2.3 Tightness Testing Procedures for Natural Gas (IGEM/UP/1B)

2.3 Tightness Testing Procedures for Natural Gas (IGEM/UP/1B)

Tightness testing is the fundamental safety check performed on gas installations to confirm that pipework, fittings, meters, and appliance controls are gas-tight and free from dangerous leaks. In domestic and small commercial premises throughout the United Kingdom, tightness testing is governed by the Institution of Gas Engineers and Managers standard IGEM/UP/1B (Tightness testing and direct purging of small Natural Gas installations).

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Edition 4 Status & Transition (2026)

IGEM/UP/1B Edition 4 was published in March 2026. Gas Safe Register ISU 133 Edition 2 (15 April 2026) confirms the industry transition:

• Edition 3 may continue to be used in parallel until 30 September 2026, after which it is withdrawn.
• Edition 4 becomes mandatory from 1 October 2026.

For ACS CCN1 candidates preparing in 2026 and beyond, learn Edition 4 first. Edition 3 historically keyed some permissible drops to meter/pipe tables (for example diameter ≤ 28 mm → 4 mbar; > 28 mm → 2.5 mbar). Edition 4 replaces that as the primary rule with an Installation Volume (IV) method and a clearer perceptible movement decision tree. Keep the procedural timings (let-by, stabilisation, test duration) — they are unchanged — but judge drops using Edition 4 logic.

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Scope and Applications of IGEM/UP/1B (Low Pressure)

Gas operatives must understand the strict scope boundaries of IGEM/UP/1B before executing a test. For low-pressure (LP) Natural Gas, UP/1B applies when all of the following are met:

• Fuel Type: Natural Gas (Methane).
• Maximum Operating Pressure (MOP): MOP at the outlet of the Emergency Control Valve (ECV) not exceeding 2 bar for Natural Gas and LPG/Air (IGEM/UP/1B Edition 4 overall scope). Within that envelope, low-pressure installations (MOP ≤ 75 mbar) and medium-pressure installations (above 75 mbar up to 2 bar, normally fitted with a Meter Inlet Valve) follow different isolation and let-by steps. Reject the exam trap that wrongly claims overall UP/1B scope is “MOP ≤ 40 mbar” (that figure belongs to the older/narrower UP/1A case, not UP/1B Edition 4).
• Maximum Pipe Diameter: Nominal pipe diameter ≤ 35 mm.
• Maximum Total Installation Volume ($IV$): Total system volume ≤ 0.035 m³ (35 litres), including meter volume.

If any installation exceeds these parameters (e.g., pipework > 35 mm or $IV$ > 0.035 m³), the operative must test under the larger commercial standard, IGEM/UP/1 or IGEM/UP/1A.

Medium pressure (MP) supplies — above 75 mbar at the ECV outlet up to 2 bar — sit outside ordinary LP tightness practice. They require additional meter installation protective devices and have extra MIV / let-by requirements; do not treat an MP meter installation as a simple LP UP/1B job without those controls.

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Test Media Selection: Fuel Gas vs. Air

The choice of testing medium depends on the installation status:

1. Existing Installations (Gas Connected): Tested using Natural Gas at working pressure supplied via the meter and Emergency Control Valve (ECV).
2. New Installations / Pipework Extensions (No Gas Connected): Tested using clean oil-free compressed air or nitrogen introduced via a hand test pump before the meter is connected or before the supply is turned on.
3. De-commissioned / Altered Systems: Tested with air or gas depending on whether the meter is fitted and commissioned.

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Complete Step-by-Step Tightness Testing Workflow

Once the let-by test has been successfully completed and passed (7.0–10.0 mbar for 1 minute), the tightness test procedure unfolds as follows:

+-----------------------------------------------------------------------+
|                TIGHTNESS TESTING STAGE-BY-STAGE (UP/1B Ed4)           |
|                                                                       |
|  STAGE 1: PRE-TEST & LET-BY PASS                                      |
|  - Confirm let-by test passed (7.0 - 10.0 mbar for 1 min).            |
|                                                                       |
|  STAGE 2: PRESSURISATION TO OPERATING PRESSURE                        |
|  - Slowly open ECV to raise gauge pressure to 20.0 - 21.0 mbar.       |
|  - Close ECV.                                                         |
|                                                                       |
|  STAGE 3: THERMAL STABILISATION (1 MINUTE)                            |
|  - Wait 60 seconds for gas temperature & pressure to equalise.        |
|  - Adjust pressure back to 20.0 mbar if drop occurred due to cooling.  |
|                                                                       |
|  STAGE 4: OFFICIAL TEST PERIOD (2 MINUTES)                            |
|  - Time for exactly 120 seconds.                                      |
|  - Observe for perceptible movement; record P1 and P2 if needed.      |
|  - Apply Edition 4 pass/fail rules (movement → IV → Table 3).         |
+-----------------------------------------------------------------------+

Step 1: Pre-Test Setup & Let-By Completion

• Ensure all gas appliances are turned off, burner controls are shut, and pilot lights are out.
• Keep Appliance Isolation Valves (AIVs) OPEN so that supply pipework right up to appliance gas controls is pressurised (unless carrying out a deliberate pipework-only re-test with appliances isolated).
• Ensure the manometer remains connected to the meter test point.

Step 2: System Pressurisation to Operating Pressure

• Open the ECV very slowly to pressurise the downstream pipework and meter up to normal working pressure: 20.0 mbar to 21.0 mbar.
• If pressurising with a hand pump (new installation with air), pump up to 20.0–21.0 mbar.
• Close the ECV (or close the test pump valve).

Step 3: The 1-Minute Thermal Stabilisation Period

• When gas or air is compressed into pipework, thermodynamic effects cause brief temperature and pressure shifts (adiabatic heating followed by cooling).
• Mandatory Wait: Wait exactly 1 minute (60 seconds) with the supply isolated.
• Adjustment: At the end of the 1-minute stabilisation period, check the gauge pressure. If the pressure dropped slightly due to cooling, top up the pressure back to 20.0 mbar using the ECV or test pump.

Step 4: The 2-Minute Tightness Test Period

• Start the stopwatch immediately after stabilisation.
• Monitor the gauge continuously for exactly 2 minutes (120 seconds).
• Record whether there is perceptible movement and, if so, the starting and ending pressures ($P_1$, $P_2$) so that $\Delta P = P_1 - P_2$ can be compared with the Edition 4 IV table (detailed in the next section).

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Edition 4 Decision Rules (Core Procedure)

Candidates must memorise this order of decisions:

1. No perceptible movement during the 2-minute test → PASS (gas tight).
2. Any perceptible movement and no smell of gas → calculate Installation Volume (IV) and compare the measured drop with Edition 4 Table 3 (IV bands).
3. If the drop is within Table 3 limits with appliances connected, isolate all appliances and repeat a pipework-only test. On pipework-only, no perceptible movement is allowed — any movement fails.
4. New install / pipework-only / only new appliances: require no perceptible movement.
5. Smell of gas → FAIL regardless of the numerical drop or IV band.
6. Perceptible movement definition: water gauge 0.25 mbar; electronic gauges typically 0.25 mbar (use 0.2 mbar if the gauge displays only one decimal place).
7. Purge volume (when later commissioning): standardised in Edition 4 as $PV = 1.5 \times IV$.

Full IV tables, NICEIC copper pipe volumes, and leak-tracing practice are covered in section 2.4.

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Environmental and Physical Factors Affecting Gauge Readings

During tightness testing, gas operatives must account for environmental variables that can distort gauge readings:

Temperature Variations

• Pipework Exposure: Pipework exposed to direct sunlight, radiant heat from space heaters, or hot water pipes will heat the trapped gas. As gas warms, pressure RISES, which can mask an actual pipework leak.
• Cold Drafts: Pipework exposed to cold drafts or outdoors will cool the gas, causing pressure to FALL, giving a false impression of a pipework leak.
• Guidance: Shield test gauges and exposed pipework from extreme heat sources or cold wind during the test period.

Altitude and Barometric Influence

• In high-rise buildings or during rapidly changing atmospheric weather fronts, barometric pressure fluctuations can cause slight gauge movement on sensitive electronic manometers.
• Ensure electronic manometers are re-zeroed if atmospheric ambient conditions shift substantially between tests.