Quote:

Corgi – Essential Gas Safety Handbook (Page 40)

Electronic combustion analysers

An electronic analyser can aid gas operatives to determine the safety, efficiency and performance of a gas appliance.

The gas industry has traditionally used the CO/CO2 ratio of the combustion products to establish safe combustion and appliance efficiency. A ratio is simply a way of expressing a mixture of a solid, liquid or gas that must be maintained or not exceeded.

British Standard 5258 states that the permitted maximum ratio of carbon monoxide to carbon dioxide in flue products is 0.02 to 1, (or 20 parts of carbon monoxide to 1000 parts carbon dioxide.) By dividing 20 by 1000 we get the ratio "0.02 to 1. This figure applies to all appliances burning natural gas, or LPG.

CO is measured in Parts Per Million (PPM) of the flue products. CO2 is measured as a percentage of the flue products. The CO and CO2 content of the combustion products is sampled and the proportions of each are then compared by the machine and shown as the ratio. The greater the CO content, the greater the degree of incomplete combustion.

In 1989 the Institution of Gas Engineers (IGE) and British Gas published a consultation paper giving their proposition for a new servicing method based on the combustion performance of an existing appliance. They proposed that if an appliance’s combustion performance was within a certain percentage of stoichiometric it would not require a full strip down service.

The limit set by the Institute of Gas Engineers and British Gas was 0.01 CO to 1 CO2. or not more than 10 parts CO in 1000 parts CO2. This is less than the British Standard figure. The trigger value they set for a full service is 0.004. Again this is less than demanded by the British Standard.

End of Quote:

Editor - Our expert has provided the following analysis. Red text is simply a copy of text being referred to and taken from the above Corgi Handbook content.

OPINION

The gas industry has traditionally used the CO/CO2 ratio of the combustion products to establish safe combustion and appliance efficiency.

The CO/CO2 ratio can only give an indication of how well combustion is taking place - hence the term combustion efficiency. The ratio itself can IN NO WAY provide any information on appliance efficiency as stated above.

Any industrial installer worth his salt will tell you that appliance efficiency will generally rise as the CO/CO2 ratio rises (less excess air, more efficient appliance because there are less losses but combustion may suffer slightly).

So what, on industrial you often reduce excess air until the point where the ratio is acceptable. Acceptable depends upon many factors but you cannot just state a standard figure you need to understand.

Some history - the CO/CO2 figure was produced way back (over 30 years ago). I think that the standards were known as combustion testing of appliances BS 2512 and BS 1250.

These were the first to quote but the figure should never have been applied without knowing the background etc. At this time there was no governor on the meter and sometimes no governor on the appliance although most appliances did have their own governor.

The regulations said that the CO/CO2 ratio must not exceed 0.02 when the appliance was subjected to overload. 20% for non governed appliances and 10% for governed. The figure could exceed 0.02 when the appliance was on turndown but the amount of CO produced could not exceed the amount produced when the appliance passed the 0.02 ratio at the overload rate. Complicated isn’t it. As with everything easy when you have been through it.

Note nothing to do with efficiency. For some reason the figures were then passed to the new appliance performance standards (EN series) 5258 but even then the figure was 'applied'. You had to know how to interpret.

Appliances are tested on a series of test gases which are designed to simulate the far extremes of supply which could be encountered on the district. A fault in gas processing etc. One of the test gases is a gas which easily produces CO and even when using this gas the appliance must pass the test.

Manufacturers have to produce appliances that pass. That is why they know all about the figures. I am pretty sure that they do not have a design procedure or test based on the ratio to predict servicing levels. Even a one shot efficiency reading means nothing.

Another lesson. I can understand empirical data being used but never as a blanket decision maker because appliances are not designed in the same way and a ratio on one is not the same as a ratio on another. It has different meaning.

A ratio is simply a way of expressing a mixture of a solid, liquid or gas that must be maintained or not exceeded.

The term ratio has nothing to do with maintained or exceeded. Pedantic but if you are going to publish make sure it does not mislead.

Products is 0.02 to 1,

a ratio is not to 1, it is stand alone. Small point but shows confusion

The CO and CO2 content of the combustion products is sampled

Unless the FGA is a laboratory machine ie Infra Red gas analyser (portables are not) the CO2 is not sampled, it is calculated from the O2 reading. Small but important point if you know what you are talking about.

The greater the CO content, the greater the degree of incomplete combustion.

Common mistake by people who don't know the crack. CO on its own can never give a meaningful answer because it is affected by excess air. That is the whole point of having a ratio. 5 ppm on one appliance may be OK, on another lethal. His statement may be factually true but is misleading.

They proposed that if an appliance’s combustion performance was within a certain percentage of stoichiometric

This does not make sense. Combustion performance means CO/CO2 ratio. Stoichiometric means exact amount of air for a given amount of gas. It is a non statement. The cost of a haircut is 5.30pm. This has the same meaning.

END of EXPERT OPINION

Editor – We do know that before we invest in any further FGA the following strict tests will first be applied to the subject and must be completely satisfied:

• There must be a relevant British Standard (not a draft) that Corgi can and do refer to.
• Corgi must produce a relevant technical feature on the subject, which must separate domestic from commercial boilers and address all issues of concern.
• Equipment manufacturers must offer product training – for a fee – that can be referred to in support and again must separate domestic from commercial boilers and address all issues of concern.

You may wish to consider applying the same tests but I can say we will be revising our list of tests shortly. We are investigating all this further and you never know, prosecutions may follow in the wake.