(For Gas Safe Engineers Only)

This procedure must only be carried out by Gas Safe Registered Engineers.
Flue integrity testing is a critical safety check carried out by Gas Safe-registered engineers to confirm that flue gases are not leaking into a boiler’s air intake. This test helps identify failures in flue seals or appliance casing that could compromise combustion quality and significantly increase the risk of carbon monoxide exposure.

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Why Test Flue Integrity?

A flue integrity test checks whether flue gases are leaking into the boiler’s air intake.
This condition compromises combustion quality and presents a serious carbon monoxide risk.

Flue integrity testing forms part of best-practice combustion analysis and should be completed whenever flue seal integrity is in doubt.

Testing the air intake is just as critical as analysing combustion at the flue terminal, as it confirms:

• Case integrity
• Flue seal integrity
• Correct separation of exhaust and intake paths

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Tools Required

• Flue Gas Analyser (FGA) with ratio mode
• Boiler capable of running in service / maximum rate mode
• Manufacturer-approved air sample access point

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Step-by-Step Flue Integrity Test Procedure

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1. Ensure the Boiler Casing Is Correctly Sealed

The boiler casing must be:

• Fully refitted
• Securely fastened
• Tested in normal operating conditions

Testing with the casing removed is not valid for flue integrity checks.

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2. Run the Boiler in Service Mode

Set the appliance to maximum rate using service mode in accordance with manufacturer’s instructions.
Allow readings to stabilise before proceeding.

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3. Insert the Analyser Probe into the Air Sample Point

• Set the analyser to ratio mode
• Insert the probe into the air intake sampling point on the flue (upstream of the combustion chamber)
• Position the probe as specified by the manufacturer

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4. Read the Flue Integrity Value

• Correct reading: 0.0000
• Any reading above 0.0000 indicates flue gases are present in the air intake

This confirms flue seal failure, casing seal failure, or compromised flue joints.

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5. Additional Indicators of Flue Seal Failure

Supporting signs may include:

• Black or grey staining on the top of the boiler
• Whistling or chattering noises from flue joints
• Noticeable CO₂ drift between minimum and maximum rate tests

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If the Flue Integrity Test Fails

If flue gases are detected in the air intake:

• Stop testing immediately
• Classify the appliance as unsafe
• Apply appropriate warning labels
• Follow the Unsafe Situations Procedure (IGEM/G/11)
• Do not recommission the appliance until faults are rectified

Engineers must follow the Unsafe Situations Procedure (IGEM/G/11) when flue integrity testing identifies contamination of the air intake.

IGEM/G/11 – Unsafe Situations Procedure (Gas Industry)
https://www.igem.org.uk/resource/igemg11/

Gas Safe Register – Technical & Safety Guidance
https://www.gassaferegister.co.uk/gas-safety/gas-safety-professionals/

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Quick Reference Summary

Check	Acceptable
Air intake ratio = 0.0000	Yes
Air intake ratio > 0.0000	No – flue integrity failed

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Support for Gas Safe Engineers

At Certificates4UK Ltd, we support Gas Safe engineers with:

• Advanced diagnostics
• Second opinions on complex faults
• Assistance with difficult servicing or integrity issues

If you operate in West Molesey, Surrey, or London, contact us for professional support