Checked 21 October 2026. Read from CCN1 Issue 20.1, which IGEM's resource library lists as Oct 2026. Certification bodies may number and word the criteria differently, so check the issue your centre assesses against.
CCN1 is the core domestic natural gas safety assessment in the Nationally Accredited Certification Scheme for Individual Gas Fitting Operatives (ACS). IGEM's Assessment page gives the plain example: to work on boilers in a domestic property an engineer needs CCN1 (core domestic gas safety) and CENWAT (Domestic Central Heating Boilers). This guide lists what the CCN1 criteria document says the assessment covers, taken from ACS.CCN1 Safety Assessment Criteria, Issue 20.1, which IGEM's resource library lists for October 2026.
ACS.CCN1 Safety Assessment Criteria Issue 20.1 and IGEM's Assessment page, read 21 Oct 2026.
- 10 topic tests for ACS Gas Assessment (UK): CCN1 and Appliance Modules, one topic at a time
- Every answer explained in full
- A full report on every attempt — what went wrong and what to revise
- Unlimited retakes — one payment, yours for good
What the criteria document says CCN1 is
The document opens with one line: CCN1 "Tests gas safety competence in core domestic gas work." Its range is "All domestic gas fittings." For an initial assessment the prerequisite is set by "ACS Entry Routes & Changeover Requirements (Guidance Note 8)"; for re-assessment it is the initial CCN1 "Or Group Competency Certificate."
Two lines matter before you revise from any list, including ours: "CBs may adopt Competence and Criteria numbering different to that used in this document", and CB documentation "may adopt wording for criteria different to that used in this document, provided the meaning is unaffected." CB stands for Certification Body. Your centre's paperwork may look different; the content is the same.
The 16 areas, in the document's order
Each area has performance criteria (things you do) and knowledge and understanding (things you explain or answer), or only one of the two. The criteria tables mark items for I (initial) or R (re-assessment) or both. The last column below is our summary of which kinds of criteria each area's table carries.
| Area | Area title (CCN1 Issue 20.1) | Criteria in the table |
|---|---|---|
| 1 | Gas safety legislation and Standards | Knowledge |
| 2 | Gas emergency actions and procedures | Knowledge |
| 3 | Products and characteristics of combustion | Practical and knowledge |
| 4 | Ventilation | Practical and knowledge |
| 5 | Installation of pipework and fittings (pipe sizes 6 mm to 35 mm) | Practical and knowledge |
| 6 | Tightness testing and purging | Practical and knowledge |
| 7 | Checking operating pressure at outlet of meter | Practical and knowledge |
| 8 | Unsafe situations, emergency notices and warning labels | Practical and knowledge |
| 9 | Operation and positioning of emergency isolation controls and valves | Practical and knowledge |
| 10 | Checking and setting appliance burner pressures and gas rates | Practical and knowledge |
| 11 | Operation and checking of appliance gas safety devices and controls | Practical and knowledge |
| 12 | Chimney Standards | Knowledge |
| 13 | Chimney inspection and testing | Practical and knowledge |
| 14 | Installation of open, balanced and fan assisted chimneys | Practical and knowledge |
| 15 | Re-establish existing gas supply and relight appliances | Practical and knowledge |
| 16 | Inspect, test, commission and maintain gas appliances | Practical, at re-assessment only |
Two details are easy to miss. In ventilation, some items are marked with an asterisk: "* denotes K&U for Initial but PC for Re-assessment", so an item you only explained at your first assessment can become something you must do at re-assessment. And the abbreviations list includes "OQ. Oral questioning", and items such as area 7's "effects of low and high flow rates on regulator outlet pressures" are marked (OQ).
What some areas actually ask for
Area titles are short; the criteria under them are specific. A few examples, quoted from Issue 20.1:
| Area | Examples of criteria (quoted) |
|---|---|
| 2. Gas emergency actions | "priorities of actions and responsibilities"; "limits of flammability"; "specific gravity and its effect in relation to air"; "hazardous ignition sources and their elimination"; "advice to occupants" |
| 3. Combustion | "inspect flame picture of burners visually"; "symptoms of CO poisoning"; "causes of activation of CO detectors and indicators"; "read and record CO/CO2 ratios for each appliance" |
| 7. Meter outlet pressure | "zero pressure gauge and connect to meter test point"; "observe and record standing pressure at test point"; "remove gauge and test point; re-seal and test for gas tightness" |
| 8. Unsafe situations | "identify unsafe situations"; "classify unsafe situations as ID & AR"; "demonstrate procedure for each unsafe situation to GIUSP"; "complete, explain, issue appropriate warning/advisory notices to appropriate persons" |
The criterion classifies unsafe situations as ID or AR, the terms of the Gas Industry Unsafe Situations Procedure (IGEM/G/11), which Gas Safe Register's Registration Policy requires every registered business to apply in its most current edition. The criteria test that you can classify a situation and act on it; the procedure itself is in the GIUSP, which you should study from the current edition.
Area 1: the regulations CCN1 names
The legislation area lists regulations of the Gas Safety (Installation and Use) Regulations 1998 by number, under the heading HSL56 (HSE's L56 book, which prints the regulations with their Approved Code of Practice). For an initial assessment it ticks regulations 2, 3, 5, 6, 7, 8, 25, 26, 35 and 36 (duties of landlords); regulation 4 (duty on employer) is listed without a tick. Both initial and re-assessment cover awareness of the rules for non-combustible materials through exterior walls in high-risk residential buildings and "Requirements for CO alarms in rented properties.", and re-assessment ticks regulation 26(9)(ca). The emergency area adds regulation 37 (escape of gas) for an initial assessment.
Two of those regulations are worth knowing word for word:
"Where a person carries out any work in relation to a gas fitting which might affect the gas tightness of the gas installation he shall immediately thereafter test the installation for gas tightness at least as far as the nearest valves upstream and downstream in the installation."
"Where a person performs work on a gas appliance he shall immediately thereafter examine— (a) the effectiveness of any flue; (b) the supply of combustion air; (c) subject to sub-paragraph (ca), its operating pressure or heat input or, where necessary, both; (ca) if it is not reasonably practicable to examine its operating pressure or heat input (or, where necessary, both), its combustion performance; (d) its operation so as to ensure its safe functioning, and forthwith take all reasonable practicable steps to notify any defect to the responsible person and, where different, the owner of the premises in which the appliance is situated ..."
The standards the document lists
Under "References and normative documents" CCN1 Issue 20.1 names manufacturers' instructions and "All relevant documents as listed in the Legislative, Normative & Informative Document List (LNIDL)", including:
| Document | Note |
|---|---|
| HSL56 | HSE's L56, which prints the Regulations with their Approved Code of Practice and guidance |
| BS 5440-1 2023 and BS 5440-2 2023 | Listed by number and year |
| IGEM/G/11 GIUSP | The document's own abbreviation list: "Gas Industry Unsafe Situation Procedure" |
| IGEM/UP/1B, IGEM/UP/17, IGEM/G/5, IGEM/G/13 | Listed by number |
| Building Regulations (ADB, ADF, ADJ) | Approved Documents; the document says these are England examples and regional differences may apply in the devolved nations |
| The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 | Listed by name |
This guide does not give test pressures, ventilation sizes or CO/CO2 action levels. Learn them from the current editions of the documents your centre lists, not from a summary.
Try one question
You have replaced a short section of installation pipework that sits between two valves. What is the least that regulation 6(6) requires you to test for gas tightness?
- A. Only the joints you made
- B. The installation at least as far as the nearest valves upstream and downstream
- C. Nothing, if every joint was made with new fittings
- D. Only the pipe between the meter and the first valve
Show the answer
B. Regulation 6(6) requires a tightness test "at least as far as the nearest valves upstream and downstream in the installation", immediately after any work that might affect gas tightness. Testing only your own joints falls short of that.
Our own question, written from the regulation quoted. It is not an ACS assessment question.
What CCN1 is not
- Not a registration. CCN1 is an assessment of competence; for registration see ( our guide to modules and registration).color:#255a45;font-weight:600">our guide to modules and registration).
- Not enough on its own for appliance work. Appliance modules such as CENWAT, CKR1 and HTR1 each list CCN1 (or an equivalent route) as a prerequisite.
- Not open to anyone who books. New entrants first complete industry-recognised training; IGEM's Guidance Note 8 sets the entry routes.
- Not for life. IGEM: engineers "are required to renew their certificates of competency every 5 years".
Frequently asked questions
What is the CCN1 assessment?
The core domestic natural gas safety assessment in the ACS scheme. Its criteria document says it "Tests gas safety competence in core domestic gas work" and its range is "All domestic gas fittings."
How many areas does CCN1 cover?
Sixteen in Issue 20.1, from gas safety legislation (area 1) to inspecting, testing, commissioning and maintaining gas appliances (area 16, at re-assessment only).
Is CCN1 a written exam or a practical?
Both kinds of criteria are in it: performance criteria you carry out and knowledge and understanding you answer. Some items are marked OQ, oral questioning; for the rest the document does not say in what form they are asked, so ask your centre for the candidate information.
Does CCN1 include combustion analysis?
Yes. Area 3 includes combustion performance analysis, with criteria such as checking the analyser and reading and recording CO/CO2 ratios, and appliance modules such as CENWAT give CCN1 "taken since April 2012" as their prerequisite.
Which regulations does CCN1 test?
For an initial assessment, area 1 ticks regulations 2, 3, 5, 6, 7, 8, 25, 26, 35 and 36 of the Gas Safety (Installation and Use) Regulations 1998, and area 2 adds regulation 37 on escapes of gas. Re-assessment ticks regulation 26(9)(ca).
ProSyllabus is an independent study site, not part of Gas Safe Register, HSE, IGEM or any ACS certification body. Rules here are quoted from the regulations and the scheme's own documents; ask your assessment centre how your own assessment will run.
Sources
- official — IGEM resource library: ACS.CCN1 Safety Assessment Criteria, Initial and Re-assessment, Domestic Natural Gas, Issue 20.1 (listed Oct 2026)
- official — IGEM: Assessment, overview of Matters of Gas Safety, ACS and other routes to competence (read 21 Oct 2026)
- official — legislation.gov.uk: GSIUR 1998, regulation 6 (General safety precautions)
- official — legislation.gov.uk: GSIUR 1998, regulation 26 (Gas appliances, safety precautions; 26(9)(ca) inserted 6 Apr 2018)
- official — HSE: L56 "Safety in the installation and use of gas systems and appliances", ACOP and guidance, fifth edition 2018 (paras 81, 85, 87, 102)
- official — IGEM resource library: assessment criteria documents (CENWAT Issue 6.2, CKR1 Issue 5.5, HTR1 Issue 5.5 and others)
- official — IGEM: ACS Entry Routes and Changeover Requirements, Guidance Note 8, version 8.1 (implementation 1 Oct 2024), linked from the Assessment page
official = a document published by the conducting body. reported = a news or coaching site we could not check against an original. Where sources disagree this page says so rather than picking one.
