Carbon Monoxide Alarm Positioning: BS EN 50292 Distances and Heights

Summary

CO alarm positioning is the single most-checked item on Gas Safe inspections and HETAS sign-offs. An alarm in the wrong place fails the regulations and gives the homeowner false reassurance — and the installer carries the liability if a CO event occurs after the install.

The rules are a combination of: where Building Regulations require an alarm (Part J), what standard the alarm must meet (BS EN 50291-1), and where the alarm must be sited within the room (BS EN 50292 — a separate guidance standard often misquoted as "the alarm standard"). All three need to be right.

The most common installer mistakes are: siting the alarm on the same wall as a gas hob (gets steam and grease, fails early), positioning it inside the dead-air zone within 150mm of the ceiling, or placing it in the wrong room entirely for a room-sealed appliance.

Key Facts

• BS EN 50291-1:2018+A1:2023 — the product standard CO alarms must meet. Look for this number on the back of the alarm body.
• BS EN 50292:2019 — the guidance standard for the selection, installation and use of CO alarms in dwellings. Sets the positioning rules summarised here.
• Building Regulations Part J (England & Wales) — makes CO alarms compulsory for new/replacement solid fuel and certain gas/oil appliance installations.
• Smoke and Carbon Monoxide Alarm (England) Regulations 2015 (amended 2022) — requires landlords to install a CO alarm in every room with a fixed combustion appliance (excluding gas cookers). The 2022 amendment removed the "solid fuel only" restriction.
• Welsh, Scottish and NI regulations — vary slightly. Scotland requires CO alarms in any room with a combustion appliance under Tolerable Standard 2019.
• Horizontal distance — 1m minimum to 3m maximum from the appliance.
• Vertical position (ceiling-mounted) — 150mm minimum below ceiling, 300mm minimum from any wall.
• Vertical position (wall-mounted) — at least the height of any doorway or window head, but at least 150mm below the ceiling.
• Distance from corner — 300mm minimum from any internal corner (dead-air zone).
• Same room — for all open-flued and flueless combustion appliances.
• Adjacent rooms — also recommended where occupants spend significant time (bedrooms adjacent to a boiler room, for example).
• Battery sealed alarms — modern standard, 7-10 year sealed battery life. End-of-life beep warns of replacement.
• CO alarm vs combination alarm — combined smoke + CO alarms exist but are not interchangeable with dedicated CO alarms for siting purposes — site them as CO alarms (low, near appliance).
• Stairwells and landings — recommended additional CO alarm where multiple combustion appliances are spread across floors.
• Sleeping areas — manufacturer guidance often advises an additional CO alarm in or near bedrooms.

Quick Reference Table — Where to Site by Room Type

Quick Reference Table — Distances to Avoid

Detailed Guidance

Where the regulations come from

Two different rule-sets apply and they often confuse installers:

1. Where you must fit a CO alarm — Building Regulations Approved Document Part J for new/replacement appliances; Smoke and CO Alarm (England) Regulations 2015 (amended 2022) for landlords; Tolerable Standard 2019 for Scotland; Building Regulations Part F (Northern Ireland).

2. How to fit it — BS EN 50292:2019 (positioning) and the alarm manufacturer's instructions.

If the regulations require a CO alarm, you fit one. If they don't, fitting one is still strongly advised — a £25 alarm protects against a £50k claim.

Why "150mm below the ceiling" matters

CO is roughly the same density as air at room temperature (CO MW = 28; air MW = 29). It does not "rise like smoke" or "sink like LPG". It mixes throughout the room within minutes.

But the dead-air zone within ~150mm of the ceiling and ~300mm of corners is real — air movement in these zones is so slow that CO can be slow to reach the sensor. Smoke alarms are designed for the smoke that does sit at the ceiling; CO alarms are designed to sample the mixed room air.

The exact position:

SECTION VIEW — CEILING MOUNTED CO ALARM
========================================

CEILING ──────────────────────────────────
                  ▲
              150mm min (300mm preferred)
                  ▼
              [CO ALARM]   ← horizontal 300mm min from any wall
                  ▲
                  │       2 m typical
                  │       (1m min — 3m max
                  │        horizontal to appliance)
                  ▼
          ╔════════════════╗
          ║   BOILER       ║
          ║   (appliance)  ║
          ╚════════════════╝

Common installer mistakes

• In a kitchen cupboard with the boiler — alarm cannot sample the room air. Always mount outside the cupboard at the right distance.
• On the same wall as the cooker — splatter and steam fouls the sensor in 6-12 months.
• In the corner above the door — looks tidy but sits inside two dead-air zones (corner + ceiling).
• In a different room — only legitimate if there is no combustion appliance in the original room. For a kitchen boiler, the alarm goes in the kitchen.
• Above a stove — heat plume from the stove disperses CO away from the sensor.
• Bedroom only for a stove downstairs — Building Regs require the alarm in the room with the appliance. A second alarm in a bedroom is supplementary, not a substitute.

Multi-storey and multi-appliance dwellings

For a house with a gas boiler on the ground floor and a stove in the lounge:

• Boiler room → CO alarm (sited as above)
• Lounge → CO alarm
• Hallway / landing → recommended additional CO alarm
• Bedroom directly above the boiler / lounge → recommended

For a HMO or rental:

• Every room with a fixed combustion appliance (excluding gas cookers per the 2022 amendment) → mandatory
• Common areas → recommended
• Landlord must test on tenant change-over

Replacing an existing CO alarm

If the existing alarm is in the wrong place, relocate it during the install. Do not leave a non-compliant alarm "because it was already there". The new install is the trigger for full compliance. Document the relocation and confirm the new position in the customer hand-over.

Audible test and end-of-life

All sealed-battery CO alarms have:

• A test button — confirms sounder and electronics. Press monthly per manufacturer.
• An end-of-life chirp — typically at 7-10 years from manufacture date (printed on back). When the chirp starts, the unit must be replaced — recalibration is not possible.

When installing, write the install date and end-of-life date on a sticker on the alarm body, and note both in the install certificate.

Frequently Asked Questions

Does a room-sealed combi boiler need a CO alarm?

Not under Approved Document Part J — room-sealed appliances draw combustion air from outside and discharge flue gases outside, so they don't normally produce a CO risk in the room. But fitting one is still recommended best practice; flues can crack, seals can perish, and a £25 alarm costs nothing compared to the risk. For private rented properties in England, the Smoke and CO Alarm Regulations 2015 (as amended 2022) require an alarm in any room with a fixed combustion appliance — which includes room-sealed boilers — unless the appliance is solely for cooking.

What's the difference between BS EN 50291 and BS EN 50292?

BS EN 50291-1:2018+A1:2023 is the product standard — what the alarm itself must do (sensitivity, sounder volume, response time, durability). BS EN 50292:2019 is the installer guidance standard — where to fit, how many to fit, how often to test. Buy alarms to 50291; install per 50292.

Can I fit a CO alarm above a doorway?

Yes, provided it is at least 150mm below the ceiling and 300mm from the corner where the door frame meets the ceiling. The door head height itself is not the constraint — the 150mm/300mm dead-air zones are. A practical position is on the wall above and beside the door, 150-300mm below the ceiling.

How many CO alarms does a typical 3-bed house need?

For a 3-bed semi with a gas combi boiler in the kitchen and a stove in the lounge: minimum 2 alarms (one per appliance room), recommended 4 (add hallway/landing and master bedroom). If gas-only with no stove and a room-sealed combi: 1 minimum (the kitchen), 2 recommended (add landing). For HMOs or landlords, follow the per-room rule plus a landing alarm.

Does the customer test it or do I?

Both. You test on install and again at commissioning. Demonstrate to the customer how to test and what the end-of-life chirp sounds like. Make this part of the hand-over — and write it on the install certificate. Many installer disputes after a CO incident come down to "did the customer know how to test?"

Regulations & Standards

• BS EN 50291-1:2018+A1:2023 — Electrical apparatus for the detection of carbon monoxide in domestic premises. Product standard for the alarm itself.

• BS EN 50292:2019 — Electrical apparatus for the detection of carbon monoxide in domestic premises — guide on the selection, installation, use and maintenance.

• Building Regulations Approved Document Part J (England & Wales) — Combustion appliances and fuel storage systems. Mandates CO alarms for new/replacement solid fuel and certain gas/oil appliances.

• The Smoke and Carbon Monoxide Alarm (England) Regulations 2015 — landlord duty; amended 2022 to cover all fixed combustion appliances except gas cookers.

• Building Regulations Part F (Northern Ireland) — equivalent NI provisions.

• Tolerable Standard (Scotland) 2019 — equivalent Scottish provisions.

• Gas Safety (Installation and Use) Regulations 1998 — duties for gas installers including CO awareness.

• Approved Document J — Combustion appliances (gov.uk)

• Smoke and CO Alarm Regulations — guidance for landlords (gov.uk)

• HSE — Carbon Monoxide in the home

• Gas Safe Register — CO awareness

• CORGI Technical Bulletin — CO alarm positioning — industry guidance

• wood burner installation pricing guide — install scenarios requiring CO alarms

• cookers bayonet fittings — cooker installations and CO considerations

• smoke detection — separate smoke alarm rules

• gas safety certificate — landlord obligations