Gas Unsafe Situations: ID, AR & NCS Classifications — What to Do

Quick Answer: Gas Safe engineers must classify any unsafe gas situation using one of three categories: Immediately Dangerous (ID) — turn off and do not use; At Risk (AR) — advise the customer of the risk and recommend action; Not to Current Standards (NCS) — does not comply with current regulations but is not immediately dangerous. Gas Safe engineers have a legal and professional duty to act on unsafe situations and must not leave an ID appliance in service. Refer to the Gas Safe Unsafe Situations procedure, which is the industry standard for all registered engineers.

Summary

The classification of unsafe gas situations is one of the most important skills a Gas Safe engineer must have. Getting this wrong — either by over-classifying a safe situation (causing unnecessary distress) or under-classifying a dangerous one (leaving someone at risk) — has real consequences: for the occupant's safety, for the engineer's professional registration, and potentially for criminal liability. The Gas Safe Unsafe Situations procedure provides a structured, documented framework for these decisions.

The three classifications — Immediately Dangerous (ID), At Risk (AR), and Not to Current Standards (NCS) — are not just labels. Each carries specific required actions. An ID classification requires the engineer to turn off and isolate the appliance, and to obtain the customer's agreement to do so (or document refusal). An AR classification requires the engineer to advise the customer in writing of the nature of the risk and recommended action. An NCS classification is an advisory note — the appliance is safe to use but should be updated to current standards at the next practicable opportunity.

It is important to understand that classifying a situation correctly requires technical knowledge, professional judgement, and documentation. Engineers must be able to explain their classification to a customer clearly, issue the correct paperwork (typically a Warning Notice), and advise on the appropriate next steps. The Gas Safe Register publishes guidance on this procedure which all registered engineers should have read and understood.

Key Facts

• Immediately Dangerous (ID) — a fault that poses an immediate risk to life or property if the appliance continues to operate; must be turned off and isolated before the engineer leaves
• At Risk (AR) — a fault that does not pose an immediate risk but could become dangerous; customer must be advised in writing; recommended action is to repair or replace the appliance
• Not to Current Standards (NCS) — does not comply with current regulations or best practice but is not dangerous; advisory only; no mandatory isolation required
• Warning Notice — a formal document issued to the customer explaining the classification, the fault, and the required action; must be given in writing
• Customer refuses to allow isolation — if an ID situation is identified and the customer refuses to allow the engineer to isolate the appliance, the engineer must document this refusal and notify the Gas Safe Register; the engineer must not leave the appliance in a dangerous condition without informing the appropriate authority
• Gas escape (significant) — always an ID situation; the national gas emergency line is 0800 111 999
• Inadequate ventilation — can be ID or AR depending on severity; an open-flue appliance with zero ventilation in a sealed room is likely ID
• Carbon monoxide risk — high CO in room air (>100ppm) from an appliance is an ID situation; evacuate and call emergency services
• RIDDOR — certain dangerous occurrences involving gas must be reported to HSE under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013
• Gas Safe engineer duty — Gas Safe registered engineers have a statutory duty under the Gas Safety (Installation and Use) Regulations 1998 to deal appropriately with unsafe situations
• Not your installation — engineers have the same duty whether or not they installed the appliance; if you attend and find an unsafe situation, you must act
• Appliance age not a classification criterion — an old appliance is not automatically unsafe; it must have a specific, identifiable defect to be classified
• Documentation — always complete a Warning Notice for ID and AR; retain a copy; photograph the defect if possible

Quick Reference Table

Detailed Guidance

Classification Decision Framework

UNSAFE SITUATION IDENTIFIED

Is there a risk of IMMEDIATE danger to life or property?
├── YES → Classify as IMMEDIATELY DANGEROUS (ID)
│   ├── Inform the customer of the danger
│   ├── Request permission to turn off/isolate
│   │   ├── Customer agrees → Isolate; label; complete Warning Notice
│   │   └── Customer refuses → Document refusal; notify Gas Safe Register
│   └── Issue ID Warning Notice; retain copy
│
└── NO → Could this situation BECOME dangerous?
    ├── YES → Classify as AT RISK (AR)
    │   ├── Explain the risk to the customer
    │   ├── Recommend immediate repair or replacement
    │   ├── Confirm in writing with AR Warning Notice
    │   └── Offer to carry out repair; do not leave without documentation
    │
    └── NO → Does it comply with current standards?
        ├── NO  → Classify as NCS
        │   ├── Advise the customer verbally and in writing
        │   └── Recommend update at next service/opportunity
        └── YES → No classification required; record as satisfactory

Immediately Dangerous (ID) Situations — Examples

Confirmed gas escape: Any confirmed gas escape from pipework or an appliance body is ID. Turn off the ECV immediately. Do not operate any electrical switches. Ventilate. Call 0800 111 999 if the escape is significant.

Severe flue gas spillage: If combustion products are spilling into the living space from an open-flue appliance, and particularly if CO readings in room air are elevated (>100ppm), the situation is ID. Turn off the appliance, ventilate, and advise the customer to vacate until the cause is rectified.

Cracked heat exchanger with confirmed high CO: A cracked heat exchanger that allows combustion products into the heating circuit or living space, confirmed by high CO readings in flue products or room air, is ID.

No effective flue: An appliance with a disconnected, missing, or completely obstructed flue cannot safely exhaust combustion products — ID. This applies equally to boilers, gas fires, and water heaters.

Uncontrollable gas release: If gas cannot be isolated because the ECV or isolation valve is defective — ID. The gas emergency service must be called.

At Risk (AR) Situations — Examples

Borderline combustion: CO/CO2 ratio outside acceptable limits but not at levels causing immediate room-air CO elevation; appliance appears to be operating but combustion is substandard. The appliance should be repaired before next use.

Inadequate ventilation: An open-flue appliance with reduced but not zero ventilation. The appliance is combusting correctly under current conditions, but if the ventilation is further reduced (e.g. by draught-proofing), a dangerous situation will result.

Pilot light repeatedly extinguishing: May indicate thermocouple failure, inadequate gas supply, or draughts. The appliance is currently off when the pilot is out, so not immediately dangerous, but is likely to become so if left.

Appliance sealing defect: A minor crack in the appliance casing that could allow flue gas ingress into the room air under adverse conditions, but where current CO readings are acceptable.

Not to Current Standards (NCS) — Examples

Missing isolation valve: An appliance should have an accessible isolation valve within 1.0m of the appliance per current standards. An old installation without this valve but with no gas safety defect is NCS.

Old pipe material: Lead gas pipes, or early plastic pipes not approved for current use, that are structurally sound and leak-free but would not be installed today.

Appliance installed too close to combustible materials: Does not meet current clearance requirements but is not causing a fire hazard; NCS.

Missing Benchmark record: A boiler installed without a Benchmark logbook completed is not dangerous but does not meet current good practice — NCS.

Warning Notice Procedure

A Warning Notice must be completed and handed to the customer for all ID and AR classifications. The notice should include:

1. Date and time of visit
2. Engineer name and Gas Safe registration number
3. Address of property
4. Description of the appliance (make, model, location)
5. Classification (ID or AR)
6. Description of the defect found
7. Action taken (isolated, labelled, etc.)
8. Recommended action for the customer
9. Customer signature (confirming they have been informed)
10. Copy retained by engineer

Labels should be attached to isolated appliances stating: "DO NOT USE — Gas Safe Warning — [date] — [engineer Gas Safe number]"

RIDDOR Reporting

Certain gas-related incidents require RIDDOR reporting to HSE:

• Unintended ignition or explosion of gas
• Unintended fire caused by gas
• Incomplete combustion of gas creating a risk of harm
• Uncontrolled release of flammable gas

Consult HSE guidance or your Gas Safe Register support line if you are unsure whether a specific incident triggers RIDDOR.

Frequently Asked Questions

Can I refuse to work on an appliance if I think it's dangerous but the customer wants me to fix it?

Yes. If an appliance is classified as ID and you have isolated it, you are not obliged to reconnect or repair it if you believe it cannot be made safe. You must explain this to the customer and document your position. You should also ensure the customer understands they should not attempt to reconnect the appliance themselves.

What if I'm not sure whether something is ID or AR?

When in doubt, err on the side of caution. A situation that you believe might be immediately dangerous should be treated as ID until you can confirm it is not. The consequences of under-classifying a genuinely dangerous situation are far more serious than over-classifying one that turns out to be AR.

Does the customer have to sign the Warning Notice?

It is best practice to obtain the customer's signature to confirm they have been informed, but a customer cannot legally prevent an engineer from issuing a Warning Notice or from isolating an ID appliance. If the customer refuses to sign, note this on the form and retain your copy. Document all communications.

Can I classify something as NCS and then leave it without issuing any paperwork?

For NCS you should still provide written advisory information — either on the service record, a specific NCS notice, or in a follow-up letter. While NCS does not require a formal Warning Notice, a written record protects both the engineer and the customer and ensures the advisory is not forgotten.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — statutory framework for gas safety in Great Britain; engineers' duties regarding unsafe situations

• Gas Safe Register — Unsafe Situations Procedure — the mandatory procedure for all Gas Safe registered engineers when an unsafe situation is found

• RIDDOR 2013 — Reporting of Injuries, Diseases and Dangerous Occurrences Regulations: gas-related incidents may be reportable

• Health and Safety at Work etc. Act 1974 — general duty of care applicable to gas engineers

• Gas Safe Register — Unsafe Situations Guidance — Official guidance and Warning Notice templates

• HSE — Gas Safety — Regulatory overview and RIDDOR guidance

• National Gas Emergency Service — 0800 111 999 — Emergency line for gas escapes

• Gas Safe Technical Bulletins — Specific technical guidance bulletins for engineers

• gas safe requirements — Gas Safe registration requirements

• flue gas analysis — Combustion testing and interpreting CO readings

• boiler servicing — Annual service checklist

• carbon monoxide — CO risks and alarm requirements