Gas Tightness Test Guide: Procedure & Permissible Drop (IGEM/UP/1B Edition 4)
New to the trade or brushing up your knowledge? Understanding how to carry out a gas tightness test correctly is essential.
It’s one of the core safety checks every engineer must perform to confirm that a gas installation is sound and leak-free before issuing a gas certificate.
In this guide, we’ll break down what a tightness test is, how to do one step-by-step, and what counts as a permissible pressure drop according to Gas Safe and IGEM/UP/1B standards.
This guide reflects IGEM/UP/1B Edition 4, which becomes mandatory on 1 October 2026. The biggest change: permissible pressure drop is now determined by Installation Volume (IV), not meter size. For a full breakdown of the Edition 4 changes and a worked IV calculation, see our IGEM/UP/1B Edition 4 guide.
What Is a Gas Tightness Test?
A gas tightness test checks whether gas pipework is free from leaks and safe to use. It’s performed using a manometer (U-gauge or digital) to measure any pressure drop in the system when it’s isolated.
Tightness tests are carried out:
- On new installations or pipework, before gas is introduced.
- If a gas leak is suspected.
- After any work or modification to existing pipework.
- During routine safety inspections or landlord checks.
- After a gas let-by test.
To carry out a gas tightness test, you either need a U-gauge (water gauge) or a digital gauge. Your flue gas analyser probably has the capability to perform a tightness test without a separate water gauge. If not, there may be an optional add-on attachment you can purchase online.
Gas Tightness Test Procedure (Step-by-Step)
Here’s how to correctly perform a gas tightness test under domestic conditions (following IGEM/UP/1B Edition 4).
Before starting, carry out a let-by test to confirm the ECV is sealing properly. A failed let-by makes the tightness test unreliable — see our gas let-by test guide for the full procedure.
1. Preparation
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- Check that all appliances are turned off and isolation valves are closed.
- Warn occupants that you’ll be carrying out a test and ensure good ventilation.
- Connect your manometer to the meter test point.
2. Set Test Pressure and Stabilise
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- For natural gas: Apply a test pressure of 20 – 21 mbar.
- For LPG: Apply 37 mbar.
- Allow the pressure to stabilise for 1 minute before timing the test.
3. Carry Out the 2-Minute Test
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- Once stabilised, time the test for 2 minutes. Under Edition 4 this is the standard duration for all systems within the UP/1B scope.
- Observe the pressure reading throughout the test period.
- A drop indicates a leak or expansion in the system.
4. Assess Against Permissible Drop (Edition 4)
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- If there is no perceptible movement of the gauge, the installation passes — proceed to Step 6 (LDF check).
- If there is any perceptible movement, calculate the Installation Volume (IV) of the system. See our Edition 4 guide for the calculation and a worked example.
- Compare the observed drop to the permissible limit for your fuel gas and IV band (see tables below).
- If the drop is within the permissible limit: proceed to Step 5 (pipework-only retest).
- If the drop is outside the permissible limit: trace and repair the escape, or disconnect the installation from the gas supply and make it safe.
5. Pipework-Only Retest (new in Edition 4)
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- Isolate all appliances from the gas supply.
- Repeat the tightness test on pipework only. There must be no perceptible movement of the gauge for this to be deemed a pass.
- Where it is not practicable to isolate appliances, carry out the test with them connected and apply the “no perceptible movement” criterion to the whole system.
- If any perceptible movement is recorded: trace and repair the escape, or make the system safe.
6. Apply Leak Detection Fluid (LDF)
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- On completion of a satisfactory tightness test, remove the gauge and refit the test nipple.
- Reintroduce the gas.
- Test all disturbed joints — plus any installation pipework between an appliance and its isolation valve — with leak detection fluid (LDF). There must be no smell of gas.
Following this gas tightness test procedure ensures compliance with Gas Safe requirements and keeps installations safe for use.
When you’re finished with the test, you’ll probably be issuing your customer a certificate. Create a CP12 online for free using our generator, or see a full list of certificates available in Gas Engineer Software.
What Is a Permissible Drop?
This is the key part of interpreting your results — understanding what’s acceptable and what’s not.
A permissible drop refers to the small pressure decrease that can occur naturally (for example, due to temperature stabilisation) and is still considered safe.
For any system where the observed drop is perceptible, you calculate the IV and look up the permissible limit for your fuel gas type.
- For new installations:
➜ No pressure drop is permissible — the system must remain stable throughout the test. - Existing installations with appliances connected:
➜ A drop within the IV-based permissible limit is acceptable — but you must to repeat the test on pipework only and confirm there is no perceptible movement
For the full method of calculating IV, see our IGEM/UP/1B Edition 4 guide.
Gas Tightness Test Permissible Drops (Edition 4)
Natural gas
| Installation Volume (IV) | Maximum permissible pressure drop |
|---|---|
| ≤ 0.005 m³ | 8 mbar |
| > 0.005 – ≤ 0.010 m³ | 4 mbar |
| > 0.010 – ≤ 0.015 m³ | 2.5 mbar |
| > 0.015 – ≤ 0.035 m³ | 1 mbar |
LPG
| Installation Volume (IV) | Maximum permissible pressure drop |
|---|---|
| ≤ 0.0025 m³ | 2 mbar |
| > 0.0025 – ≤ 0.005 m³ | 1 mbar |
| > 0.005 – ≤ 0.01 m³ | 0.5 mbar |
| > 0.01 – ≤ 0.035 m³ | No perceptible movement |
LPG/Air
| Installation Volume (IV) | Maximum permissible pressure drop |
|---|---|
| ≤ 0.025 m³ | 1.5 mbar |
| > 0.025 – ≤ 0.035 m³ | 0.5 mbar |
Edition 3 reference
In domestic natural gas systems:
| Type of Installation / Meter Designation | Pipework Diameter | Maximum Permissible Pressure Drop |
| No Meter (AECV only, e.g. flat) | ≤ 28 mm | 8 mbar |
| > 28 mm ≤ 35 mm | 4 mbar | |
| Ultrasonic (≤ 6 m³/h, e.g. E6) | ≤ 28 mm | 8 mbar |
| > 28 mm ≤ 35 mm | 4 mbar | |
| Diaphragm (≤ 6 m³/h, e.g. U6, G4) | ≤ 28 mm | 4 mbar |
| > 28 mm ≤ 35 mm | 2.5 mbar | |
| Diaphragm (> 6 m³/h ≤ 16 m³/h, e.g. U16, G10) | ≤ 35 mm | 1 mbar |
In domestic LPG systems:
| Type of Installation / Meter Designation | Pipework Diameter | Maximum Permissible Pressure Drop |
| No Meter (ECV/AECV only, e.g. flat) | ≤ 35 mm | 1.5 mbar |
| Diaphragm (≤ 6 m³/h, e.g. U6, G4) | ≤ 35 mm | 1.5 mbar |
| Diaphragm (> 6 m³/h ≤ 16 m³/h, e.g. U16, G10) | ≤ 35 mm | 0.5 mbar |
If you notice a larger or continuous drop beyond these limits, it indicates a leak, and you’ll need to locate and repair it before re-testing.
Common Mistakes (and How to Avoid Them)
Even experienced engineers can make simple errors that affect results:
- Skipping stabilisation time — always allow 1 minute before timing.
- Using uncalibrated equipment — check your manometer regularly.
- Incorrect test duration — follow the guidance for system size and gas type.
- Recording errors — write down readings clearly or, better yet, log them digitally.
- Using Edition 3 tables after 30 September 2026 — the meter-size-based tables are superseded by Installation Volume-based tables.
- Skipping the pipework-only retest — under Edition 4, if your initial test shows any perceptible drop but is within the IV-based permissible limit, you must isolate appliances and retest the pipework on its own.
Recording results directly in your digital certificates (via Gas Engineer Software) ensures you always have a complete, traceable record linked to the job.
Recording and Certification
After completing the test:
- Record your initial and final pressures.
- Note the duration of the test and any pressure changes.
Include your name, Gas Safe number, and date of test.
If you’re using paper pads, this can be time-consuming and easy to lose. Many engineers now prefer using digital gas safety certificates, which automatically link readings to the customer record and help you stay compliant.
💡 Gas Engineer Software lets you record tightness test readings quickly on-site, generate certificates, and store them securely — no paper, no missed details, no stress.
FAQs:
What pressure should a gas tightness test be carried out at?
20 mbar for natural gas and 37 mbar for LPG (domestic systems).
How long should the test last?
Under IGEM/UP/1B Edition 4, the timed test is 2 minutes following a 1-minute stabilisation period. The pipework-only retest after that is also a 2-minute check.
What’s an acceptable drop on a gas tightness test?
Under IGEM/UP/1B Edition 4 it depends on the Installation Volume (IV) of the system and the fuel gas type. For natural gas the permissible drop ranges from 8 mbar (IV ≤ 0.005 m³) down to 1 mbar (IV > 0.015 – ≤ 0.035 m³). For LPG the thresholds are tighter still. Whatever the initial result, Edition 4 also requires a pipework-only retest where there must be no perceptible movement. See the tables above for full details.
Can I use a digital manometer?
Yes. Note that under Edition 4, “no perceptible movement” has specific numerical definitions for electronic gauges: 0.25 mbar maximum on a standard-resolution gauge, and 0.2 mbar on a gauge that reads to one decimal place. Make sure your gauge meets the resolution required for the test you’re performing.