ACS CCN1 Core Domestic Gas Safety Assessment — Questions and Answers

Question 1: What should an engineer do if a gas appliance cannot achieve the correct working pressure during commissioning?

Do not commission the appliance and investigate the cause of the low pressure (Correct answer)
Commission the appliance and note the pressure deficit in the paperwork
Inform the customer and leave the appliance in service at reduced input
Adjust the appliance gas valve to compensate for the low pressure

Correct answer: Do not commission the appliance and investigate the cause of the low pressure

An appliance that cannot achieve its design working pressure must not be commissioned, as incorrect gas pressure leads to incomplete combustion, poor efficiency, and potentially dangerous operating conditions.

Question 2: Which of the following is an example of a RIDDOR-reportable dangerous gas fitting?

A boiler that needs its annual service
A gas cooker that has a slightly noisy burner
A flue terminal that is positioned too close to an opening window, allowing combustion products to re-enter the property (Correct answer)
A gas meter box with cosmetic damage to the door

Correct answer: A flue terminal that is positioned too close to an opening window, allowing combustion products to re-enter the property

A flue terminal positioned too close to an opening window that allows combustion products to re-enter the property is a dangerous situation that could lead to carbon monoxide poisoning. This must be classified under GIUSP (likely AR or ID depending on the specifics) and, if considered dangerous to the extent it could cause death, loss of consciousness, or hospitalisation, must be reported under RIDDOR.

Question 3: Why must electrical switches NOT be operated when a gas escape is suspected?

Switches emit RF signals that affect gas sensors
Switches can interfere with the gas meter's safety shut-off
Switches are gas-operated and will increase the flow
Electrical switches create sparks that can ignite the gas-air mixture, causing an explosion or fire (Correct answer)

Correct answer: Electrical switches create sparks that can ignite the gas-air mixture, causing an explosion or fire

Even low-energy electrical switches create small sparks when opened or closed. If gas has accumulated to within its flammable range (5-15% in air for natural gas), this spark can ignite the mixture, causing a fire or explosion.

Question 4: What is the purpose of a flue gas analysis test carried out during commissioning of a gas boiler?

To check the gas supply pressure
To test the condensate drainage system
To verify the electrical connections are correct
To measure the carbon monoxide, carbon dioxide, and flue gas temperature to verify safe and efficient combustion (Correct answer)

Correct answer: To measure the carbon monoxide, carbon dioxide, and flue gas temperature to verify safe and efficient combustion

A flue gas analysis (FGA) measures the levels of CO, CO₂, O₂, and flue gas temperature in the products of combustion. This verifies that the appliance is burning efficiently and safely, with combustion products within acceptable limits. The CO/CO₂ ratio must be within safe limits (maximum 0.004 for most boilers).

Question 5: What safety feature prevents a gas appliance from operating if the fan in a fanned-flue system fails?

The room thermostat
The timer programmer
An air pressure switch (APS) that proves fan operation before allowing the gas valve to open (Correct answer)
The overheat thermostat

Correct answer: An air pressure switch (APS) that proves fan operation before allowing the gas valve to open

An air pressure switch (APS) is a safety device that detects whether the fan is running and creating sufficient draught before allowing the gas valve to open. If the fan fails to operate or cannot generate adequate pressure differential, the APS will not make and the gas valve will remain closed, preventing the burner from firing without a safe means of flue gas evacuation.

Question 6: After a gas escape has been confirmed and the supply turned off, when may the gas supply be restored?

After 30 minutes to allow gas to disperse
Only after the source of the escape has been identified and repaired by a Gas Safe Registered engineer, and a tightness test confirms the installation is gas-tight (Correct answer)
As soon as the occupant requests it
Immediately if the smell has cleared

Correct answer: Only after the source of the escape has been identified and repaired by a Gas Safe Registered engineer, and a tightness test confirms the installation is gas-tight

Gas must not be restored until the escape source has been identified and repaired by a Gas Safe Registered engineer, and a tightness test confirms the installation is secure. Restoring gas without these steps risks further escape.

Question 7: What is the ideal CO₂ reading for natural gas when an appliance is correctly set up for complete combustion?

7-8%
5-6%
12-14%
9-10% (Correct answer)

Correct answer: 9-10%

For natural gas (methane), the ideal CO₂ reading is approximately 9-10% when combustion is complete and the appliance is correctly adjusted. This indicates the correct air-to-gas ratio is being achieved. A CO₂ reading significantly below this range suggests excess air, while above it may indicate incomplete combustion.

Question 8: A fanned draught flue system operates by using what method to discharge combustion products?

Natural convection (buoyancy of hot gases)
An electrically powered fan to push or pull combustion products through the flue (Correct answer)
A venturi effect created by wind passing over the terminal
Compressed air injection

Correct answer: An electrically powered fan to push or pull combustion products through the flue

A fanned draught (fan-flued) system uses an electrically powered fan to move combustion products through the flue to the terminal. This allows more flexible flue routing (including horizontal runs and longer distances) compared to natural draught systems. Most modern condensing boilers use fanned draught flue systems.

Question 9: Which of the following situations would be classified as 'At Risk' (AR) under GIUSP?

A gas pipe that has been completely severed and is releasing gas
A gas cooker with a missing flame supervision device that was never fitted as original equipment
An appliance with a flue that is incorrectly terminated but with no current evidence of fume spillage into the room (Correct answer)
A gas fire producing significant carbon monoxide with visible sooting and staining

Correct answer: An appliance with a flue that is incorrectly terminated but with no current evidence of fume spillage into the room

An incorrectly terminated flue with no current evidence of spillage would typically be classified as At Risk (AR). While not immediately spilling products of combustion, the incorrect termination means the flue could fail under certain conditions (e.g., adverse wind). A severed pipe or significant CO production would be ID classifications.

Question 10: What component must be fitted where a gas flue pipe passes through a combustible material such as a timber floor or wall?

A fire-resistant sleeve with a minimum 25 mm air gap (Correct answer)
A gas isolation valve
A pressure relief valve
A condensate trap

Correct answer: A fire-resistant sleeve with a minimum 25 mm air gap

Where a gas flue pipe passes through combustible material (such as a timber floor, ceiling, or wall), a non-combustible fire-resistant sleeve must be fitted. There must be a minimum 25 mm air gap between the flue pipe and the combustible material, filled with non-combustible material if necessary. This prevents heat transfer from the flue igniting the surrounding structure.

Question 11: What is the correct immediate action if a gas smell is detected in a property?

Do not operate any switches, open windows and doors, turn off the gas at the meter if safe, get everyone out, and call 0800 111 999 (Correct answer)
Open windows only and call the landlord
Turn on extraction fans to clear the gas
Turn off appliances at their own controls and wait 30 minutes

Correct answer: Do not operate any switches, open windows and doors, turn off the gas at the meter if safe, get everyone out, and call 0800 111 999

On detecting a gas smell: do not operate any electrical switches (which can ignite gas), ventilate by opening windows and doors, turn off the gas at the emergency control valve (meter) if safe, evacuate, and call 0800 111 999 from outside.

Question 12: What is the correct approach for purging a very long gas pipe run in a domestic installation?

Purge all at once from the meter with a very large gas volume
Purge in sections, venting each section to the outside atmosphere, confirming gas presence at each stage before moving to the next (Correct answer)
Leave the gas on for 15 minutes and allow natural purging to occur
Purge by lighting appliances in sequence from the furthest to the nearest

Correct answer: Purge in sections, venting each section to the outside atmosphere, confirming gas presence at each stage before moving to the next

Long pipe runs should be purged in sections, venting each section to the outside and confirming gas presence before proceeding. This ensures complete purging throughout and minimises the volume of gas discharged at any one point.

Question 13: When installing gas pipework, what is the minimum distance it should be kept from electrical cables where they run parallel?

25 mm (Correct answer)
10 mm
150 mm
50 mm

Correct answer: 25 mm

Gas pipework should be installed at least 25 mm away from electrical cables where they run parallel. Where they cross, the gas pipe should ideally pass behind the electrical cable. This separation prevents potential damage from heat and reduces ignition risk in the event of a gas escape.

Question 14: Which document must be completed and left with the customer after commissioning a new gas boiler?

CORGI registration form
Gas Safe registration certificate
Benchmark commissioning checklist (Correct answer)
Local authority building notice

Correct answer: Benchmark commissioning checklist

The Benchmark commissioning checklist must be completed and handed to the customer as proof that the appliance has been correctly installed, commissioned, and is operating to the manufacturer's specification.

Question 15: Under the Health and Safety at Work etc. Act 1974, what is the primary duty placed upon employers regarding gas work?

To ensure, so far as is reasonably practicable, the health and safety of employees and others (Correct answer)
To use only brand-new components on every job
To provide the cheapest gas installations possible
To complete work as quickly as possible

Correct answer: To ensure, so far as is reasonably practicable, the health and safety of employees and others

Section 2 of the Health and Safety at Work etc. Act 1974 places a general duty on employers to ensure, so far as is reasonably practicable, the health, safety, and welfare at work of all employees. Section 3 extends this duty to non-employees who may be affected by the undertaking.

Question 16: At what intervals must copper gas pipework be supported (clipped) in a domestic installation?

Support intervals are at the installer's discretion
Every 3 metres regardless of pipe size or orientation
Every 1.2 metres for horizontal runs and every 1.8 metres for vertical runs (15 mm copper) — per BS 6891 (Correct answer)
Only at changes of direction

Correct answer: Every 1.2 metres for horizontal runs and every 1.8 metres for vertical runs (15 mm copper) — per BS 6891

BS 6891 specifies support intervals for domestic gas installations. For 15 mm copper pipe, supports are required at maximum 1.2 m intervals on horizontal runs and 1.8 m on vertical runs. Correct support prevents sagging and joint stress.

Question 17: What type of thermostat is designed to shut off the gas supply to a boiler if the water temperature exceeds a safe limit?

Overheat thermostat (limit thermostat) (Correct answer)
Programmer timer
Frost thermostat
Room thermostat

Correct answer: Overheat thermostat (limit thermostat)

An overheat thermostat (also called a limit thermostat or high-limit thermostat) is a safety device that shuts off the gas supply to the boiler if the water temperature exceeds a predetermined safe limit (typically around 85-90°C). This prevents the system from reaching dangerously high temperatures and pressures. It is a non-self-resetting device that requires manual reset.

Question 18: How often must a landlord arrange a gas safety check on appliances in a rented property?

Every 12 months (Correct answer)
Only when a new tenant moves in
Every 24 months
Every 6 months

Correct answer: Every 12 months

The Gas Safety (Installation and Use) Regulations 1998 require landlords to have gas appliances, fittings, and flues checked at intervals of not more than 12 months by a Gas Safe registered engineer. A Landlord Gas Safety Record (LGSR) must be issued after each check.

Question 19: When commissioning a gas appliance, the engineer must check the burner pressure. What tool is used for this measurement?

A combustion analyser only
A digital manometer or U-gauge connected to the burner pressure test point (Correct answer)
A multimeter
A spirit level

Correct answer: A digital manometer or U-gauge connected to the burner pressure test point

Burner pressure (also called injector pressure or manifold pressure) is measured using a digital manometer or U-gauge connected to the burner pressure test point on the gas valve. For natural gas, the typical burner pressure is around 10-12 mbar for most appliances, but the correct setting is specified in the manufacturer's data plate.

Question 20: What type of leak detection fluid must NOT be used on gas pipework?

Proprietary leak detection sprays are always safe for all gas types
Any liquid containing oil or grease — these can contaminate oxygen systems and must not be used on oxygen-enriched gas systems. Standard gas installations use approved leak detection fluids or electronic detectors (Correct answer)
Washing-up liquid — this is approved and the most effective detection fluid
Only electronic detectors are permitted — no fluids are allowed

Correct answer: Any liquid containing oil or grease — these can contaminate oxygen systems and must not be used on oxygen-enriched gas systems. Standard gas installations use approved leak detection fluids or electronic detectors

Standard leak detection fluids (including washing-up liquid) are acceptable on natural gas installations. However, oil or grease-containing fluids must never be used on oxygen-enriched or medical gas systems. Electronic gas detectors are the preferred method for gas installations.

Question 21: What are the three elements required for combustion to take place, commonly known as the 'fire triangle'?

Gas, air, and a flue
Gas, electricity, and water
Fuel, nitrogen, and pressure
Fuel, oxygen, and ignition (heat) (Correct answer)

Correct answer: Fuel, oxygen, and ignition (heat)

The fire triangle consists of fuel, oxygen (from the air supply), and a source of ignition (heat). All three must be present simultaneously for combustion to occur. Removing any one element will extinguish the flame or prevent ignition.

Question 22: A gas engineer needs to check the standing pressure at a domestic meter. What reading should they expect under normal conditions?

11 mbar
35 mbar
21 mbar (Correct answer)
75 mbar

Correct answer: 21 mbar

The normal standing pressure (static pressure) at a domestic gas meter in the UK should be approximately 21 mbar. This is measured with all appliances turned off (no gas flowing). The working pressure (with appliances operating) should not drop below 19 mbar at the meter. A significant deviation may indicate a supply problem.

Question 23: What is a 'pressure governor' (regulator) in a domestic gas installation and where is it located?

A pressure governor is only fitted to commercial installations
A device inside each individual gas appliance
A device in the gas main under the road
A device that reduces the high pressure from the gas main to the lower working pressure (typically 20-21 mbar) required by domestic appliances — located at or in the meter assembly (Correct answer)

Correct answer: A device that reduces the high pressure from the gas main to the lower working pressure (typically 20-21 mbar) required by domestic appliances — located at or in the meter assembly

The pressure governor (regulator) is part of the meter assembly. It reduces the gas supply pressure from the higher distribution main pressure to the lower working pressure (approximately 20-21 mbar) required by domestic appliances.

Question 24: What type of joint is NOT permitted for use on gas pipework concealed within a wall cavity?

Compression joints (Correct answer)
Welded joints
Brazed joints
Soldered capillary joints

Correct answer: Compression joints

Compression joints (and any other type of mechanical joint that could work loose) are not permitted on gas pipework that is to be concealed within wall cavities or where access is restricted. Only permanent joints such as soldered capillary, brazed, welded, or press-fit joints with appropriate certification for gas should be used in concealed locations.

Question 25: What is 'spillage' in the context of an open-flued gas appliance?

The leakage of flue gases from the appliance or flue back into the room, potentially introducing CO into the living space (Correct answer)
The discharge of unused gas to atmosphere during start-up
An overflow of gas beyond the appliance's burner
Leakage of condensate from a condensing boiler

Correct answer: The leakage of flue gases from the appliance or flue back into the room, potentially introducing CO into the living space

Spillage occurs when flue gases escape from the draught diverter or flue into the room rather than discharging to outside. This can introduce CO and other combustion products into the living space, creating a serious health risk.

Question 26: What is the role of a room thermostat in a domestic gas central heating system?

To sense the room air temperature and signal the boiler to switch on or off to maintain the desired temperature (Correct answer)
To measure the gas pressure
To monitor the outdoor temperature
To control the hot water temperature

Correct answer: To sense the room air temperature and signal the boiler to switch on or off to maintain the desired temperature

A room thermostat measures the air temperature in the room where it is installed and sends a signal to the boiler to fire or shut down to maintain the set temperature. Modern room thermostats may be digital, programmable, or smart (e.g., Nest, Hive). They should be installed on an interior wall at approximately 1.5 m height, away from direct heat sources and draughts.

Question 27: What is the correct standing (static) gas pressure for natural gas at a domestic meter outlet?

17 mbar
10 mbar
21 mbar (Correct answer)
37 mbar

Correct answer: 21 mbar

The standard standing pressure for natural gas at a domestic meter outlet is 21 mbar, measured with no gas flowing; working pressure when gas is flowing is typically 20 mbar.

Question 28: What instrument is used to verify that gas has fully purged air from new pipework and reached the appliance?

Purging is verified by smell alone
A gas detector or combustible gas indicator at the appliance connection point (Correct answer)
A manometer at the appliance
A pressure gauge at the meter

Correct answer: A gas detector or combustible gas indicator at the appliance connection point

A combustible gas indicator (or gas detector) is used at the appliance connection to confirm that gas has displaced all air and reached the point of use. Smell alone is unreliable and insufficient.

Question 29: What is a draught diverter and what is its function?

A device that increases the draught to improve combustion
A valve that prevents back-flow of gas from the flue into the burner
A device that controls the rate of air supply to the burner
A component on the flue of an open-flued appliance that prevents down-draughts from extinguishing the burner, while allowing room air to dilute flue gases (Correct answer)

Correct answer: A component on the flue of an open-flued appliance that prevents down-draughts from extinguishing the burner, while allowing room air to dilute flue gases

A draught diverter is fitted between the appliance flue outlet and the main flue. It breaks the direct draught connection, preventing down-draughts from extinguishing the burner, and allows room air to dilute the flue gases.

Question 30: Under RIDDOR, who must a Gas Safe registered engineer report a dangerous gas fitting to?

The local fire brigade
The local authority environmental health department
The HSE (Health and Safety Executive) via the online RIDDOR reporting system (Correct answer)
The gas appliance manufacturer

Correct answer: The HSE (Health and Safety Executive) via the online RIDDOR reporting system

Under RIDDOR regulation 6(2), Gas Safe registered engineers must report any dangerous gas fitting to the HSE using the online reporting form at www.hse.gov.uk/riddor. The report must be made as soon as reasonably practicable. The gas transporter (e.g., National Gas Emergency Service) should also be notified if there is an immediate gas escape.

Question 31: What is the minimum distance a room-sealed balanced flue terminal must be from an openable window or air vent?

1200mm
150mm
300mm (Correct answer)
600mm

Correct answer: 300mm

BS 5440 Part 1 requires a room-sealed appliance flue terminal to be at least 300mm from any openable window, air vent, or opening to prevent combustion products from re-entering the building.

Question 32: Why is checking the gas rate important during appliance commissioning?

To verify the appliance is consuming gas within its designed input (Correct answer)
To detect gas leaks in the pipework
To set the correct thermostat calibration
To test the accuracy of the gas meter

Correct answer: To verify the appliance is consuming gas within its designed input

Checking the gas rate by timing the meter confirms that the appliance is operating within its designed input rating, ensuring efficient combustion and safe operation in line with the manufacturer's specification.

Question 33: What is the purpose of purging a gas installation after completing pipework installation or modification?

To check the burner pressure of connected appliances
To remove air from the pipework and replace it with gas, preventing a potentially explosive air-gas mixture (Correct answer)
To test the tightness of the installation
To measure the gas flow rate

Correct answer: To remove air from the pipework and replace it with gas, preventing a potentially explosive air-gas mixture

Purging removes the air from newly installed or modified gas pipework and replaces it with gas. This is essential because a mixture of air and gas within the pipe could form an explosive mixture. Purging must be carried out safely in accordance with IGE/UP/1B, ensuring proper ventilation and that no ignition sources are present.

Question 34: What does a gas tightness test confirm?

That the correct gas pressure is reaching the appliances
That the gas installation pipework contains no leaks — confirmed by applying a test pressure and observing for pressure drop (Correct answer)
That the pipework is correctly sized for the appliance load
That all appliances are correctly commissioned

Correct answer: That the gas installation pipework contains no leaks — confirmed by applying a test pressure and observing for pressure drop

A gas tightness test applies a test pressure to the pipework and monitors for any pressure drop over a specified period. No pressure drop confirms the installation is gas-tight and free from leaks.

Question 35: What is 'vitiation' in the context of gas appliance operation?

The progressive reduction of oxygen in a room due to an appliance consuming air for combustion (Correct answer)
The process of igniting a gas burner
The corrosion of flue components
The measurement of gas pressure at the meter

Correct answer: The progressive reduction of oxygen in a room due to an appliance consuming air for combustion

Vitiation occurs when an open-flued or flueless appliance consumes oxygen from the room for combustion, progressively reducing the oxygen level in the space. This can lead to incomplete combustion, increased carbon monoxide production, and ultimately a dangerous situation if adequate ventilation is not provided.

Question 36: What are the three requirements for complete combustion of natural gas?

Sufficient fuel, sufficient oxygen (air), and adequate temperature (ignition temperature maintained) (Correct answer)
Fuel, flue gas, and a pilot light
Fuel, spark ignition, and water vapour
Fuel, nitrogen, and a catalyst

Correct answer: Sufficient fuel, sufficient oxygen (air), and adequate temperature (ignition temperature maintained)

Complete combustion requires three elements: sufficient fuel (gas), sufficient oxygen from the air supply, and maintenance of a temperature above the ignition threshold. Deficiency in any of these results in incomplete combustion.

Question 37: What is the purpose of the Gas Industry Unsafe Situations Procedure (GIUSP)?

To manage customer complaints about gas bills
To set pricing standards for gas work
To regulate the manufacture of gas appliances
To provide a standardised procedure for engineers when they encounter unsafe gas installations (Correct answer)

Correct answer: To provide a standardised procedure for engineers when they encounter unsafe gas installations

The Gas Industry Unsafe Situations Procedure (GIUSP) provides a standardised framework for Gas Safe registered engineers to classify and deal with unsafe gas installations. It categorises situations as Immediately Dangerous (ID), At Risk (AR), or Not to Current Standards (NCS), with specific actions required for each classification.

Question 38: What does a yellow or orange flame on a natural gas burner indicate?

The correct operational colour for a gas burner
Incomplete combustion — insufficient air supply causing carbon particles to glow as the flame burns luminously (Correct answer)
High gas pressure causing excess fuel combustion
The appliance is in its warm-up phase and will correct automatically

Correct answer: Incomplete combustion — insufficient air supply causing carbon particles to glow as the flame burns luminously

A natural gas burner should produce a predominantly blue flame. A yellow or orange flame indicates incomplete combustion due to insufficient air, producing luminescent carbon particles (soot) and CO.

Question 39: Before purging a gas installation that has been disconnected for more than 12 months, what additional step is required?

Contact the gas transporter for permission to reconnect
A full tightness test of the installation before introducing gas, to confirm no deterioration has occurred during the period of non-use (Correct answer)
Simply turn on the gas and purge as normal
A visual inspection of accessible pipework only

Correct answer: A full tightness test of the installation before introducing gas, to confirm no deterioration has occurred during the period of non-use

Pipework that has been inactive for extended periods may have developed leaks due to joint deterioration or corrosion. A full tightness test before introducing gas is essential to confirm integrity before purging.

Question 40: When carrying out a flue flow test (spillage test) on an open-flued appliance, where should the smoke match be held?

At the base of the draught diverter or canopy (Correct answer)
At the ventilation grille
At the gas meter
At the flue terminal on the outside of the building

Correct answer: At the base of the draught diverter or canopy

The smoke match or smoke pellet should be held at the base of the draught diverter or canopy of the appliance. The smoke should be drawn upwards into the flue if the flue is operating correctly. If the smoke spills outward into the room, it indicates that the flue is not drawing properly and combustion products may be entering the living space.

Question 41: What material is commonly used for flexible flue liners in gas installations within masonry chimneys?

316 grade stainless steel (Correct answer)
Aluminium alloy
Plastic PVC
Galvanised mild steel

Correct answer: 316 grade stainless steel

316 grade stainless steel is the standard material for flexible flue liners used in gas installations within masonry chimneys. It provides excellent resistance to the corrosive condensates produced by gas combustion and has the durability required for the expected lifespan of the installation. It must conform to BS EN 1856-2.

Question 42: What safety precaution is taken after purging gas into the atmosphere outside?

Apply a pressure test immediately after purging
Confirm the purge point is clear of ignition sources, the gas is fully dispersed before any ignition sources are introduced, and the area is ventilated (Correct answer)
Light a match at the purge point to confirm gas is present
Allow immediate reoccupation of the building

Correct answer: Confirm the purge point is clear of ignition sources, the gas is fully dispersed before any ignition sources are introduced, and the area is ventilated

After purging gas to atmosphere, the purge point and nearby area must be kept clear of ignition sources until the gas has fully dispersed. Good ventilation speeds dispersal. Only when gas is confirmed to have dispersed should ignition sources be reintroduced.

Question 43: What is the minimum internal diameter of a flue liner suitable for a standard domestic gas boiler?

125 mm (Correct answer)
100 mm
75 mm
150 mm

Correct answer: 125 mm

The minimum internal diameter for a flue liner serving a standard domestic gas boiler is 125 mm (approximately 5 inches). This applies to both rigid and flexible flue liners installed in masonry chimneys. The liner must be suitable for gas and must be installed in accordance with the appliance manufacturer's instructions and BS 5440-1.

ACS CCN1 Core Domestic Gas Safety Assessment

The ACS CCN1 is the UK's core domestic gas safety certification required for Gas Safe Register membership. It covers gas legislation, combustion, ventilation, pipework, flue systems, unsafe situations procedures, and meter controls through written multiple-choice assessments.

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Sources & references

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igem.org.uk — Eligibility, test delivery, score validity, exam information · verified Nov 2026
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