The high excess air in domestic boilers

[wavetrain]

A link with some relevant info: http://www.engineeringtoolbox.com/boiler-combustion-efficiency-d_271.html

A combustion print out in my hand for a Keston boiler gives XAIR 36% at high rate and 45% at low rate.

Is the reason for having this amount of excess air to keep the CO as low as practicable, even though by doing this, the temperature that condensing occurs is lowered and therefore also the efficiency?

 

[Agile]

As explained in that link, greater efficiency is achieved by having a slight excess of air, about 6% suggested as optimum for G20.

You have to ve very careful that you are correctly interpreting your FGA readout. They do not measure all those parameters but just a few and then calculate the rest of the results shown.

That figure of 36% or 45% seems very high and I wonder if the boiler has been correctly set up???

Tony

 

[Gasguru]

Excess air is not the same as excess oxygen. Optimum combustion may be with around excess oxygen of 3%...this equates to excess air of around 15% due to the 20.9% oxygen content of air. Running with less than 3% oxygen and you risk incomplete combustion.

Forget looking at engineering websites...they normally talk about large commercial boilers with oxygen sensors providing feedback. This enables them to run very close to optimum without the danger of high carbon monoxide/hydrocarbon production. ie they can get very close to the curve peak but not over it.

For domestic boilers a margin of safety has to be deployed. No domestic boilers have oxygen trim (except one of the Viessmans) so manufacturers have the combustion with more excess air than ideal to provide safer and reliable combustion. They also have to allow for lack of servicing and ageing of components etc. Remember fully pre-mixed burners (as used on the Keston) are always less safe than natural draught pre-aerated burners (that run with much higher levels of excess air).

Your Keston should be around 30 to 40% excess air. What are the CO2 figures?...should be between 8.3 to 8.8 % and the O2 figure probably around 5 to 6 %. The C55 mix is not set very close to the optimum rate ie. it's kept a bit leaner than others.

 

[wavetrain]

On the high rate it is 5.5% O2, 8.8% CO2, CO ppm 92, XAIR 36%. The FGA belonged to the Keston engineer when he checked this boiler.

Here is a link to an interesting pdf www.hse.gov.uk/gas/domestic/pdf/fluegasanalyser.pdf

where all kinds of appliances have been tested. It bears out what Gasguru has written, they need to keep the CO down as much as possible even though the appliance is a little less efficient as a result of all that extra excess air.[/url]

 

[ChrisR]

3%...this equates to excess air of around 15% due to the 20.9% oxygen content or air

.

Interesting figures in the green columns in the first link

 

[Gasguru]

I've amended typo "or" to "of" to make sense now.