What a flue gas analyser measures, what it cannot check, and the conditions that make a combustion reading worth recording.
What the analyser is for
FGA is the usual shorthand for a flue gas analyser. It samples the products of combustion and reports values such as oxygen (O2), carbon monoxide (CO) and carbon dioxide (CO2), and from CO and CO2 it gives the CO/CO2 ratio. At the specified test conditions, these figures help judge combustion against the appliance limits and can reveal excess CO that a flame picture may miss. A good sample does not mean every bit of fuel burnt completely, or that the appliance and flue are safe.
The analyser does not replace the other checks. Gas rate comes from a separate consumption check. Pressures and tightness require the appropriate pressure connections and procedure, using a manometer or an analyser equipped for that separate function; they do not come from the flue-gas sample. Gas readings do not tell you which part has failed either.
Conditions for a valid reading
- Warm the appliance through. Combustion changes during start-up, so early figures are not representative.
- Run it at the firing rate the manufacturer specifies for the test. Combustion readings are normally taken at maximum (full) rate.
- Follow the maker’s required firing rates, including low fire on many modulating boilers. Let combustion settle at each rate before sampling. Low-fire results alone cannot describe high-fire operation, just as a high-fire result cannot replace a required minimum-rate check.
- Use the sampling position and casing condition specified for that appliance.
- Wait for steady readings before recording them.
Judging the result
Compare the readings with the manufacturer's published commissioning data and the appliance's maximum CO/CO2 ratio. Acceptable figures differ between models and burner types. Check the ratio and the CO concentration separately: passing one does not cancel failure of the other.
Some evidence outranks a good-looking number:
- Visible sooting or lazy flames still need investigating when the reading is in range, because the reading, the instrument or the test conditions could be at fault.
- A clean blue flame does not prove safe combustion: the eye cannot measure CO.
- Readings that swing wildly or look implausible must not be used. Stop and check the instrument and sampling first.
- If an occupant feels unwell when the appliance runs, stop testing, get everyone into fresh air and obtain medical and emergency advice. A later competent investigation may include room-air measurements once it is safe; a flue sample tells you nothing about the CO people breathed in the room.
Keep a baseline
Record the actual readings. Commissioning readings form a known-good baseline that later services can compare against to spot gradual deterioration. A noticeable increase in ratio warrants investigation. First check that firing rate, sample position and instrument condition are comparable; a confirmed rise under matching conditions can indicate deteriorating combustion.
Exam cues
- In a valid sample, a lower ratio suggests cleaner combustion and a higher one suggests poorer combustion. Check the CO figure and every other required test before judging the appliance.
- Choosing a different firing rate to get a convenient result does not complete the test.