What Is a Gas Tightness Test and When Do You Need One?

What a Gas Tightness Test Is

A gas tightness test is a pressure test of the gas installation — the network of pipework, valves, and connections that carries natural gas from the meter to the appliances within a property. The test works by pressurising the installation with gas and monitoring whether the pressure holds over a set period. Any pressure drop indicates gas escaping from a leak somewhere in the system.

The test procedure follows the requirements of IGEM/UP/1B (the industry guidance for domestic gas installations) and is carried out using a pressure manometer — a precision gauge capable of measuring gas pressure in millibars. The general process is:

1. All gas appliances are isolated at their individual isolation valves (or turned off). The gas is live from the meter to the isolation points.
2. The manometer is connected to a test point on the gas pipework — typically at the meter outlet or at a test nipple on the main installation pipe.
3. The installation is pressurised to a working test pressure (typically 20–50 mbar for domestic low-pressure natural gas installations).
4. The pressure is monitored over a defined period — one minute for a visual check, two minutes for the formal test per IGEM/UP/1B guidance for domestic installations.
5. If the pressure holds within the permitted tolerance, the installation passes. If the pressure drops measurably, the installation fails and the leak must be located and repaired before gas can be supplied to the appliances.

The test specifically checks the pipework and connections — not whether individual appliances are working correctly (that is a commissioning check, not a tightness test). An installation can pass a gas tightness test even if an appliance has a fault, because appliances are isolated during the test.

When a Gas Tightness Test Is Required

After Any Gas Installation Work

Whenever a Gas Safe engineer carries out gas installation work — fitting a new boiler, extending or modifying gas pipework, adding a new gas appliance (hob, fire, or meter bypass) — a gas tightness test must be carried out before the new work is commissioned and the gas supply is restored to the appliances. This is a mandatory requirement under the Gas Safety (Installation and Use) Regulations 1998. No responsible Gas Safe engineer will commission an installation without first confirming the pipework is gas tight.

During a Gas Safety Certificate Inspection

A Gas Safety Certificate (CP12) — required annually for landlords — involves the Gas Safe engineer inspecting and testing all gas appliances in the property. The inspection includes checking the gas tightness of the installation as part of the assessment. If the tightness check reveals a pressure drop indicating a leak, the engineer will investigate to locate the fault before completing the certificate. A gas installation with an identified leak cannot receive a satisfactory Gas Safety Certificate.

When You Smell Gas

If a gas smell is detected in a property, the immediate action is to call the National Gas Emergency Service on 0800 111 999, ventilate the property, and evacuate if the smell is strong. The emergency service will attend, isolate the gas supply at the meter if a leak is confirmed, and make the property safe. A Gas Safe engineer must then carry out a full gas tightness test before the gas supply is restored. You cannot simply turn the gas back on after a suspected leak has been isolated without confirming by test that the installation is sound.

Before Re-Commissioning After a Period of Isolation

If a property has been without gas for an extended period — an unoccupied property, a flat following a major renovation where the gas was isolated, or a property following a gas incident — the installation should be tightness tested before the gas is turned back on and appliances commissioned. Pipework in an empty or abandoned property can develop corrosion at fittings, and joints left uninspected for years may have deteriorated. A tightness test confirms the pipework is sound before any appliances are lit.

When Buying a Property with Gas

A gas tightness test as part of the purchase process is strongly advisable when buying any property with a gas installation, particularly older properties or those that have been vacant. A gas survey carried out by a Gas Safe engineer before exchange of contracts confirms the installation's integrity and identifies any faults that should be remediated before completion or factored into the purchase price. This is not a legal requirement for purchase, but it is prudent practice — particularly for older London properties where pipework may be original and decades old.

What Happens If the Test Fails

A failing gas tightness test — indicated by a measurable pressure drop over the test period — means there is a gas leak somewhere in the installation. The engineer cannot restore the gas supply and commission the appliances until the leak is located and repaired.

Locating the leak follows the pressure drop: the engineer systematically isolates sections of the installation or uses leak detection solution (a soapy liquid that bubbles at a leaking joint when gas is applied) or electronic gas detectors to locate the source. Common leak points include:

• Compression fittings that have worked loose over time or were not made up correctly during original installation
• Corroded unions at appliance connections
• Old-style threaded connections that have degraded seals (PTFE tape deteriorates over time, particularly at connections exposed to vibration)
• Flexible gas hoses to appliances that have developed cracks or deteriorated seals

Once the leak is identified, the engineer reseals or replaces the failed joint or fitting, then re-tests to confirm the installation now holds pressure. Only after a satisfactory tightness test can the gas be restored and appliances commissioned.

If a leak cannot be located in accessible pipework, or if there is evidence of leak in buried or concealed pipework, more invasive investigation — lifting floors or opening walls — may be required. This is rare but does occur in older London properties with original pipework that has never been replaced.

Cost

A gas tightness test is typically included as a standard component of any Gas Safe work — it is part of a boiler installation, a gas safety inspection, or an appliance service. As a standalone test in isolation (for example, confirming an installation before purchasing a property), the typical cost is £80–£120 for a Gas Safe engineer's visit, test, and written report confirming the result. Some engineers charge a callout fee plus a modest add-on for the formal written confirmation; confirm the cost and deliverables when booking.

Gas Safe Requirement: Only Registered Engineers

Gas tightness tests must be carried out by Gas Safe registered engineers. This is not a test that can be carried out by a homeowner, an unregistered operative, or a plumber without gas registration. The test involves working with live gas supply at the meter, pressurising the installation, and making technical assessments about the meaning of the results. Under the Gas Safety (Installation and Use) Regulations 1998, any work on a gas installation — including testing — must be carried out by a registered engineer.

Verify any engineer's Gas Safe registration at gassaferegister.co.uk before booking. A Gas Safe registered engineer can provide a copy of the test result — for a formal landlord's Gas Safety Certificate, the tightness test result is recorded on the certificate. For a standalone pre-purchase test, a written report from the engineer confirming the result and the date should be requested and retained.