Carbon Monoxide Safety for Chimney Sweeps: CO Alarm Requirements, Testing After Sweeping and Legal Duties

Summary

Carbon monoxide is a colourless, odourless gas produced by incomplete combustion. It is one of the most significant hazards associated with solid fuel, oil, and gas appliances, and blocked or poorly swept chimneys are a leading cause of CO incidents in domestic properties. Chimney sweeps are frequently the only trade professional who enters a property with a combustion appliance each year — placing them in a unique position to identify CO risks and advise customers.

The legal landscape for CO alarms has tightened significantly since 2022. The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022, which came into force in England and Wales on 1 October 2022, extended mandatory CO alarm requirements to all privately rented and social housing properties that contain any gas appliance (not just solid fuel). This widened the scope considerably. In Scotland, Building Standards require a CO alarm for all new or replacement combustion appliances regardless of fuel type.

A common misconception among sweeps is that CO alarm advice is the landlord's or installer's problem, not theirs. In practice, sweep bodies (HETAS, NACS, and the Guild) all require or strongly recommend that sweeps note CO alarm status on their certificate. Failure to do so — and failure to advise a customer where no alarm exists — creates professional and reputational risk. It also leaves the customer unprotected.

Key Facts

• CO is invisible and odourless — early symptoms mimic flu without fever: headache, nausea, dizziness, confusion
• Immediate threat level: 400 ppm can be life-threatening within 3 hours; 1,600 ppm within 1 hour; 12,800 ppm within 3 minutes
• Building Regulations Part J 2022: Requires CO alarm in any room where a new or replacement solid fuel appliance is installed in England and Wales
• Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022: Extends CO alarm requirements to all gas appliances in rented dwellings (England and Wales) from 1 October 2022
• Scottish Building Standards (Section 3.17): CO alarm required for all new or replacement combustion appliances (solid fuel, gas, oil) — not limited to rented property
• Northern Ireland: Building Regulations Technical Booklet L — CO alarm requirements apply; check current edition
• Placement: Manufacturers typically recommend within 1–3 m horizontally of the appliance; at ceiling height (at least 150 mm from ceiling per most manufacturers' guidance) or at sleeping height for bedrooms
• Not above the appliance: CO alarms should not be placed directly above the appliance or cooker — this risks false alarms from cooking fumes
• Sensor types: Electrochemical sensor (most common, reliable, 5–7 year lifespan) or biomimetic sensor (less common in domestic use)
• Lifespan: Most CO alarms have a 7-year operational lifespan — end-of-life alarm should be replaced, not tested and re-hung
• Sweep's duty to test: Best practice (required by HETAS and Guild codes of practice) is to test the CO alarm during the sweep visit using the test button
• Certificate notation: CO alarm status (present/absent/tested/not working) should be recorded on every sweep certificate
• Landlord duty: Private landlords in England must provide CO alarms in rooms with gas, oil, or solid fuel appliances (since October 2022)
• Chimney blockage as CO cause: Bird nests, debris, cowl failure, creosote build-up can all cause partial or total flue blockage leading to CO spillage
• Spillage test: Smoke pellet or smoke match test can indicate whether flue gases are being drawn up the flue or spilling into the room

Quick Reference Table

Detailed Guidance

Why Chimney Sweeps Are the Key Point of Defence

Most combustion appliance owners have their chimney swept once or twice a year. This is frequently the only time a trained professional examines the appliance and flue. Sweeps are therefore in the front line for identifying CO risk — not just through sweeping, but through visual inspection of the appliance, flue, and surrounding area.

Common causes of CO that a sweep may identify include: a blocked or partially blocked flue (bird nest, debris, collapsed masonry, cowl failure); visible creosote build-up that indicates poor draw or incomplete combustion; visible cracks or gaps in the flue or appliance; signs of spillage (soot staining around the appliance door or on the wall above the appliance); and absence of a functioning CO alarm.

When any of these conditions are found, the sweep should refuse to certify the appliance as safe, document the defect on the certificate, and advise the customer not to use the appliance until the defect is remedied. This is not optional for registered sweeps — it is part of the code of practice for HETAS, NACS, and Guild members.

CO Alarm Requirements: What the Law Says

The legal position in England differs between owner-occupied and rented properties. For rented properties (both private and social housing), the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 require landlords to install CO alarms in rooms with gas, oil, or solid fuel appliances. This is the landlord's legal obligation — not the sweep's. However, the sweep has a professional duty to advise the landlord or tenant if no alarm is present.

For owner-occupied homes, there is no statutory duty on the homeowner to install a CO alarm in England (as of April 2026), except where Part J applies to a new appliance installation. In practice, the absence of a CO alarm in an owner-occupied home with a solid fuel appliance is a significant safety risk. Sweeps should advise customers of this clearly and note the absence on the certificate.

In Scotland, Building Standards require a CO alarm for all new or replacement combustion appliances regardless of property tenure — the obligation is on the person carrying out the installation. Existing appliances without CO alarms in rented properties in Scotland are also subject to landlord duties under Scottish housing legislation.

CO Alarm Placement: Getting It Right on Site

The optimal placement for a CO alarm is:

• Horizontal distance: Within 1–3 m of the appliance. The alarm does not need to be immediately adjacent — CO disperses quickly and a nearby location is sufficient.
• Height: Manufacturers' guidance varies. Most domestic alarms are designed to be mounted at ceiling height (CO is roughly the same density as air and disperses uniformly) or at head height in a sleeping area. A common installation height is 150 mm below the ceiling. Do not place it directly above a cooker or hob where steam and cooking fumes could cause false alarms.
• Not on the ceiling directly above the appliance: Avoid locations where heat from the appliance could affect the sensor.
• Bedroom alarms: In bedrooms with a combustion appliance, or where the flue passes through, a CO alarm at breathing height (chest height when lying down) provides earlier warning.

When advising customers on placement, sweeps should defer to the manufacturer's installation instructions for the specific alarm model. These vary between products and are the primary authority.

What to Do When the CO Alarm Sounds During a Sweep Visit

If the CO alarm activates during a sweep visit, the sweep must treat it as a real alarm. The following procedure is appropriate:

1. Stop work immediately
2. Open windows and doors to ventilate the property
3. Evacuate all occupants (people and pets) from the property
4. Do not use any electrical switches or naked flames
5. Call the Gas Emergency Service on 0800 111 999 if a gas appliance is involved, or contact the appropriate emergency service
6. Do not re-enter the property until it has been declared safe

The alarm may be activated by CO from the sweeping process itself — for example, if a flue blockage is disturbed and gas is temporarily released into the room. Even if this is the suspected cause, the procedure is the same: evacuate and ventilate before continuing.

The Spillage Test

A smoke match or smoke pellet test can help identify whether a flue is drawing correctly before the sweep begins and after the sweep is complete. The test involves lighting a smoke source near the appliance opening (or in the throat of the fireplace) and observing whether the smoke is drawn upward into the flue or spills back into the room.

Spillage into the room indicates a draw problem — which may be caused by a blockage, a negative pressure in the property, an inadequate flue height, or a flue diameter that is too large for the appliance. Spillage is a significant indicator of CO risk and should be documented on the sweep certificate. If spillage is confirmed, the sweep should advise the customer not to use the appliance until the cause is investigated.

Record Keeping and Certificate Documentation

Every sweep certificate should record CO alarm status, including:

• Whether a CO alarm was present in the room
• Whether the alarm was tested (test button pressed)
• Whether the alarm functioned correctly on test
• If no alarm was present, confirmation that the customer was advised and the date of advice

This documentation serves two purposes. First, it protects the customer by creating a clear record that the issue was flagged. Second, it protects the sweep: if a CO incident occurs after a sweep visit, the sweep's certificate demonstrating that they identified the absence of an alarm and advised accordingly is an important professional defence.

Frequently Asked Questions

Am I legally required to advise customers about CO alarms?

There is no specific statute that imposes a legal duty on chimney sweeps to advise about CO alarms. However, HETAS, NACS, and Guild codes of practice all require or strongly recommend it. Failure to advise could also give rise to a negligence claim if a customer suffers harm from CO exposure that was foreseeable and the sweep had an opportunity to warn them. Professional indemnity insurance is important protection here.

What if a customer refuses to have a CO alarm installed?

If the customer refuses, the sweep should note the refusal on the certificate ("Customer advised CO alarm absent; customer declined advice") and consider whether they are willing to issue a certificate for the visit. Some sweeps will not issue a certificate recommending continued appliance use where no CO alarm is present. This is a matter of professional judgment and individual business policy. At a minimum, the advice given and the customer's response should be documented.

How often should a CO alarm be tested?

Customers should test CO alarms monthly using the test button, per most manufacturers' guidance. The alarm should also be replaced at the end of its operational lifespan — typically 7 years for electrochemical sensors. The end-of-life date is usually printed on the alarm or packaging. When a sweep visits, testing the alarm during the visit is best practice.

Can I carry a CO alarm to give to customers who don't have one?

Yes — many sweeps carry basic CO alarms to sell or supply to customers who do not have one. This is a commercially sensible practice and a meaningful safety contribution. Make sure any alarm supplied meets EN 50291-1 (the European standard for domestic CO alarms) and that you provide the manufacturer's installation instructions.

Does a bird nest in the chimney cause CO?

Yes. A bird nest — particularly a jackdaw or starling nest — can partially or completely block a flue. Even a partial blockage can cause combustion gases, including CO, to back-spill into the room instead of exhausting up the flue. Bird nests must be removed as part of the sweep, and sweeps should check for evidence of nest activity (twigs, feathers, droppings near the appliance) before beginning work.

Regulations & Standards

• Building Regulations Part J (England and Wales) — CO alarm requirement for new solid fuel appliance installations

• Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 (SI 2022/707) — mandatory CO alarms in all rented homes with combustion appliances, England

• Scottish Building Standards Section 3.17 — CO alarm requirement for all new or replacement combustion appliances

• Northern Ireland Building Regulations Technical Booklet L — combustion appliance standards including CO

• EN 50291-1:2018 — European standard for electrotechnical systems in residential and similar premises — domestic CO alarms

• HSE HSG150 — Health and Safety in Construction (ladder/working at height safety during chimney work)

• Gas Safety (Installation and Use) Regulations 1998 — relevant when sweeping gas flues

• HSE Carbon Monoxide guidance — hse.gov.uk/gas/domestic/co-alarms.htm

• Private Housing (Tenancies) (Scotland) Act 2016 — landlord duties in Scotland including CO alarm provision

• HSE — hse.gov.uk/gas/domestic/co-alarms.htm (CO alarm guidance)

• GOV.UK — gov.uk/government/publications/smoke-and-carbon-monoxide-alarms (Smoke and CO Alarm Regulations guidance)

• Scottish Government — gov.scot/publications/fire-and-carbon-monoxide-alarms (Scottish requirements)

• HETAS — hetas.co.uk (CO safety guidance for sweep members)

• National Association of Chimney Sweeps — nacsuk.org.uk (CO safety code of practice)

• St John Ambulance — sja.org.uk/get-advice/first-aid-advice/carbon-monoxide-poisoning (CO symptoms and first aid)

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