Flue Gas Analysis: What the Law Requires and What to Do With a Bad Reading

This page covers Great Britain (England, Wales and Scotland), where the Gas Safety (Installation and Use) Regulations 1998 apply. Northern Ireland has its own regulations, which this page does not cover. It is correct as at 30 September 2026.

When the law requires a combustion check

Regulation 26(9) says that anyone who works on a gas appliance must, straight after the work, examine:

• the effectiveness of any flue
• the supply of combustion air
• its operating pressure or heat input, or both where necessary
• its operation, so that it works safely

Since 6 April 2018 there is a further item, regulation 26(9)(ca). If it is not reasonably practicable to examine the operating pressure or heat input, you must examine the appliance’s combustion performance instead. HSE says this applies where there is no meter or meter display to measure heat input directly and the operating pressure cannot be measured (L56, paragraph 245).

Separately, HSE’s list of checks for installation, commissioning, maintenance and safety check work says combustion performance should be measured where the manufacturer specifies it (L56, Appendix 3, paragraph 2(f)). So for many appliances the manufacturer’s instructions are what make a flue gas analyser reading part of the job.

The regulation 26(9) checks are not needed after directly disconnecting an appliance, or after purging that does not affect its safety (regulation 26(10)).

Where the pass and fail figures come from

Neither the regulations nor L56 give a carbon monoxide limit, a CO/CO2 ratio, or CO2, O2 or efficiency ranges. L56 points engineers to the manufacturer’s instructions and the data badge for appliance settings (Appendix 3, paragraph 2(e)), and to the manufacturer’s instructions and appropriate standards for checks after work (paragraph 240).

Work from the figures in the manufacturer’s instructions for that appliance. Where you need figures beyond those, they are set by industry standards and procedures that are sold rather than published openly. Use the current edition. This page does not quote them because they could not be checked against an openly published official source.

When a reading means the appliance is unsafe

L56 treats an appliance as unsafe when both of these apply (paragraph 280):

• there is a fault, such as maladjustment or a servicing deficiency, in the appliance, its flue or ventilation, or its gas supply, which has caused or is likely to cause incomplete combustion, products of combustion not being safely removed, too little oxygen for the people in the room, a gas leak, or another danger such as fire
• that result has caused, or is likely to cause, death or serious injury

Whether a particular appliance is dangerous is a judgement for a competent person, taking account of the circumstances (paragraph 283). An appliance that is merely below current standards, and needs minor improvement, is not “unsafe” for the purposes of regulation 34 (paragraph 282). The Gas Industry Unsafe Situations Procedure gives examples, and L56 refers to it throughout. It is published through Gas Safe Register.

What to do with an unsafe appliance

Regulation 34(3) says an engineer who knows or suspects an appliance is dangerous must straight away take all reasonably practicable steps to tell the responsible person for the premises and, if different, the owner of the appliance. HSE’s guidance on how to do this (L56, paragraphs 286 to 288):

1. Tell the user, and the owner if different, in writing that the appliance is unsafe, could cause death or serious injury, and that using it is an offence. In rented homes the owner is the landlord, or the managing agent where the landlord has passed gas safety to them.
2. Try to get their agreement to repair or disconnect it.
3. If they refuse, you have no legal power to disconnect it. Label it as unsafe, and close the appliance shut-off device if one is fitted and they agree.
4. If you still cannot get agreement to repair or disconnect it, contact the Gas Emergency number, 0800 111 999, for natural gas, or the gas supplier for LPG.

Once told, the responsible person must not use the appliance or let anyone else use it (regulation 34(1)). In rented homes this duty covers tenants, landlords and managing agents (L56, paragraph 281).

Reporting to HSE

Under RIDDOR 2013, regulation 11(2), an employer or self-employed person in the class approved by HSE (in practice, Gas Safe registered) who has enough information to decide a gas fitting is, or could have been, likely to cause death, loss of consciousness or a hospital visit through gas leakage, incomplete combustion or poor removal of combustion products must report it to HSE within 14 days. HSE says a report will generally be needed where disconnecting the appliance is considered necessary (L56, paragraph 290).

Suspected carbon monoxide or a smell of gas

If you smell gas or suspect carbon monoxide from a natural gas appliance on the mains, call the National Gas Emergency Service on 0800 111 999 (L56, paragraph 327). The law treats carbon monoxide from incomplete combustion as an escape of gas (regulation 37(8)). The responsible person for the premises must shut off the supply straight away, and tell the gas supplier if the escape or smell continues (regulation 37(2) and (3)). The supply must not be turned back on, except to repair it, until steps have been taken to stop it happening again (regulation 37(4)).

Landlord gas safety checks

A landlord’s annual safety check must include the regulation 26(9) examinations (regulation 36(9)). Combustion performance is part of that check when operating pressure or heat input cannot practicably be examined. The record must include, among other things, any safety defect found, any remedial action taken, and confirmation that the check met regulation 36(9). It must be kept until there have been two further checks (regulation 36(3)(c)).

What to record

The regulations do not list analyser readings as a required entry. If you took readings, record what you measured and the manufacturer’s figures you compared them with, along with any defect, who you told and what you did about it.

Sources

• Gas Safety (Installation and Use) Regulations 1998, regulation 26
• Regulation 34, regulation 36 and regulation 37
• HSE, L56: Safety in the installation and use of gas systems and appliances (fifth edition, PDF download)
• RIDDOR 2013, regulation 11

Published 2026-07-27. Corrected and updated 2026-09-30. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.