What Does a Gas Safe Engineer Check During a Gas Safety Inspection?

Gas Safety Inspection vs Boiler Service: An Important Distinction

Before describing what a gas safety inspection covers, it is important to understand what it does not cover. A gas safety inspection — the check required annually for all tenanted properties under the Gas Safety (Installation and Use) Regulations 1998 — is a safety assessment of the gas installation and all connected appliances. It is not a boiler service. It does not include the internal cleaning, combustion analysis, and component inspection that a full annual boiler service entails. An engineer carrying out only a gas safety inspection will check that the boiler operates safely; they will not necessarily clean the burner, check the heat exchanger condition, or verify that the boiler is operating at peak efficiency. For landlords, it is good practice to combine the annual gas safety inspection with an annual boiler service in a single visit — but these are two distinct activities, and a CP12 certificate does not evidence that the boiler has been serviced.

Appliance Checks: What the Engineer Tests

For each gas appliance in the property — the boiler, any gas fire, and a gas cooker if present — the engineer carries out a series of standardised checks. These include a gas tightness test, which verifies that the appliance and its connections do not leak gas when pressurised; a burner pressure test, measuring the gas pressure at the burner against the manufacturer's specified operating pressure; verification that all safety devices on the appliance operate correctly — for a boiler, this includes testing the overheat thermostat cutout, the flame-failure device, and the pressure relief valve; a visual inspection of the appliance condition, checking for signs of corrosion, physical damage, or deterioration that could compromise safe operation; and confirmation that the appliance has adequate ventilation for safe combustion where required. For gas cookers, the hob and oven burners are each lit and checked for correct flame pattern and gas rate.

Flue Inspection

The flue — the pipe or duct that carries combustion products from the appliance to the outside of the building — is one of the most safety-critical components of any gas installation. A defective or obstructed flue can cause carbon monoxide to spill into the living space. During the gas safety inspection, the engineer inspects the visible sections of the flue for physical damage, signs of condensate leakage, correct joint sealing, and adequate support. For room-sealed (balanced flue) boilers, which are standard in modern London flats and houses, the engineer checks the flue terminal position against the minimum clearances specified in the Gas Safety Regulations — clearances from windows, doors, ventilation openings, and building corners that ensure combustion products are discharged safely. An obstructed or incorrectly positioned flue terminal is one of the most common causes of an ID (immediately dangerous) classification on a CP12.

Gas Tightness Test: What It Is and How It Works

The gas tightness test is the most definitive check in the gas safety inspection. The engineer uses a calibrated manometer connected to the gas installation to test for leaks under two conditions: the standing pressure test checks for leaks with all appliances off and the gas valve open; the working pressure test checks for leaks and pressure drop with the appliances under load. A properly executed gas tightness test will detect even very small leaks in appliance connections, pipe joints, valves, and flexible connectors. The test takes 2 to 3 minutes and, unlike the use of a gas leak detection fluid or electronic sniffer (which can miss slow leaks), provides a definitive result. Any drop in pressure during the standing or working test indicates a leak that must be investigated and rectified before the certificate can be issued.

Classification of Findings: ID, AR, and NCS

The Gas Safe Register and associated industry guidance specify three classifications for hazardous findings. An ID classification — immediately dangerous — means that the engineer has found a condition that presents an immediate risk to life. On finding an ID issue, the engineer must disconnect the appliance (or advise the occupant to do so where disconnection is not immediately possible) and the appliance cannot be used until the fault is rectified. Common ID findings include a suspected gas leak that cannot be immediately located, a severely blocked flue, a cracked heat exchanger, or an appliance producing elevated carbon monoxide in the room. An AR classification — at risk — means that the appliance or installation presents a significant risk that does not meet the immediate danger threshold but that must be rectified promptly. The engineer records the AR finding on the CP12 and advises the occupant in writing. An NCS classification — not to current standards — is an advisory note where the installation does not meet current standards but does not present a safety risk; correction is recommended but not mandatory.

The CP12 Record Format

The Gas Safety Record (CP12) lists each gas appliance inspected separately, recording the appliance type, make, model, location, and the result of each check carried out. It identifies any ID, AR, or NCS findings and the action taken. The engineer's Gas Safe registration number is included, which enables the landlord and tenant to verify the engineer's credentials independently at GasSafeRegister.co.uk. The certificate also records the date of inspection, the property address, and the engineer's signature and certification. A valid CP12 must be issued on the same day as the inspection and must reflect the actual condition of the installation at the time of inspection — post-dating or pre-dating a CP12 is fraudulent.

How Long Does a Gas Safety Inspection Take?

For a typical London flat with a single combi boiler and no additional gas appliances, a gas safety inspection takes approximately 45 to 60 minutes. Where additional appliances are present — a gas hob, gas fire, or a system boiler with a hot water cylinder — allow an additional 15 to 20 minutes per appliance. An HMO with shared gas appliances may take 90 minutes or more. The combination of a gas safety inspection and an annual boiler service in a single visit typically takes 75 to 90 minutes for a combi boiler, as the service elements (combustion analysis, internal component checks, system pressure assessment) add to the base inspection time.

What Landlords Should Prepare Before the Visit

To ensure the inspection proceeds smoothly and that the engineer can complete all checks, landlords and tenants should: ensure all gas appliances are accessible (boiler panel open, gas cooker not blocked by appliances or objects, gas fire surround accessible); inform tenants that access is required and confirm the appointment is convenient; ensure the boiler is not locked out in fault mode (reset if necessary before the engineer arrives so the appliance can be tested in normal operation); and for older properties with gas fires that have pilot lights, ensure the pilot is lit. A boiler that cannot be accessed because of a cluttered cupboard, or a gas fire that cannot be tested because it is covered by furniture, will result in an incomplete inspection and may require a return visit at additional cost.