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. On an existing U6/G4 installation with appliances, the permissible drop is 4 mbar for pipework up to 28mm and 2.5 mbar over 28mm; a new pipework-only test permits no drop.

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, AR and NCS mean?

Under the Gas Industry Unsafe Situations Procedure (IGEM/G/11), Immediately Dangerous (ID) is an immediate danger to life or property, At Risk (AR) is a fault that could become dangerous, and Not to Current Standards (NCS) is safe to use but does not meet current standards.

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.

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2026 Statistics

Key Facts: ACS CCN1 Exam

100

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 of 4 mbar up to 28mm and 2.5 mbar over 28mm on a U6/G4 meter), 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). CCN1 is typically re-assessed every 5 years. This free prep includes 100 research-based practice questions with explanations and an AI tutor.

Sample ACS CCN1 Practice Questions

Try these sample questions to test your ACS CCN1 exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ 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?

A.The Gas Act 1986

B.The Gas Safety (Installation and Use) Regulations 1998 (GSIUR)

C.The Pressure Systems Safety Regulations 2000

D.The Construction (Design and Management) Regulations 2015

Explanation: The Gas Safety (Installation and Use) Regulations 1998 (GSIUR) is the principal legislation governing the installation, maintenance and use of gas fittings and appliances in domestic premises. It requires anyone carrying out gas work to be competent and on the Gas Safe Register.

2Under GSIUR 1998, a person carrying out gas work for hire or reward in domestic premises must be registered with which body?

A.CORGI

B.The Gas Safe Register

C.OFGEM

D.The Health and Safety Executive

Explanation: Since 1 April 2009 the Gas Safe Register replaced CORGI as the official gas registration body in Great Britain. Regulation 3 of GSIUR requires businesses and operatives working on gas for reward to be on the Gas Safe Register.

3What is the approximate gross calorific value (CV) of UK natural gas used when calculating heat input?

A.19.5 MJ/m3

B.29.3 MJ/m3

C.39.5 MJ/m3

D.93.0 MJ/m3

Explanation: UK natural gas has a gross calorific value of approximately 39.5 MJ/m3 (about 10.76 kWh/m3). This figure is used in the heat-input formula to convert a measured gas rate in m3/h into kW.

4When gas rating a domestic appliance on a metric (m3) meter, what does the timed test dial reading allow you to calculate?

A.The standing pressure only

B.The gas rate in m3/h, from which heat input in kW can be derived

C.The flue draught

D.The CO/CO2 ratio

Explanation: Gas rating times one revolution (or a known volume) of the meter dial. Gas rate (m3/h) = volume x 3600 / time(s). Multiplying the gas rate by the calorific value (kWh/m3) gives the gross heat input in kW, which is compared with the data plate.

5What is the standard nominal operating (working) pressure for natural gas at the outlet of a domestic meter?

A.7 mbar

B.21 mbar

C.37 mbar

D.50 mbar

Explanation: Domestic natural gas is supplied at a nominal working pressure of 21 mbar (commonly stated as 20-21 mbar) at the meter outlet. The meter regulator controls the supply down to this pressure.

6Before applying a tightness test on a domestic installation with a U6/G4 meter, why is a let-by test performed first?

A.To purge air from the pipework

B.To confirm the emergency control valve (ECV) is holding and not letting gas pass

C.To set the meter regulator

D.To measure appliance gas rate

Explanation: The let-by test checks that the emergency control valve seals correctly. With the ECV closed and around 7-10 mbar applied, the gauge must show no rise over one minute; a rise means the valve is letting by and the tightness test cannot be relied upon.

7On a domestic natural gas installation, what test pressure is applied during a tightness test using a U6/G4 meter?

A.7 mbar

B.10 mbar

C.20-21 mbar

D.30 mbar

Explanation: The tightness test on natural gas is carried out at the normal working pressure of 20-21 mbar. The system is raised to this pressure, allowed to stabilise, then timed for the test period.

8What is the duration of the timed tightness test (after stabilisation) on a domestic installation?

A.30 seconds

B.1 minute

C.2 minutes

D.5 minutes

Explanation: After raising the system to 20-21 mbar and allowing a 1-minute stabilisation period, the timed tightness test on a domestic installation is run for 2 minutes, during which the permissible pressure drop is assessed.

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?

A.No drop permitted

B.1 mbar

C.4 mbar

D.8 mbar

Explanation: For an existing installation on a U6/G4 meter with pipework up to 28mm and appliances connected, a maximum drop of 4 mbar is permitted over the 2-minute test (IGEM/UP/1B). For pipework over 28mm the allowance is reduced to 2.5 mbar.

10On a NEW domestic installation with no appliances connected (pipework only), what pressure drop is permitted during the tightness test?

A.No pressure drop is permitted

B.Up to 4 mbar

C.Up to 2.5 mbar

D.Up to 8 mbar

Explanation: On a new installation tested as pipework only (no appliances), no pressure drop is permissible over the 2-minute test. Any detectable drop indicates a leak that must be traced and rectified before commissioning.

About the ACS CCN1 Practice Questions

Verified exam format metadata for ACS CCN1 Core Domestic Natural Gas Safety Assessment is pending. The practice questions above remain available while official exam length, timing, passing score, fee, and administrator details are reviewed.

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