Flue Gas Analysis

What is Flue Gas Analysis? Understanding This Vital Boiler Test

If you’ve ever had your boiler serviced by a Gas Safe engineer, you might have noticed them using a handheld device with a small tube inserted into your boiler’s flue. That’s not just a random test — it’s called a flue gas analysis, and it plays a critical role in both your home’s safety and your boiler’s efficiency.

In this guide, we’ll break down exactly what flue gas analysis is, why it’s important, and how it’s performed during a boiler service by the engineers at My Home Heating in Tewkesbury and across Gloucestershire.

What is Flue Gas Analysis?

Flue gas analysis is the process of testing the gases released by your boiler during combustion. These gases exit your property through the flue (the exhaust pipe). By analysing the composition of these gases, an engineer can determine whether your boiler is burning fuel correctly and operating safely and efficiently.

The main gases tested are:

• Carbon Monoxide (CO) – a toxic, odourless gas that must be kept at safe levels
• Carbon Dioxide (CO₂) – used to evaluate combustion quality
• Oxygen (O₂) – excess oxygen indicates inefficient combustion
• Flue Temperature – hotter gases may indicate heat loss
• CO/CO₂ Ratio – a critical safety measurement of combustion performance

Why is Flue Gas Analysis Important?

Flue gas analysis is one of the most important diagnostic tools we use during a boiler service. Here’s why:

• Safety: High levels of carbon monoxide (CO) can be extremely dangerous. The test detects unsafe CO output before it becomes a hazard.
• Efficiency: A properly adjusted boiler burns less gas, saves money, and reduces emissions.
• Appliance Health: Abnormal readings can indicate blockages, air leaks, or faulty parts before they cause major damage or breakdown.
• Warranty Compliance: Many boiler manufacturers require annual flue gas analysis as part of the service log to maintain warranty validity.

How Is Flue Gas Analysis Carried Out?

At My Home Heating, every gas boiler service includes a flue gas analysis (where the model allows). Here’s how we carry out the process step-by-step:

1. Warm-Up: We ensure the boiler has been running for a few minutes so the combustion process is stable.
2. Access the Test Point: Most modern boilers have a dedicated test point (sampling hole) on the flue or combustion chamber.
3. Insert the Analyser: We insert a flexible probe from our calibrated flue gas analyser into this test point.
4. Measure the Gases: The device draws in a sample of flue gases and provides digital readings of CO, CO₂, O₂, and temperature.
5. Evaluate Readings: We compare the results against manufacturer tolerances and Gas Safe guidelines.
6. Make Adjustments: If the CO/CO₂ ratio is outside the safe limit, we may adjust air/fuel ratios or clean components (burners, injectors).
7. Record Results: The results are logged in your boiler’s service record. Some models allow the analyser to connect digitally for updates.

What Is a Safe CO/CO₂ Ratio?

One of the most critical results from flue gas testing is the CO/CO₂ ratio. This value indicates how cleanly and safely your boiler is burning gas. A ratio above 0.004 is considered a potential safety concern and requires action or shut down. Below that level, the combustion is deemed safe.

In some cases, the appliance may need to be decommissioned if the ratio exceeds 0.008 or if CO levels are dangerously high, even if the flame appears “normal.” This is one of the reasons only Gas Safe engineers should ever attempt servicing or adjusting gas appliances.

What Tools Are Used in Flue Gas Analysis?

We use advanced flue gas analysers from trusted manufacturers like Testo or Kane. These instruments are regularly calibrated to ensure their readings are accurate and compliant with Gas Safe requirements. The analysers typically have:

• CO/CO₂ sensors
• Temperature probes
• Pressure gauges
• Digital printouts or wireless sync for reports

What Happens If a Problem Is Detected?

If our flue gas readings suggest incomplete combustion, unsafe CO levels, or blocked ventilation, we’ll immediately shut down the appliance and explain the issue. We’ll then offer clear, professional advice on how to rectify the fault – whether it’s a simple adjustment, a deep clean of internal parts, or in more serious cases, a recommendation to replace the unit.

How Often Should Flue Gas Analysis Be Done?

We recommend flue gas testing as part of your annual boiler service. For landlords, it’s a legal requirement as part of your Gas Safety Certificate (CP12). Regular testing ensures continued safe operation and early detection of any combustion faults.

Flue Gas Analysis for Landlords & Letting Agents

If you’re a landlord or managing agent in the Tewkesbury area, flue gas analysis is a required part of your annual gas safety inspections. We offer discounted rates for landlords and portfolio properties and can provide digital reports on the spot. Our Gas Safe registration ensures all legal requirements are met.

Book Your Boiler Service & Flue Test in Tewkesbury

Whether your boiler’s due for its annual check, or you’ve noticed odd smells, inconsistent heat, or increased fuel bills — a professional service with flue gas analysis can diagnose and prevent more serious problems. Based in Tewkesbury, My Home Heating provides friendly, local, and fully certified gas servicing across Gloucestershire.

In Summary

Flue gas analysis isn’t just a technical detail — it’s the frontline defence against gas leaks, carbon monoxide poisoning, and energy waste. As part of your annual service, it gives you peace of mind that your boiler is safe, efficient, and built to last. Trust the experts at My Home Heating to handle it with professionalism, care, and attention to detail.