CO and Undiluted CO emissions

[i2ispa]

Hi,

Just had my boiler serviced, its a baxi back boiler 4 years old. The engineer has analysed the emissions and given me a piece of paper with the CO at 90ppm and the Undiluted CO at 300 ppm and ringed these items.

He tells me I need a new boiler.

Is this correct.
How easy is it to get these readings wrong.
Is this unusual for a 4 year old boiler.
Are there any adjustments which can be made at the boiler to improve on these figures.

The same happened to a colleague of mine (using same engineer), however his boiler was around 20 years old.

Advice appreciated.

Thankyou.

 

[chrishutt]

You shouldn't take on trust such figures. The analysis might not have been done properly (did he let the boiler run for 10 minutes or so to get up to temperature) and the analyser might not be properly calibrated.

You haven't told us if there were other figures on the print-out - in particular the CO:CO2 ratio. Did he service the boiler first? It could be a scam to persuade you to have a new boiler fitted. Get a second opinion from another heating engineer.

 

[oilman]

90ppm is no problem, but I don't know what the difference is between that and "Undiluted" CO.

In general the amount of information he is using to tell you a new boiler is needed is undiluted bilge. Don't have him back.

I only do oil boilers, but people have to fight with me before I agree to put in a new boiler. There is usually not much that cant be fixed at a reasonable cost.

 

[chrishutt]

90ppm is no problem, but I don't know what the difference is between that and "Undiluted" CO.

If the first reading (90 ppm) is taken from the secondary flue (via draught diverter), it needs to be adjusted to what it would be before the primary flue gases were diluted from the draught diverter. The analyser calculates this undiluted CO figure from the other measurements. In this case the dilution in the secondary flue was 70%, hence 300 ppm undiluted CO.

 

[oilman]

Is there an adjustment to get the fuel/air mixture correct? or is it just dirty burner jets?

Alternatively, did the guy do something deliberately to get this CO figure?

 

[oilman]

This is facinating, are there so few people who carry out these tests?

 

[Gasguru]

Easy to fake the results - just hold a cloth over the primary air port on the burner - I often show customers the resultant yellow flames just so they know how to distinguish between safe and clean combustion and the result of no servicing and risk of CO.

 

[chrishutt]

Is there an adjustment to get the fuel/air mixture correct? or is it just dirty burner jets?

On a newish gas Back boiler poor combustion is most likely caused by the primary air intakes being blocked by dust and pet hairs. That's why I asked if the boiler had been serviced, but the OP seems to have lost interest!

 

[oilman]

Thanks guys. I'd like to think I'm wrong, but this may be another "only ever posts once".

 

[i2ispa]

Sorry Guys,

I have been away.

The tests were carried out as part of a service and Landlords Safety test.

The tester was implying a new boiler was needed, "on its way out" was the phrase. When challenged he said "I am only letting you know, the boiler is still working".

Obviously with the property being tenanted I must be sure that everything is OK and legal.

The full readings are:

Fuel Natural Gas
O2 Cal 3.0%
CO2 Max 11.9%

Ambient Temp 20.8C
Fluegas temp 99.6C
O2 15.0%
CO 92ppm
%CO2 3.4%
Undiluted CO 322ppm

I have the Landlords Safety test Safety record Sheet, but with the CO and Udiluted CO marked as a fault.

Are these readings OK to carry on using the boiler. Can i just wait until the next service or should I seek another test.

Thanks.

 

[oilman]

I don't know about gas, but I'd like an explanation for the 15% O2 figure when the CO2 ia 11.9% or even 3.4%.

These figures are so far out of the norm for oil measurements, I'd be suspecting something. If it was oil and the CO2 is 3.4% then there's lots of air going in, cooling the flame, and giving rise to the high CO. The flame has to be hot so the CO (which is a flammable gas) can oxidise and produce the heat.

 

[chrishutt]

As I explained above, the figures are based on measurements from the secondary flue, after dilution with air at the draught diverter. For example the 3.4% CO2 in the secondary flue becomes 11.9% when you compensate for the 71.4% dilution. Likewise the diluted CO of 92 ppm becomes 322 ppm undiluted.

The important figure is the CO:CO2 ratio, which is not given in the print out. However it's easy to calculate from the figures above. From the secondary flue (diluted) figures it's 92/34,000 = 0.0027 and from the undiluted figures it's 322/119,000 = 0.0027 (the same, as one would expect). This ratio is within the acceptable limits (up to 0.004 I think), although it could doubtless be improved if you got a competent person to service the boiler properly.

If the LGSR records those results as faults (NCS presumably?) I should send a copy to Corgi and ask them to please see that their registered operatives are properly trained in the use of FGAs in the future.

 

[i2ispa]

Thank you for your help.

 

[i2ispa]

Just another point.

How would an engineer improve the CO to CO2 ratio on a back boiler.

Thanks again.

 

[chrishutt]

By servicing it. However your CO:CO2 ratio is nothing to worry about, being comfortably under 0.004. Send the paperwork to Corgi if you want confirmation of this.

It appears that your gas man didn't understand how to interpret the results of a Flue Gas Analysis, or deliberately misrepresented them to persuade you to have a new boiler fitted. One must therefore doubt whether he serviced the boiler properly, either through ignorance of the correct procedures or sheer idleness. A back boiler would take at least an hour to do properly. How long did he spend on it?