Flue Gas Analysis, what exactly is it?

A flue gas analyser measures and displays the products of combustion from gas appliances. Additionally, they can measure the ambient air quality in rooms or buildings.


As domestic service engineers responsible for installation, management and maintenance of gas appliances we always use a flue gas analyser.

This blog briefly explains the purpose of analysers, what they can be used to do and also the understanding of the information they display. We always emphasise that this is not a substitute for regular servicing of appliances. We also stress that you check with your engineer that the equipment they are using has been calibrated within 12 months, it has been known for this equipment to be purchased second hand and uncalibrated machines are likely to be giving totally false readings.

Using a Flue Gas Analyser Flue Gas Analyers should be used for the following two purposes:

1) Appliance testing

A Flue gas analyser measures flue gases – You place its’ probe in the chimney or appliance exhaust and see what levels of Oxygen (O2), Carbon Monoxide (CO), Carbon Dioxide (CO2) and flue gas temperatures are present.

Some analysers offer additional information such as boiler efficiency, data logging, differential pressure and other gas measurements. They also offer the option to either print or store the measurements for later review. This is a useful option and gives us an opportunity to leave a copy of the analysis with our customers.


2) Ambient air monitoring

A Flue gas analyser also measures carbon dioxide and carbon monoxide levels in room air. This is invaluable, for example, when trying to find out whether there is CO leaking from an appliance or pinpointing why a CO alarm is being triggered.

Appliances such as boilers, heaters, fires and cookers are like vehicle engines, mixing air and fuel and creating “products of combustion”. If these products, such as Carbon Monoxide “the Silent Killer”, are not safely removed they can kill or injure people. Using a flue gas analyser lets you check this is happening both safely and efficiently. Previous blogs have already highlighted the deadly nature of carbon monoxide!

Domestic Boilers and Hot Water Heaters Flue gas analysers usually display a CO/CO2 ratio. This shows the ratio of carbon monoxide and carbon dioxide in the exit flue (chimney) of the appliance.

Fires and Cookers

Products of combustion from these appliances are generally measured to establish the amount of CO within, this is measure in ppm (parts per million). There are strict levels on appliances that these figures should fall within without further investigation being required

Ambient Air Monitoring

Our analyser will monitor ambient levels of CO in any room. This is carried out by first switching the analyser on in fresh, outdoor air to set its’ zero value, following the manufacturer’s instructions. We then take the analyser into the room to be monitored.


Don’t forget that CO levels in rooms can occur from being adjacent to a busy road, car park or garage, as well as from smokers, outdoor barbeques, adjacent properties sharing the same flue systems, etc. This is also taken into consideration when determining whether there is a problem and we always consult the regulations that apply when performing appliance installation and service.

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