7.3 CO/CO₂ Ratio Action Levels (BS 7967) & GIUSP Actions

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

7.3 CO/CO₂ Ratio Action Levels (BS 7967) & GIUSP Actions

Key Takeaways Explain This Topic AI

CO/CO₂ ratio = CO (ppm) ÷ (CO₂ (%) × 10,000); it normalises combustion quality for excess-air dilution.

BS 7967 Table 3 sets appliance-specific action levels — there is no single universal statutory 0.004 ceiling for every appliance.

Typical examples: central heating boiler with an air/gas ratio valve often uses 0.0040; boiler without AGRV often uses 0.0080; some water heaters use higher published action levels (e.g. 0.0200) — manufacturer instructions override when stated.

Above the relevant action level, investigate/service per MIs and BS 7967; classify under GIUSP (ID/AR) based on the actual hazard (e.g. spillage into the room is Immediately Dangerous).

CPA1 competence (embedded in CCN1 since April 2012) requires correct analyser use and ratio interpretation, not memorising one number for all appliances.

Last updated: July 2026

7.3 CO/CO₂ Ratio Thresholds (0.004 Limit) & Action Procedures

ACS CCN1 Exam Notice: Calculating and interpreting the CO/CO₂ ratio is a core practical skill evaluated during ACS assessments. Candidates must master the ratio formula, apply the correct BS 7967 Table 3 action level for the appliance type (and any manufacturer limit), and execute the correct Gas Industry Unsafe Situations Procedure (GIUSP) actions.

Exam correction: BS 7967 Table 3 is appliance-specific (e.g. boiler with AGRV often 0.0040 ; boiler without AGRV often 0.0080 ). Manufacturer limits override.

The CO/CO₂ ratio (also known as the combustion ratio or combustion performance index) compares the concentration of toxic carbon monoxide (CO) to carbon dioxide (CO₂) in flue gases. Because absolute CO ppm readings vary depending on flue gas dilution by excess air, the CO/CO₂ ratio provides an absolute, dilution-independent index of combustion efficiency and safety.

The Mathematical Formula & Worked Calculations

Carbon monoxide is measured in parts per million (ppm) , whereas carbon dioxide is measured as a percentage (%) . To calculate the ratio, both values must be expressed in identical units. Since 1% is equal to 10,000 ppm , the percentage of CO₂ is multiplied by 10,000.

The Standard Calculation Formula CO/CO 2 Ratio = CO (ppm) CO 2 ( % ) × 10 , 000 \text{CO/CO}_2 \text{ Ratio} = \frac{\text{CO (ppm)}}{\text{CO}_2 (\%) \times 10,000} CO/CO 2 ​ Ratio = CO 2 ​ ( % ) × 10 , 000 CO (ppm) ​ +-----------------------------------------------------------------------+

• | WORKED CO/CO₂ RATIO CALCULATIONS |
• | |
• | Example A (Pass): CO = 32 ppm, CO₂ = 8.0% |
• | 32 / (8.0 x 10,000) = 32 / 80,000 = 0.0004 |
• | |
• | Example B (Limit): CO = 240 ppm, CO₂ = 6.0% |
• | 240 / (6.0 x 10,000) = 240 / 60,000 = 0.0040 |
• | |
• | Example C (Fail): CO = 560 ppm, CO₂ = 7.0% |
• | 560 / (7.0 x 10,000) = 560 / 70,000 = 0.0080 |
• +-----------------------------------------------------------------------+
• Step-by-Step Worked Examples
• Worked Example 1: Modern Condensing Boiler

FGA Measured Values: CO = 28 ppm \text{CO} = 28 \text{ ppm} CO = 28 ppm , CO 2 = 9.3 % \text{CO}_2 = 9.3\% CO 2 ​ = 9.3% .

Calculation: Ratio = 28 9.3 × 10 , 000 = 28 93 , 000 = 0.00030 \text{Ratio} = \frac{28}{9.3 \times 10,000} = \frac{28}{93,000} = 0.00030 Ratio = 9.3 × 10 , 000 28 ​ = 93 , 000 28 ​ = 0.00030

Evaluation: Pass. Modern high-efficiency boilers typically operate between 0.00010 and 0.00080.

Worked Example 2: Boundary Threshold Case

FGA Measured Values: CO = 200 ppm \text{CO} = 200 \text{ ppm} CO = 200 ppm , CO 2 = 5.0 % \text{CO}_2 = 5.0\% CO 2 ​ = 5.0% .

Calculation: Ratio = 200 5.0 × 10 , 000 = 200 50 , 000 = 0.00400 \text{Ratio} = \frac{200}{5.0 \times 10,000} = \frac{200}{50,000} = 0.00400 Ratio = 5.0 × 10 , 000 200 ​ = 50 , 000 200 ​ = 0.00400

Evaluation: Upper safe operational limit under BS 7967. Servicing is strongly recommended.

Worked Example 3: Defective Appliance

FGA Measured Values: CO = 720 ppm \text{CO} = 720 \text{ ppm} CO = 720 ppm , CO 2 = 8.0 % \text{CO}_2 = 8.0\% CO 2 ​ = 8.0% .

Calculation: Ratio = 720 8.0 × 10 , 000 = 720 80 , 000 = 0.00900 \text{Ratio} = \frac{720}{8.0 \times 10,000} = \frac{720}{80,000} = 0.00900 Ratio = 8.0 × 10 , 000 720 ​ = 80 , 000 720 ​ = 0.00900

Evaluation: Severe combustion failure ( > 0.0080 > 0.0080 > 0.0080 ). Unsafe installation under GIUSP.

BS 7967 Action Threshold Boundaries

BS 7967 establishes three distinct combustion performance zones based on the calculated CO/CO₂ ratio: 0.0000 ------------- 0.0040 ----------------- 0.0080 ------------------> Higher

• | SATISFACTORY | UNACCEPTABLE | IMMEDIATELY DANGEROUS |
• | (Normal Range) | (Service Required) | (Isolate Appliance) |

Satisfactory Zone (Ratio ≤ 0.0040 \le 0.0040 ≤ 0.0040 ):

Combustion performance is acceptable.

No emergency intervention required. Normal routine servicing as per manufacturer instructions.

Unacceptable Zone ( 0.0040 < Ratio ≤ 0.0080 0.0040 < \text{Ratio} \le 0.0080 0.0040 < Ratio ≤ 0.0080 ):

Combustion performance is unsatisfactory and deteriorating.

The gas engineer must investigate the cause: check gas burner pressure, clean burner ports, inspect primary aeration shutters, and verify flue integrity.

If servicing restores the ratio below 0.0040, the appliance may remain in service.

If servicing cannot reduce the ratio below 0.0040, the appliance must be classified under GIUSP.

Dangerous Zone (Ratio > 0.0080 > 0.0080 > 0.0080 ):

Severe incomplete combustion and high toxicity risk.

Appliance presents an immediate risk to life or health.

Mandatory immediate isolation under GIUSP.

GIUSP Categorization & Action Procedures

Under the Gas Industry Unsafe Situations Procedure (GIUSP - IGEM/G/11) , combustion faults detected via FGA sampling must be categorized as follows:

GIUSP Classification Technical Threshold / Condition Mandatory Engineering Action

Immediately Dangerous (ID) Ratio > 0.0080 > 0.0080 > 0.0080 , OR flue spillage into room, OR CO entering property Disconnect & Isolate Gas Supply with user permission; fit ID warning label; issue Warning Notice

At Risk (AR) Ratio 0.0040 − 0.0080 0.0040 - 0.0080 0.0040 − 0.0080 unrectifiable, but NO immediate flue spillage into room Turn Off & Disconnect with user permission; fit AR warning label; issue Warning Notice

Not to Current Standards (NCS) Ratio ≤ 0.0040 \le 0.0040 ≤ 0.0040 , but installation lacks modern minor features Advise customer in writing; no isolation required

Comprehensive Decision & Action Matrix

• Measured CO (ppm) Measured CO₂ (%) Calculated Ratio BS 7967 Status GIUSP Classification Required Action
• 30 ppm 8.5% 0.00035 Satisfactory Normal Operation Complete annual service report
• 160 ppm 4.0% 0.00400 Upper Limit Satisfactory Clean burner & re-test
• 300 ppm 6.0% 0.00500 Unacceptable At Risk (if unrectifiable) Investigate & clean; isolate if unrectified
• 480 ppm 6.0% 0.00800 Unacceptable At Risk (if no spillage) Servicing required; isolate if unrectified
• 850 ppm 8.5% 0.01000 Dangerous Immediately Dangerous Disconnect gas supply immediately
• Mandatory Legal & Isolation Protocols

When an appliance is classified as At Risk (AR) or Immediately Dangerous (ID) based on combustion performance, the engineer must execute the following statutory steps:

Obtain Customer Consent: Explain the dangerous combustion findings clearly to the user/landlord and request permission to isolate the gas supply.

Physical Isolation:

Turn off the appliance isolation valve or main gas meter valve.

Disconnect the gas supply pipework to the appliance and fit a gas-tight blanking plug or cap.

Warning Labeling: Affix an official high-visibility ID or AR Warning Label directly onto the appliance controls or isolation valve.

Issue Documentation: Complete a formal Gas Safety Warning Notice (MPWN) detailing the measured CO/CO₂ ratio, specific fault, GIUSP classification, and exact remedial work required.

Customer Refusal Protocol: If the customer refuses permission to isolate an Immediately Dangerous (ID) appliance:

Do NOT force entry or engage in physical altercations.

Immediately contact the Gas Emergency Service Provider (ESP) (0800 111 999) to request emergency disconnection at the meter/street.

Complete the Warning Notice stating "Permission to isolate refused" and leave it on site.

Document the incident thoroughly in company records and notify Gas Safe Register.

BS 7967 Action Levels Are Appliance-Specific

Do not treat 0.0040 as a single legal limit for every appliance. Common Table 3 examples taught for ACS include:

• Appliance type (examples) Typical BS 7967 action level (when MIs do not state otherwise)
• Central heating boiler with air/gas ratio valve (AGRV) 0.0040
• Central heating boiler without AGRV 0.0080

Instantaneous / storage water heater (flued or flueless examples in Table 3) Higher published levels may apply (e.g. 0.0200 )

Always prefer the manufacturer's stated CO/CO₂ or CO₂ commissioning limits . If only a CO₂ figure is published, satisfy that figure and stay at or below the relevant Table 3 action level.

GIUSP link

Exceeding an action level triggers investigation and possible GIUSP classification based on actual risk . Active spillage of combustion products into the room is typically Immediately Dangerous (ID) regardless of the exact ratio number.