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A Gas Safety Certificate (CP12) is not a tick-box exercise. For Gas Safe engineers, it is a structured safety inspection designed to confirm that gas installations are safe, compliant, and suitable for continued use.

This engineer-focused guide covers tightness testing, meter checks, pipework inspection, carbon monoxide safety, and detailed appliance checks for cookers, boilers, and gas fires. It is intended to support consistent inspection standards across Surrey and London.

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Tightness & Let-By Testing at the Meter

Before any appliance work begins, the integrity of the gas installation must be confirmed.

Required Tests & Pressures

• Let-By Test
  Duration: 1 minute
  Acceptable pressure: 6–10 mbar
• Stabilisation Test
  Duration: 1 minute
  Acceptable pressure: 18–21 mbar
• Tightness Test
  Duration: 2 minutes
  Acceptable pressure: 18–21 mbar

Any pressure drop outside tolerance must be investigated and resolved before proceeding.

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Meter & Emergency Control Valve Checks

The meter installation must be compliant, accessible, and clearly labelled.

Checks include:

• Main equipotential bonding present
• Meter, ECV and appliance isolation labels fitted
• Regulator outlet pressure below 30 mbar
• Emergency Control Valve turns fully off and is accessible
• No signs of damage, corrosion or tampering

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Gas Pipework Inspection

Pipework must be assessed visually and practically throughout the installation.

Key points:

• Correct pipe sizing for appliance load
• Suitable isolation valves fitted
• No inaccessible fittings
• Minimum 25mm clearance from electrical wiring
• No contact with sharp edges or cables

Undersized or poorly routed pipework must be recorded and classified correctly.

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Pre-Appliance Safety Assessment

Before working on any appliance, confirm:

• Appliance is safe to touch
• Appliance was operational before inspection
• No Immediately Dangerous or At Risk conditions present

Any unsafe condition must be actioned before testing continues.

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Carbon Monoxide Safety

Carbon monoxide protection is a critical part of every Gas Safety Certificate CP12.

Engineers must confirm:

• CO alarms fitted where flues pass through habitable rooms
• Alarms installed to manufacturer’s instructions
• Correct mounting position and clearances
• Alarms within service life and not expired

Where required, linked alarms must be verified and recorded.

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Cooker Safety Checks

Ventilation

• Adequate ventilation provided
• Openable window present where required
• Bedroom installations checked against manufacturer instructions

Visual Inspection

• Stable blue flame picture
• No scorching, heat damage or spillage
• Clearances maintained
• No combustible materials nearby

Connections

• Rigid or approved flexible connection used
• Bayonet point facing downwards
• Hose fitted in a U-shape
• No stretching, kinking or stress

Additional checks may include door seal tests, safety lid operation, and flue gas analysis where applicable.

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Boiler Safety Checks

Visual Inspection

• No corrosion, cracks or splits
• Gaskets and seals intact
• Insulation panels fitted correctly
• Condensate sump condition acceptable

Safety Devices

• Flame Supervision Device tested where fitted
• Gas isolation confirmed effective
• Manufacturer instructions followed at all times

Combustion & Flue

• Flue Gas Analysis at high and low rate
• Case seal integrity confirmed
• Flue joints secured and sealed
• Concealed flues provided with inspection access
• Terminal guards fitted where required

System & Performance Checks

• Condensate pipe plastic and correctly sized
• Gas rate and working pressure verified
• Burner pressure checked on SE boilers

Electrical

• Correct fuse rating (typically 3A)
• Test points verified