Gas Tightness Test Guide: Procedure & Permissible Drop (IGEM/UP/1B Edition 4)

Gas Tightness Test Procedure (Step-by-Step)

Here’s how to carry out a gas tightness test on a domestic installation, following IGEM/UP/1B Edition 4.

Step 1. Preparation & Let-By Test

• Do a visual inspection of the installation.
• Make sure each appliance is set up for the test. Leave appliance isolation valves open, turn off all control taps, put out any permanent pilots, and lift cooker lids to the upright position.
• Inform occupants and make sure there’s good ventilation.
• Turn off the appropriate valve: the ECV, AECV or MIV.
• Connect your manometer to the pressure test point.
• Carry out a let-by test. Adjust the pressure to between 7 and 10 mbar, then watch the gauge for 1 minute. Any perceptible rise means the valve is letting by. Trace and repair it, or make the installation safe, before you go any further. A failed let-by makes the tightness test result unreliable. See our gas let-by test guide for the full procedure.

2. Set Test Pressure and Stabilise

Once the let-by test has passed, raise the pressure to the tightness test pressure (TTP) for the fuel gas, then turn off the gas supply. Allow the pressure to stabilise for 1 minute before timing the test.

3. Carry Out the 2-Minute Test

• • Once the pressure has stabilised, time the test for 2 minutes.
  • Watch the gauge throughout the test period.

4. Assess Against Permissible Drop (Edition 4)

• • No perceptible movement on the gauge and no smell of gas: the installation passes. Go to Step 6 (LDF check).
  • New installations, or pipework with only new appliances connected: there must be no perceptible movement. Any movement is a fail, so trace and repair the escape, or make the installation safe.
  • Existing appliances connected and perceptible movement: calculate the Installation Volume (IV) of the installation. See our Edition 4 guide for the method, or use our Installation Volume Calculator to get the result quickly.
  • Compare the observed drop with the permissible limit for your fuel gas and IV band (see the tables below).
  • Drop within the permissible limit and no smell of gas: go to Step 5 (pipework-only retest).
  • Drop outside the permissible limit, or a smell of gas: 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)

• • Close the appliance isolation valves so that all appliances are isolated from the gas supply.
  • Repeat the tightness test on the pipework only. It passes if there’s no perceptible movement of the gauge and no smell of gas.
  • Where it isn’t practicable to isolate an appliance, test with it connected and apply the “no perceptible movement” criterion to the whole system.
  • If there’s any perceptible movement or a smell of gas, trace and repair the escape, or make the system safe.
  • Once the retest has passed, open all the appliance isolation valves again.

6. Apply Leak Detection Fluid (LDF)

• • Once the tightness test has passed, remove the gauge and refit the test nipple.
  • Turn the gas back on.
  • Test all disturbed joints with leak detection fluid (LDF), plus any installation pipework between an appliance and its isolation valve. There must be no smell of gas.

7. Purge If Needed and Record Results

• • If the installation is new, or any gasways have been exposed to air, purge the installation. Our gas purging procedure guide covers it step by step, including how to work out purge volume under Edition 4.
  • Record your results.

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 GES.

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

FAQs

What pressure should a gas tightness test be carried out at?

On natural gas the test pressure is 20–21 mbar. If you can’t reach 20 mbar, you can test at a minimum of 18 mbar followed by operating pressure checks.

On LPG and LPG/Air the test pressure depends on the installation’s operating pressure. On LPG it also depends on whether you’re testing through the regulator:

• LPG/Air: 13–14 mbar at a 14 mbar operating pressure, or 20–21 mbar at 21 mbar.
• LPG at 28 mbar: 20–21 mbar with the regulator, or 27–28 mbar without.
• LPG at 30 mbar: 28–29 mbar with the regulator, or 29–30 mbar without.
• LPG at 37 mbar: 30–31 mbar with the regulator, or 36–37 mbar without.

How long should the test last?

Allow 1 minute for temperature stabilisation, then time the test for 2 minutes. If you need a pipework-only retest, repeat the same test with the appliances isolated.

What’s an acceptable drop on a gas tightness test?

On new installations, or pipework with only new appliances connected, no drop is acceptable. There must be no perceptible movement of the gauge.

On installations with existing appliances connected, Edition 4 sets the permissible drop by Installation Volume (IV) and fuel gas:

• Natural gas: from 8 mbar (IV ≤ 0.005 m³) down to 1 mbar (IV > 0.015 – ≤ 0.035 m³).
• LPG: from 2 mbar (IV ≤ 0.0025 m³) down to no perceptible movement (IV > 0.01 – ≤ 0.035 m³).
• LPG/Air: 1.5 mbar (IV ≤ 0.025 m³) or 0.5 mbar (IV > 0.025 – ≤ 0.035 m³).

A drop within the limit isn’t an automatic pass. You have to isolate the appliances and retest the pipework on its own, and that retest must show no perceptible movement. There must also be no smell of gas at any stage. See the tables above for every IV band.

Can I use a digital manometer?

Yes. Edition 4 defines “no perceptible movement” for each type of gauge:

• Water (fluid) gauge: a movement of 0.25 mbar or less isn’t perceptible.
• Electronic gauge: a maximum of 0.25 mbar.
• Electronic gauge that reads to one decimal place: a maximum of 0.2 mbar.