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A tightness test checks a gas installation is sound, and it is a job for a competent Gas Safe registered engineer. After a let-by test at the Emergency Control Valve, a test pressure of about 20 to 21 mbar is applied to a natural gas installation, left to stabilise for about 1 minute, then timed for 2 minutes.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed July 2026
A tightness test, sometimes called a soundness test, checks that a gas installation holds pressure and has no leaks. It is a task for a competent, Gas Safe registered engineer working to the current industry procedure, not something to attempt without the right training and equipment.
Before the tightness test itself comes a let-by test. With the appliances off, the Emergency Control Valve is closed and the manometer is watched for a period. If the pressure rises, the valve is letting by, meaning it is not sealing properly, and that has to be dealt with first.
On a natural gas installation, a test pressure of about 20 to 21 mbar is applied through a calibrated gauge.
The pressure is allowed to stabilise for about 1 minute, so temperature and pressure settle before timing begins.
The test is then timed for 2 minutes and the pressure watched for any drop.
The permissible pressure loss depends on the installation volume and the meter, and is read from the current procedure, IGEM/UP/1B. A new installation must show no drop, while a small drop may be acceptable on an existing one.
Always use a calibrated gauge and work to the current procedure. A tightness test is a job for a competent, Gas Safe registered engineer, and this guide is a high-level overview, not a substitute for the IGEM/UP/1B procedure or proper training.
Last reviewed July 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.