Document J (2022) puts them in the same room as the appliance, 1 to 3 m from it horizontally, on the ceiling at least 300 mm from a wall, or high on a wall above doors and windows but not within 150 mm of the ceiling.
- Interlink them so every alarm sounds when one does. Radio links are accepted.
- Indoors only: Ajax rates FireProtect 2 for 0 to 50 °C, so keep it out of unheated spaces that can freeze.
What do the UK rules require?
- Scotland, all homes: smoke alarms in the living room and every hallway and landing, a heat alarm in the kitchen, all ceiling-mounted and interlinked, plus a CO alarm in any room with a carbon-fuelled appliance such as a boiler (gov.scot).
- England, rented homes: under the Smoke and Carbon Monoxide Alarm (England) Regulations, as amended from 1 October 2022, private and social landlords must provide a smoke alarm on every storey with living accommodation and a CO alarm in any room with a fixed combustion appliance other than a gas cooker.
- Wales, rented homes: the Renting Homes (Fitness for Human Habitation) (Wales) Regulations 2022 call for a mains-wired, interlinked smoke alarm on each storey and a CO alarm in each room with a gas, oil or solid-fuel appliance.
Ajax lists the mains AC versions as BS 5839-6 Grade D1 (mains with a sealed backup battery) and several sealed-battery SB versions as Grade F1. The Home Office guidance expects D1 and accepts F1 only as a stopgap of around two to three years. This is a summary, not legal advice.
Which FireProtect 2 version do you need?
Pick the sensors, then the power: AC runs on 110 to 240 V mains with a sealed backup battery and is wired in by an electrician (the type for Welsh lets); SB needs no wiring and Ajax quotes up to 10 years before the unit is replaced; RB takes two CR123A cells, rated by Ajax at up to 7 years.
| Model | Suits | Our price inc VAT |
|---|---|---|
| FireProtect 2 SB (Heat) | Kitchen, utility room | £111.93 |
| FireProtect 2 SB (Heat/CO) | Kitchen with a boiler | £150.00 |
| FireProtect 2 AC (Heat/Smoke) | Hallway, landing, living room | £80.77 |
| FireProtect 2 SB (Heat/Smoke) | Hallway, landing, bedroom | £122.30 |
| FireProtect 2 (Heat/Smoke/CO) | Living room with a gas fire or stove | AC £105.00, RB £150.00 |
| FireProtect 2 AC (CO) | Room with a boiler | £95.76 |
Prices include VAT, delivery is extra, and prices can change. Compare them with our other standalone smoke and heat detectors.
Do you need a hub? Not to raise the alarm: each detector has its own 85 dB siren. Interlinking and phone alerts are set up through a hub such as the Ajax Hub 2 (2G) panel (£161.53, up to 100 Ajax devices). Ajax says one alarm then sets off every fire detector on the system within 20 seconds, so where alarms must be interlinked, plan on a hub. The same hub can run an Ajax burglar alarm and a LeaksProtect flood detector. For commercial premises Ajax makes a separate EN54 Line (see our Ajax wireless fire alarms).
Our take
Match the sensor to the room first: smoke where you escape and sleep, heat where you cook, CO where fuel burns. Combined units save ceiling points, and mains suits jobs where the rules call for it. Zenith Security Co. has supplied security equipment since 2012. We supply Ajax alarms rather than fit them, and we can recommend a qualified installer depending on the job and area.
Want help with your list? Call 01902 902251, contact us online or visit our Wolverhampton trade counter. We ship nationally to England, Scotland and Wales.
Smoke, heat and CO alarms: quick questions
Can I put a smoke alarm in the kitchen?
It is not advised. Steam and cooking fumes are a common cause of false alarms, so UK guidance puts a heat alarm there instead.
Is a combined smoke and CO alarm a good idea?
Yes, where one spot suits both: on the ceiling for smoke and 1 to 3 m from the appliance for CO. In a kitchen, choose Heat/CO instead.
How long do FireProtect 2 alarms last?
Ajax rates the CO sensor and the sealed-battery versions at up to 10 years. SFRS advises replacing smoke and heat alarms every ten years.
Do Ajax fire alarms need Wi-Fi?
No. Each detector sounds its own siren without a hub or internet. Interlinking needs an Ajax hub, and phone alerts need that hub online.
Source: Ajax Systems blog, 30 Dec 2024, with MHCLG fire statistics (19 Aug 2026), the Scottish Fire and Rescue Service, gov.scot, Home Office fire safety guidance, Approved Document J and legislation.gov.uk for England and Wales. Device figures are Ajax's own.
