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Central Heating

Flue reading analysis

I have a flue reading of 18.6 I am told it should be 20.9 and have been told flue seal leaking .. can you explain and confirm this information is correct please

5 Answers from MyBuilder Heating Engineers

Best Answer

Yes this sounds correct. Sounds as if this is an analysis of the air intake to the flue which should ideally have an oxygen reading of around 20.9% or a minimum of 20.6% oxygen in air ratio/mix. As you state your reading is too low, this indicates the flue seals have perished and the air intake is mixing in the flue with the exhaust gases.. essentially starving the boiler of oxygen for combustion and will cause the boiler to burn incorrectly and be dangerous. I hope your engineer has capped of the appliance until this can be remedied. Some manufacturers allow the flue seals to be replaced.. however most of the time the whole flue needs changing.


Answered 27th Feb 2020

I completely.agree with the reply given!
The boiler should be isolated from gas and a immediately dangerous notice issued with the users permission, if no permission given then the gas carrier should be informed about the findings


Answered 11th Mar 2020

Mike Kitchen MT Gas & Heating
I too completely agree with the answer
The customer should be made aware of the problem and in the interests of safety the boiler should be capped, to be absolutely sure a new flue should be fitted to the boiler and new readings taken with the analyser to ensure the boiler is working effectively and safely


Answered 14th Mar 2020

You need to be more specific with your details


Answered 22nd Feb 2020

Ive bought a new Kane FGA. Its reading 19.9 outside and 19.9 in the flue. Ive rang them as i had to get my m8 out with his fga and they said thats acceptable, i told them it wasnt and wanted it recalibrated. Is this acceptable?


Answered 19th Sep 2022

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