2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

ACS CCN1 Core Domestic Natural Gas Safety Assessment Exam

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1 Chapter 0: Introduction & ACS CCN1 Overview

0.1 ACS CCN1 Assessment Format, Open-Book Strategy & Gas Safe Path 0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

2 Chapter 1: Gas Safety Legislation & Regulations

1.1 Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — Operative Duties 1.2 Gas Safe Register Rules, Competence Categories & Operative ID Cards 1.3 Landlord Gas Safety Duties & CP12 / Safety Check Records (Regulation 36) 1.4 Building Regulations (Part J), HASWA & Health and Safety on Site

3 Chapter 2: Tightness Testing & Let-By Testing

2.1 Gas Meter Types (U6/G4/E6), Emergency Control Valves (ECVs) & MIVs 2.2 Let-By Testing Procedures & Electronic / U-Tube Manometer Setup 2.3 Tightness Testing Procedures for Natural Gas (IGEM/UP/1B) 2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

4 Chapter 3: Gas Rating, Pressure & Calorific Value

3.1 Gas Pressure Standards: Standing, Working & Meter Outlet Pressures 3.2 Gas Rating Domestic Appliances — Imperial & Metric Meter Calculations 3.3 Gross Calorific Value (39.5 MJ/m³), Gross/Net Heat Input & Efficiency 3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting

5 Chapter 4: Pipework Sizing, Installation & Purging

4.1 Gas Pipework Materials: Copper, Steel, PE & Corrugated Stainless Steel (CSST) 4.2 Pipe Sizing Calculations & The 1 mbar Maximum Pressure Drop Rule 4.3 Pipework Jointing, Sleeving, Corrosion Protection & Concealed Routes 4.4 Main Equipotential Bonding (within 600mm) & Direct Purging (IGEM/UP/1B)

6 Chapter 5: Ventilation Requirements for Domestic Gas

5.1 Ventilation Fundamentals & BS 5440-2 Standard Requirements 5.2 Ventilation Calculations for Open-Flued Appliances (5 cm²/kW Rule) 5.3 Ventilation for Flueless Appliances, Cookers & Appliance Compartments 5.4 Adventitious Air, Free Area Grille Sizing & Ventilation Defects

7 Chapter 6: Flue Types, Siting & Spillage Testing

6.1 Flue Classifications: Open-Flued (Type B), Room-Sealed (Type C) & Flueless (Type A) 6.2 Flue Terminal Siting, Clearance Distances & Protection (BS 5440-1) 6.3 Flue Flow Testing Procedures & Smoke Pellets Usage 6.4 Spillage Testing Procedures under Normal & Adverse/Worst-Case Conditions

8 Chapter 7: Combustion Analysis & Carbon Monoxide Safety

7.1 Complete vs Incomplete Combustion & Flame Picture Analysis 7.2 Electronic Flue Gas Analysers (FGA) & BS 7967 Testing Protocols 7.3 CO/CO₂ Ratio Action Levels (BS 7967) & GIUSP Actions 7.4 Carbon Monoxide Poisoning Symptoms, Causes & CO Alarm Rules (BS EN 50291)

9 Chapter 8: Emergency & Unsafe Situations (GIUSP / RIDDOR)

8.1 Gas Industry Unsafe Situations Procedure (GIUSP / IGEM/G/11) Overview 8.2 Immediately Dangerous (ID) Situations: Definition, Actions & Warning Labels 8.3 At Risk (AR) Situations & Advising Not to Current Standards (NCS) 8.4 Gas Escape Actions (0800 111 999), Isolation & RIDDOR Reporting Regulations

2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

Key Takeaways Explain This Topic AI

Meter IVm values: U6/G4 diaphragm = 0.008 m³; E6 ultrasonic = 0.0024 m³; U16 = 0.025 m³ — never treat E6 as 0.008 m³.

NICEIC copper pipe volumes: 15 mm = 0.00014 m³/m; 22 mm = 0.00032 m³/m; 28 mm = 0.00054 m³/m; 35 mm = 0.00084 m³/m; fittings ≈ 10% of IVp.

IGEM/UP/1B Edition 4 Table 3 (natural gas) keys maximum permissible drop to Installation Volume: ≤0.005 → 8 mbar; >0.005–≤0.010 → 4 mbar; >0.010–≤0.015 → 2.5 mbar; >0.015–≤0.035 → 1 mbar.

Any perceptible movement with appliances connected that is within Table 3 still requires a pipework-only re-test with no perceptible movement; smell of gas is always a fail, and new work / pipework-only tests also require no perceptible movement.

Uncorrected leaks exceeding limits are managed under GIUSP (ID or AR) with isolation, warning notices, and labels.

Last updated: July 2026

2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

Once a 2-minute tightness test is completed under IGEM/UP/1B Edition 4 , the gas operative must evaluate whether any recorded pressure movement is within acceptable safety thresholds. Determining whether a gas system passes or fails requires calculating the Installation Volume (IV) , checking for the smell of gas , applying the Edition 4 perceptible-movement rules, and executing systematic diagnostic leak tracing if a leak is present.

Calculating Total Installation Volume (IV)

The Installation Volume ( I V IV I V ) represents the combined internal volume of the gas meter and all downstream pipework up to appliance controls (plus an allowance for fittings). Total I V = I V m + I V p ( + fittings allowance ) \text{Total } IV = IV_m + IV_p (+ \text{fittings allowance}) Total I V = I V m ​ + I V p ​ ( + fittings allowance )

Where:

I V m IV_m I V m ​ = meter badged volume (see table below — meter type matters ).

I V p IV_p I V p ​ = ∑ ( Length of pipe in metres × Internal volume per metre ) \sum (\text{Length of pipe in metres} \times \text{Internal volume per metre}) ∑ ( Length of pipe in metres × Internal volume per metre ) .

Fittings : allow approximately 10% of I V p IV_p I V p ​ for elbows, tees, and couplings unless a more detailed volume schedule is used.

Meter Internal Volumes ( I V m IV_m I V m ​ )

• Meter Type Technology Badged Volume ( I V m IV_m I V m ​ )
• U6 / G4 Diaphragm 0.008 m³ (8 litres)
• E6 Ultrasonic smart 0.0024 m³ (2.4 litres)
• U16 Diaphragm (commercial) 0.025 m³ (25 litres)

Do not group U6, G4, and E6 as all 0.008 m³. E6 ultrasonic meters are 0.0024 m³ .

Internal Pipe Volumes per Metre (NICEIC copper values)

• Copper Pipe Size (Outside Diameter) Internal Volume per Metre ( v p v_p v p ​ )
• 15 mm 0.00014 m³/m (0.14 L/m)
• 22 mm 0.00032 m³/m (0.32 L/m)
• 28 mm 0.00054 m³/m (0.54 L/m)
• 35 mm 0.00084 m³/m (0.84 L/m)
• Worked Calculation Example

Problem : A property contains a U6 meter ( I V m = 0.008 m 3 IV_m = 0.008\text{ m}^3 I V m ​ = 0.008 m 3 ), 5 metres of 28 mm copper pipe, 12 metres of 22 mm copper pipe, and 8 metres of 15 mm copper pipe. Calculate the total Installation Volume ( I V IV I V ), including a 10% fittings allowance on pipe volume, and confirm it falls within the IGEM/UP/1B scope limit ( I V ≤ 0.035 m 3 IV \le 0.035\text{ m}^3 I V ≤ 0.035 m 3 ).

Solution :

I V m = 0.008 m 3 IV_m = 0.008\text{ m}^3 I V m ​ = 0.008 m 3

I V 28 = 5 m × 0.00054 m 3 / m = 0.00270 m 3 IV_{28} = 5\text{ m} \times 0.00054\text{ m}^3/\text{m} = 0.00270\text{ m}^3 I V 28 ​ = 5 m × 0.00054 m 3 / m = 0.00270 m 3

I V 22 = 12 m × 0.00032 m 3 / m = 0.00384 m 3 IV_{22} = 12\text{ m} \times 0.00032\text{ m}^3/\text{m} = 0.00384\text{ m}^3 I V 22 ​ = 12 m × 0.00032 m 3 / m = 0.00384 m 3

I V 15 = 8 m × 0.00014 m 3 / m = 0.00112 m 3 IV_{15} = 8\text{ m} \times 0.00014\text{ m}^3/\text{m} = 0.00112\text{ m}^3 I V 15 ​ = 8 m × 0.00014 m 3 / m = 0.00112 m 3

Straight pipe I V p = 0.00270 + 0.00384 + 0.00112 = 0.00766 m 3 IV_p = 0.00270 + 0.00384 + 0.00112 = 0.00766\text{ m}^3 I V p ​ = 0.00270 + 0.00384 + 0.00112 = 0.00766 m 3

Fittings ≈ 10 % × 0.00766 = 0.000766 m 3 10\% \times 0.00766 = 0.000766\text{ m}^3 10% × 0.00766 = 0.000766 m 3

Total I V = 0.008 + 0.00766 + 0.000766 = 0.01643 m 3 \text{Total } IV = 0.008 + 0.00766 + 0.000766 = \mathbf{0.01643\text{ m}^3} Total I V = 0.008 + 0.00766 + 0.000766 = 0.01643 m 3

Conclusion : Since 0.01643 m 3 ≤ 0.035 m 3 0.01643\text{ m}^3 \le 0.035\text{ m}^3 0.01643 m 3 ≤ 0.035 m 3 , the installation is within IGEM/UP/1B scope. Looking ahead to Table 3, this IV sits in the > 0.015 – ≤ 0.035 band (maximum 1 mbar drop if movement is perceptible and there is no smell of gas).

E6 contrast : Replacing the U6 with an E6 ( I V m = 0.0024 m 3 IV_m = 0.0024\text{ m}^3 I V m ​ = 0.0024 m 3 ) on the same pipework gives I V ≈ 0.0024 + 0.00843 = 0.01083 m 3 IV \approx 0.0024 + 0.00843 = 0.01083\text{ m}^3 I V ≈ 0.0024 + 0.00843 = 0.01083 m 3 — a different Table 3 band ( > 0.010 – ≤ 0.015 → 2.5 mbar ). Meter volume errors therefore change the permitted drop.

IGEM/UP/1B Edition 4 Permissible Drops (Table 3 — Natural Gas)

Edition 4 keys maximum permissible pressure drop during the 2-minute test to Installation Volume , not to a simple pipe-diameter split.

• Installation Volume I V IV I V (m³) Maximum Permissible Drop (mbar)
• ≤ 0.005 8
• > 0.005 – ≤ 0.010 4
• > 0.010 – ≤ 0.015 2.5
• > 0.015 – ≤ 0.035 1
• Edition 4 Pass/Fail Sequence (Apply in Order)

No perceptible movement → PASS (gas tight). Perceptible movement ≈ 0.25 mbar on a water gauge; electronic gauges typically 0.25 mbar ( 0.2 mbar if one-decimal display).

Any perceptible movement and no smell of gas → calculate I V IV I V ; compare measured drop with Table 3.

If within Table 3 limits with appliances connected → isolate all appliances and repeat a pipework-only test. Pipework-only must show no perceptible movement (any movement = fail).

New install / pipework-only / only new appliances → no perceptible movement required (do not use Table 3 relaxations).

Smell of gas → FAIL regardless of IV band or numerical drop.

Transition Note (Edition 3)

During the parallel-use period until 30 September 2026 , Edition 3 historically presented meter/pipe tables that many operatives summarised as ≤ 28 mm → 4 mbar and > 28 mm → 2.5 mbar for existing installations with no smell of gas. From 1 October 2026 , Edition 4 is mandatory: learn the IV Table 3 method above. Do not treat the old diameter-only matrix as the primary 2026+ exam rule.

Diagnostic Procedures for Leak Location

When a tightness test fails (pressure drop exceeds the Table 3 limit, pipework-only shows movement, or gas smell is detected), the engineer must locate and repair the escape using systematic tracing methods. +-----------------------------------------------------------------------+

• | LEAK TRACING & DIAGNOSTIC STEPS |
• | |
• | 1. VISUAL INSPECTION: Check joints, meter unions, compression nuts. |
• | 2. APPLIANCE ISOLATION METHOD: Close individual AIVs / all AIVs and |
• | re-test — Edition 4 pipework-only requires no perceptible move. |
• | 3. LEAK DETECTION FLUID (LDF): Spray BS EN 14291 compliant fluid |
• | on joints; observe active micro-bubbling. |
• | 4. ELECTRONIC SNIFFER: Sweep sensor tip at 25mm/sec above pipe run |
• | (Natural Gas is lighter than air and rises). |
• +-----------------------------------------------------------------------+
• Soap Solution / Leak Detection Fluid (BS EN 14291)

Use only approved Leak Detection Fluid (LDF) complying with BS EN 14291 .

DO NOT use domestic washing-up liquid, as it contains chlorides and high salt content that induce stress corrosion cracking on copper pipework and brass fittings.

Apply LDF onto compression joints, capillary solder elbows, threaded fittings, and test point nipples. Watch for growing foam bubbles indicating escaping gas.

Electronic Sniffers & Combustible Gas Detectors

Electronic gas detectors utilize semi-conductor sensors tuned to detect methane hydrocarbons in parts per million (ppm).

Buoyancy Rule : Natural Gas has a relative density of approximately 0.60 relative to air ( 1.0 1.0 1.0 ), making it lighter than air . Escaping gas rises upwards.

Sniffing Technique : Move the detector probe along the upper side of horizontal pipe runs and above fittings at a slow rate of 25 mm per second .

Appliance Isolation Method

Close all individual Appliance Isolation Valves (AIVs) for a pipework-only re-test.

Under Edition 4, pipework-only must show no perceptible movement . Continued movement means the leak is on the fixed installation pipework (or meter), not inside an appliance.

Unsafe Situations Procedure (GIUSP / IGEM/G/11)

If an uncorrected gas leak causes a tightness test failure and cannot be immediately rectified:

Immediately Dangerous (ID) : An unisolated leak or open end passing gas. The operative must isolate the ECV, disconnect/cap the supply, attach an ID label, and issue a Gas Safety Warning Notice (CP14) .

At Risk (AR) : An installation fault where continued operation presents a safety hazard. Isolate with customer permission, attach an AR label, and issue a CP14 Warning Notice.