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

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.

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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

$\text{CO/CO}_2 \text{ Ratio} = \frac{\text{CO (ppm)}}{\text{CO}_2 (\%) \times 10,000}$

+-----------------------------------------------------------------------+
|                   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: $\text{CO} = 28 \text{ ppm}$, $\text{CO}_2 = 9.3%$.
• Calculation: $\text{Ratio} = \frac{28}{9.3 \times 10,000} = \frac{28}{93,000} = 0.00030$
• Evaluation: Pass. Modern high-efficiency boilers typically operate between 0.00010 and 0.00080.

Worked Example 2: Boundary Threshold Case

• FGA Measured Values: $\text{CO} = 200 \text{ ppm}$, $\text{CO}_2 = 5.0%$.
• Calculation: $\text{Ratio} = \frac{200}{5.0 \times 10,000} = \frac{200}{50,000} = 0.00400$
• Evaluation: Upper safe operational limit under BS 7967. Servicing is strongly recommended.

Worked Example 3: Defective Appliance

• FGA Measured Values: $\text{CO} = 720 \text{ ppm}$, $\text{CO}_2 = 8.0%$.
• Calculation: $\text{Ratio} = \frac{720}{8.0 \times 10,000} = \frac{720}{80,000} = 0.00900$
• Evaluation: Severe combustion failure ($> 0.0080$). Unsafe installation under GIUSP.

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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)    |

1. Satisfactory Zone (Ratio $\le 0.0040$):

   • Combustion performance is acceptable.
   • No emergency intervention required. Normal routine servicing as per manufacturer instructions.

2. Unacceptable Zone ($0.0040 < \text{Ratio} \le 0.0080$):

   • 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.

3. Dangerous Zone (Ratio $> 0.0080$):

   • Severe incomplete combustion and high toxicity risk.
   • Appliance presents an immediate risk to life or health.
   • Mandatory immediate isolation under GIUSP.

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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:

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Comprehensive Decision & Action Matrix

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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:

1. Obtain Customer Consent: Explain the dangerous combustion findings clearly to the user/landlord and request permission to isolate the gas supply.
2. 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.
3. Warning Labeling: Affix an official high-visibility ID or AR Warning Label directly onto the appliance controls or isolation valve.
4. 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.
5. 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:

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.