Free Practice Questions for ACS CCN1
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Key Facts: ACS CCN1 Exam
118
Practice Questions
OpenExamPrep
Open book
Theory + Practical
ACS / Gas Safe
20-21 mbar
Tightness Test Pressure
IGEM/UP/1B
0.004
Max CO/CO2 Ratio
BS 5440 / Gas Safe
Every 5 years
Re-assessment Cycle
ACS
Gas Safe
Required for Registration
Gas Safe Register
ACS CCN1 (Core Domestic Natural Gas Safety) is the core assessment a gas operative must hold to register with the Gas Safe Register for domestic natural gas work in Great Britain. It is open book, combining written theory (multiple-choice, short-written and scenario questions) with an observed practical, and covers the Gas Safety (Installation and Use) Regulations 1998, tightness testing and let-by (20-21 mbar over 2 minutes, with permissible drops set by IGEM/UP/1B — U6/G4 meter tables under Edition 3, installation volume under Edition 4 from 1 October 2026), gas rating and calorific value, pipework, ventilation (BS 5440-2), flueing and spillage testing, combustion and CO/CO2 analysis (0.004 ratio limit), and the Gas Industry Unsafe Situations Procedure (ID/AR — NCS was withdrawn in 2016). CCN1 is typically re-assessed every 5 years. This free prep includes 118 research-based practice questions with explanations and an AI tutor.
Sample ACS CCN1 Practice Questions
Try these sample questions to review concepts for the ACS CCN1 exam. Each question includes a detailed explanation. Start the interactive quiz above for the full 118+ question experience with AI tutoring.
1Which set of regulations places the primary legal duty on gas operatives to ensure domestic gas appliances and installations are safe?
2Under GSIUR 1998, a person carrying out gas work for hire or reward in domestic premises must be registered with which body?
3What is the approximate gross calorific value (CV) of UK natural gas used when calculating heat input?
4When gas rating a domestic appliance on a metric (m3) meter, what does the timed test dial reading allow you to calculate?
5What is the standard nominal operating (working) pressure for natural gas at the outlet of a domestic meter?
6Before applying a tightness test on a domestic installation with a U6/G4 meter, why is a let-by test performed first?
7On a domestic natural gas installation, what test pressure is applied during a tightness test using a U6/G4 meter?
8What is the duration of the timed tightness test (after stabilisation) on a domestic installation?
9On an existing installation with a U6/G4 meter and pipework up to 28mm with appliances connected, what is the maximum permissible pressure drop over the 2-minute tightness test?
10On a NEW domestic installation with no appliances connected (pipework only), what pressure drop is permitted during the tightness test?
About the ACS CCN1 Exam
The ACS CCN1 Core Domestic Natural Gas Safety Assessment is the core competence assessment required for Gas Safe registration in domestic natural gas work. It is an open-book assessment combining written theory (multiple-choice, short-written and scenario questions) with an observed practical, covering GSIUR 1998, tightness testing, gas rating, ventilation, flueing, combustion analysis and unsafe situations.
Exam sponsor: Gas Safe Register via ACS-approved assessment centres (e.g. LCL Awards, BPEC). The requirements and fees below concern the certification or admission exam, separate from our free practice resources.
Assessment
Open-book written theory (multiple-choice, short-written and scenario questions on GSIUR, tightness testing, ventilation, flueing and combustion) plus an observed practical assessment; each subject area must be passed. This practice bank is 118 selected-response items for the theory knowledge.
Time Limit
Varies by ACS centre and assessment package
Passing Score
Each subject area must be passed (centres typically require around 80% on theory components)
Exam / Certification Fees
Approximately £350-£600 (varies by centre; initial vs re-assessment differ)
Exam sponsor websiteFees, eligibility, and exam policies can change. Confirm them with the exam sponsor before applying or paying.
Official sources
Our practice resources: topics covered
We aim to reflect publicly available exam outlines and topic information in our study resources. Coverage, format, and difficulty may differ from the actual exam, and we cannot guarantee that every detail is accurate or current. Confirm exam requirements, fees, and policies with the official exam sponsor.
Gas Safety Legislation (GSIUR 1998)
GSIUR 1998, Gas Safe registration, landlord duties (Regulation 36 / CP12), and operative responsibilities
Tightness Testing & Let-By
Let-by test of the ECV, tightness test at 20-21 mbar over 2 minutes, and permissible pressure drops per IGEM/UP/1B (Edition 4, effective 1 October 2026, sets drops by installation volume rather than U6/G4 meter tables)
Gas Rating & Calorific Value
Gas rating, gross CV (about 39.5 MJ/m3), heat input, natural gas properties, working and standing pressure
Pipework Sizing & Installation
Pipework materials, 1 mbar maximum pressure drop, sleeving, purging, and equipotential bonding within 600mm of the meter
Ventilation Requirements
BS 5440-2 ventilation, the 5 cm2/kW open-flue rule above 7kW, adventitious air, air vents, and flueless-appliance ventilation
Flue Types, Flue Flow & Spillage Testing
Open-flued, room-sealed and flueless appliances, flue flow and spillage testing, and terminal siting (BS 5440-1)
Combustion & CO/CO2 Ratio Analysis
Complete vs incomplete combustion, flue gas analysis, the 0.004 CO/CO2 ratio limit, CO hazards and flame picture
Emergency & Unsafe Situations (GIUSP / RIDDOR)
GIUSP ID/AR (NCS was withdrawn in July 2016), warning notices, the ECV, gas escape procedures (0800 111 999), and RIDDOR reporting
Preparing for the ACS CCN1 Exam
What You Need to Know
- Passing score: Each subject area must be passed (centres typically require around 80% on theory components)
- Assessment: Open-book written theory (multiple-choice, short-written and scenario questions on GSIUR, tightness testing, ventilation, flueing and combustion) plus an observed practical assessment; each subject area must be passed. This practice bank is 118 selected-response items for the theory knowledge.
- Time limit: Varies by ACS centre and assessment package
- Exam / certification fees: Approximately £350-£600 (varies by centre; initial vs re-assessment differ) Official sources
Using Our Practice Resources
- Work through all 118 available questions
- Review every answer and explanation
- Track weak areas and revisit them
- Use our AI tutor for tough concepts
ACS CCN1: Suggested Study Strategy
Frequently Asked Questions
What is the ACS CCN1 assessment and why do I need it?
CCN1 is the Core Domestic Natural Gas Safety assessment. It is the core competence required to register with the Gas Safe Register for domestic natural gas work, so you must hold it (alongside relevant appliance modules) to work legally on domestic gas in Great Britain.
How is the CCN1 assessment structured?
CCN1 is open book and combines written theory (multiple-choice, short-written and scenario questions) with an observed practical. Each subject area must be passed, and theory components at most centres require around 80%.
What pressures and pressure drops should I know for the tightness test?
The tightness test on natural gas uses 20-21 mbar over a 2-minute timed test after a 1-minute stabilisation. Under IGEM/UP/1B Edition 3 (valid until 30 September 2026), the permissible drop on an existing U6/G4 installation with appliances is 4 mbar for pipework up to 28mm and 2.5 mbar over 28mm; a new pipework-only test permits no drop. From 1 October 2026, Edition 4 sets permissible drops by installation volume instead of meter size.
What CO/CO2 ratio is considered safe in combustion analysis?
A CO/CO2 ratio of 0.004 or below indicates safe, complete combustion for a modern domestic appliance. A ratio above 0.004 suggests incomplete combustion and a potential carbon monoxide hazard that must be investigated.
What do the GIUSP classifications ID and AR mean?
Under the current Gas Industry Unsafe Situations Procedure (IGEM/G/11 Edition 2, amended to July 2025), Immediately Dangerous (ID) is an immediate danger to life or property and At Risk (AR) is a fault that could become dangerous. Not to Current Standards (NCS) was removed from the procedure in July 2016 and is no longer recorded on warning notices.
How often must CCN1 be renewed?
ACS gas competencies such as CCN1 are typically re-assessed every 5 years to keep operatives current with standards and to maintain Gas Safe registration.