Gas Let-By Test: What They Are, How to Do One, and What Happens If It Fails
The ECV looks like the simplest part of the job. Close it, isolate the supply, and get on with the test. But if it’s letting gas through when it should be sealed, every pressure reading you take after that point is meaningless — you’re watching a gauge on a system that’s still being fed.
The gas let-by test is how you rule that out before you start with a gas tightness test. It takes a few minutes, it’s expected by Gas Safe, and skipping it leaves you exposed – so let’s run through the process.
The let-by test itself is unchanged under IGEM/UP/1B Edition 4 (published March 2026, mandatory from 1 October 2026) — but the tightness test that follows has significant new requirements around Installation Volume. For the full breakdown, see our IGEM/UP/1B Edition 4 guide.
What is the gas let-by test?
The let-by test checks whether the Emergency Control Valve (ECV) — the valve at the meter that shuts off the gas supply — is sealing properly when it’s closed.
“Let-by” means exactly what it sounds like: gas letting by a valve that’s supposed to be blocking it. An ECV can deteriorate over time, and a faulty one may allow a small but steady flow of gas through even in the closed position. The let-by test catches this before you run any further checks on the installation.
When do you carry out the let-by test?
The short answer is before anything else. The let-by test is the first check in the sequence — it comes before the gas tightness test, not after.
The reason matters: if the ECV is letting gas by, your tightness test results will be unreliable. Any pressure drop you’re supposed to be detecting could be masked by gas slowly feeding into the system from a faulty valve. You’ll think the installation is sound when it isn’t.
IGEM/UP/1B and Gas Safe guidance both expect the let-by check to be completed before you proceed with a tightness test as part of the standard procedure.
What equipment do you need?
- A U-gauge (water manometer) or calibrated electronic manometer (your flue gas analyser may include a digital manometer function, although this varies from model to model. Learn more in our buying guide.)
- Appropriate fittings for your test point connection
You’re likely already carrying this for the tightness test. No additional kit required.
Above: Create your certificates in Gas Engineer Software to store all notes, customer info, and records in one place.
Gas let-by test procedure — step by step
- Turn off all gas appliances and make sure nothing is drawing from the system.
- Close the ECV — the emergency control valve at the meter. This is what you’re testing.
- Connect your digital manometer/U-gauge to the test point downstream of the ECV. This is typically the test nipple on the meter installation.
- Reduce the downstream pressure to 7–10 mbar.
- Allow a stabilisation period of 1 minute. This lets the pressure settle and accounts for any temperature-related variation in the readings.
- Monitor the gauge over the test period. The timed test is 1 minute under IGEM/UP/1B. You’re looking for a pressure rise, not a drop. A rising reading after stabilisation indicates gas is passing through the ECV.
- Record your result — whether the test passed, the pressure reading, and the time. Keep this tied to the job record.
What is the let-by allowance?
The let-by allowance is the threshold that determines whether a result is a pass or a fail. A small, stable reading within tolerance is acceptable — but a sustained pressure rise over the test period is not.
The precise allowance figure depends on the meter type installed and the current version of IGEM/UP/1B. Always check the relevant Gas Safe guidance for your specific installation before deciding a borderline result is within spec. If the gauge is moving after stabilisation, treat it seriously — a rising pressure is not a stable installation.
Edition 4 has given “no perceptible movement” specific numerical definitions that are worth knowing: 0.25 mbar maximum on a water gauge, 0.25 mbar on a standard-resolution electronic gauge, and 0.2 mbar on an electronic gauge that reads to one decimal place. These definitions apply across UP/1B Edition 4 wherever “no perceptible movement” is called for.
What to do if the gas let-by test fails
A confirmed let-by means a faulty ECV. At that point, the installation is classified as At Risk (AR) under the Gas Industry Unsafe Situations Procedure.
- Do not proceed with the tightness test or any further work on the installation until the issue is resolved.
- Notify the Gas Emergency Contact Centre (For LPG, the supplier) — this is a requirement. The network operator needs to be aware of a leaking ECV on their side of the meter.
- The ECV must be repaired or replaced before the installation can be returned to use. Depending on who owns the valve, this typically falls to the gas transporter’s emergency team.
- Document everything — the test result, the time, who you notified, and when. If a customer disputes anything later, you want a clear written record. This is quick and easy with job notes on Gas Engineer Software.
A let-by on the ECV is not your fault, but handling it incorrectly is. Following the procedure protects the customer and protects you.
Find more information about procedures in unsafe situations using the official IGEM guide.
Let-by test vs tightness test — what’s the difference?
They’re related, but they test different things.
The let-by test checks whether the ECV itself is sealing. It’s a test of the valve that separates the network from the internal installation.
The tightness test checks the integrity of the gas installation downstream — the pipework, fittings, and connections after the meter.
They run in sequence: let-by first, tightness test second. A failed let-by means you don’t move to the tightness test. You can’t get a meaningful result from an installation that isn’t properly isolated.
For the full tightness test walkthrough — including permissible drops by installation volume under Edition 4 and how to handle borderline results — see our gas tightness test guide.
How Gas Engineer Software can help
Keeping accurate records of both tests — the date, the pressures, and the outcome — is easy to let slide across a busy week. Gas Engineer Software lets you log test results against the job, timestamped and tied to the job record, so everything’s there if you ever need it for a Gas Safe inspection or a customer query.