Gas Safety Certificate Inspection — What Happens During the Visit
A Gas Safety Certificate inspection takes 30-60 minutes and covers every gas appliance in the property. This guide explains exactly what the engineer checks, what faults they look for, and what happens if something fails.
Before the Inspection
The engineer will need:
- Access to all gas appliances — boiler, gas cooker, gas fires, gas boiler for hot water
- Access to the gas meter and isolation valve
- Adequate ventilation to the appliances (the inspector will check this)
- Heating and hot water to be operational (they need to fire the boiler)
Ensure all appliances are accessible and the property is free enough for the engineer to work. A gas safety inspection cannot be completed if appliances are blocked or if the boiler does not fire (a fault may prevent gas safety completion).
What the Engineer Checks — Appliance by Appliance
Boiler / Central Heating Appliance
- Gas inlet pressure measurement — checks gas is being supplied at correct working pressure (20 mbar for natural gas)
- Ignition check — boiler fires normally
- Burner pressure measurement — confirms the gas valve delivers correct pressure at the burner
- Flue gas test (combustion analysis) — CO and CO₂ in the flue gases must be within acceptable limits
- Flue inspection — visual check that the flue pipe and terminal are intact, unobstructed, and correctly fitted
- Ventilation check — adequate air supply to the combustion chamber
- Safety device operation — overheat thermostat, pressure relief valve check
Gas Cooker
- Gas tightness check at the cooker connection
- Each burner ignites and burns with correct flame colour (blue, no yellow tips)
- Stability — cooker is properly positioned and cannot tip
- Flexible hose condition — check for cracks, perishing, or inadequate length
Gas Fire
- Visual inspection of flue and fireback
- Gas pressure check
- Spillage test (convective test with smoke to confirm flue draws correctly)
- Safety device operation
Fault Codes Explained
If the engineer finds a fault, it is classified as:
- ID (Immediately Dangerous): The appliance presents an immediate risk of injury or death. The engineer will warn you and advise you to have it switched off and not used. They will mark it as ID on the record. If the hazard is so serious that immediate action is required, they may request permission to disconnect it at source.
- AR (At Risk): The appliance is not immediately dangerous but could become dangerous under certain conditions. The engineer records it, advises remediation, and the CP12 is issued with the fault noted.
What You Receive
A signed Gas Safety Record (CP12) listing all appliances inspected, their condition, test results, and any faults noted. The CP12 must be given to tenants within 28 days and retained by the landlord for 2 years.
Frequently asked questions
How long does a Gas Safety Certificate inspection take in London?
A standard inspection covering one combi boiler takes 30-45 minutes. Add 15 minutes per additional appliance (gas cooker, gas fire). A full property with boiler, cooker, and two gas fires typically takes 60-90 minutes.
What happens if my boiler fails the gas safety inspection?
If the boiler has an Immediately Dangerous (ID) fault, the engineer will advise that it must not be used until repaired. You cannot obtain a CP12 until the fault is fixed. A subsequent inspection after repair is needed to issue the certificate. An At Risk (AR) fault allows the CP12 to be issued with the fault noted — you are expected to arrange repair promptly.
Can a Gas Safety Certificate be issued if one appliance fails?
The CP12 can be issued for the appliances that pass, with the failing appliance noted as ID or AR. However, an ID appliance means the CP12 will show that appliance as unsafe — you must arrange repair and re-inspection of that appliance. Many engineers prefer not to issue a CP12 while an ID fault is present.
Does the tenant need to be present during the gas safety inspection?
No — the landlord or their agent can arrange access. However, the tenant has the right to be present if they wish. The engineer must have reasonable access — and tenants are legally required to allow access for gas safety inspections with proper notice (24 hours written notice is standard).