flue gas analysis - too much O2

[Jennie]

Hi all,
Could I check I correctly understand fga readings taken off an old Microgenus combi.
The ratio is great (0.0003%). CO2 is only 3.4% (should be 7.62), O2 is 15% (should be closer to 7%, according to the boiler manual). CO was 33ppm.
The ratio passes the gas safe requirements.
However, the readings suggest the boiler is under-gassed. Solution: check incoming gas pressure to the boiler, and also the burner pressure (which I don't have - but let's assume it's low).
Would a low burner pressure would be cause for the boiler to fail the certificate?
Apparently, the gas rate was spot on. But surely it can't be - if the boiler isn't burning enough gas to use up all the oxygen.
Also, how much work would you do on a boiler if the landlord only wants the gas safety certificate? Ie, they don't want to spend money on having it cleaned, adjusted, etc, despite the engineer's better judgement. Or if a 5-yr burner seal is due renewal, but no visual damage? Obviously, if it fails the certificate, the work must be done (or the boiler condemned).
Thanks for your advice,
Jennie

 

[Chalked]

Have you had your fga on LPG? Wouldn't be the first time I've done it!

 

[Last Plumber]

Your readings are not quite correct. Is your analyser working and if so, did you pre purge it in fresh air?

 

[Jennie]

Oh ... yeah, I did have it on LPG. I realised once I got home. How much does this change the readings?
The gas rate was good, the flame picture great.

 

[Last Plumber]

33/3.4/10000=.000971
or
3.4 x 10000=34000
33/34000=.000971

 

[Last Plumber]

Oh ... yeah, I did have it on LPG. I realised once I got home. How much does this change the readings?
The gas rate was good, the flame picture great.

I would go back and re check that !

 

[aplombier]

Thought O2 should be 20.9 % not 7 % or am I reading things wrong

 

[Last Plumber]

Thought O2 should be 20.9 % not 7 % or am I reading things wrong

It is in fresh air but in flue whilst the appliance is burning it will be a lot less.

 

[Darren Jackson]

If inlet is within parameters, gas rating is good, seals intact, integrity 20.9% flame picture good then all should be well to pass. Just need to double check the FGA is set up for correct fuel n go back to get an accurate ratio to record on the CP12.

 

[Reg Man]

26(9) Minimum gas safety checks. Burner pressure/Gas rate where appropriate both.