Free tool

Flue gas ratio calculator

The flue gas ratio calculator is a free tool that checks a CO/CO₂ ratio reading against the 0.0040 action level for correct combustion. Enter your analyser’s CO and CO₂ readings to get the combustion ratio, see how far it sits from the action level, and check it against the chart below.

CO (ppm)

CO₂ (%)

Enter the measured carbon monoxide (ppm) and carbon dioxide (%) from your flue gas analyser. The ratio is worked out in your browser, so your readings are never sent or saved.

A screening figure for qualified engineers, not a pass/fail verdict. Always work to the appliance manufacturer’s commissioning data and the current Gas Safe / BS 7967 procedure. Gas work must be carried out by an appropriately Gas Safe registered engineer.

CO/CO₂ ratio chart

Find your CO₂ reading along the top and your CO reading down the side. The cell where they meet is the combustion ratio, rounded to four decimal places. Cells marked “over” are above the 0.0040 action level and call for investigation rather than a signature.

CO/CO₂ combustion ratio for common analyser readings. Cells marked “over” exceed the 0.0040 action level in Gas Safe TB 143.

CO (ppm)	CO₂ 7%	CO₂ 8%	CO₂ 9%	CO₂ 10%	CO₂ 11%
10	0.0001	0.0001	0.0001	0.0001	0.0001
20	0.0003	0.0003	0.0002	0.0002	0.0002
40	0.0006	0.0005	0.0004	0.0004	0.0004
60	0.0009	0.0008	0.0007	0.0006	0.0005
80	0.0011	0.0010	0.0009	0.0008	0.0007
100	0.0014	0.0013	0.0011	0.0010	0.0009
150	0.0021	0.0019	0.0017	0.0015	0.0014
200	0.0029	0.0025	0.0022	0.0020	0.0018
300	0.0043 (over)	0.0037	0.0033	0.0030	0.0027
400	0.0057 (over)	0.0050 (over)	0.0044 (over)	0.0040	0.0036

Treat the chart as a sanity check on the arithmetic. A ratio inside the limit on an appliance that is spilling, or on one whose manufacturer sets a tighter figure, is still a problem.

How do you work the ratio out by hand?

Divide CO by CO₂ once both are in the same units. Take a reading of 90 ppm CO at 8.5% CO₂, which is a normal enough set of figures on a correctly set condensing boiler:

Put both on the same basis. 90 ppm of CO is 90 parts per million. 8.5% CO₂ is 8.5 parts per hundred, which is 85,000 parts per million.
Divide one by the other: 90 ÷ 85,000 = 0.0010588.
Round to four decimal places: 0.0011.
Against the 0.0040 action level that is 26%, so there is plenty of room before the limit.

The shortcut in the units your analyser actually shows is CO ÷ (10,000 × CO₂), because multiplying the CO₂ percentage by 10,000 turns it into ppm in one step. That is the sum the calculator and the chart both run.

What does the ratio tell you?

How completely the appliance is burning its fuel. Carbon monoxide divided by carbon dioxide is a sensitive indicator, because as a burner drifts out of adjustment or a heat exchanger fouls, CO climbs relative to CO₂ and the ratio rises well before anything is obvious in the room.

Gas Safe Technical Bulletin 143, written with the Heating and Hotwater Industry Council, sets the action level at 0.0040 alongside a CO level at or below 350 ppm. A ratio at or below 0.0040 is within the accepted limit for a correctly operating appliance; above it, investigate against the manufacturer’s data and the current standard. TB 143 is clear that the boiler manufacturer’s guidance must be followed where it exists, and that makers may set tighter action levels of their own.

TB 143’s procedure is written for commissioning a newly installed condensing boiler with air/gas ratio control valve technology. It does not cover service and maintenance work that involves adjusting that valve, which TB 126 covers. Gas Safe TB 157 later confirmed that natural gas containing up to 20 vol% hydrogen needs no change to TB 143.

For a fuller refresher, read flue gas analysis explained and what flue gas analyser readings mean.

What do typical readings look like?

On a correctly set natural gas condensing boiler, CO₂ sits in the high single figures and CO stays low and steady. The exact numbers belong to the appliance, not to the fuel: TB 143 itself publishes no CO₂ band, only the CO and ratio action levels, so the commissioning table in the manual for that model is the reference.

For example, Worcester Bosch gives a CO₂ reading of between 9.0 and 9.8% at maximum rated output for the natural gas Greenstar 4000, and between 8.2 and 9.0% at minimum rate (and at least 0.2 lower than the maximum reading taken), with a CO reading below 250 ppm at both, measured 10 minutes after firing. On LPG the same table asks for 10.4 to 11.0% at maximum and 10.0 to 10.4% at minimum. A different model will have different figures, so check the manual.

Analysers report O₂ next to CO₂ and the two move in opposite directions on the same sample: more excess air means more oxygen and less carbon dioxide. Most UK manuals set the target in CO₂ rather than O₂, so for many boilers there is no published O₂ band to work to. The O₂ figure the manuals do print is the flue integrity check, taken at the air inlet sample point with the case on and the boiler at maximum: Worcester asks for O₂ at or above 20.6% and CO₂ below 0.2% there, and the same two figures appear in the Vaillant ecoTEC plus and Ideal Logic manuals. Anything else there means combustion products and inlet air are mixing, and the flue needs sorting before the combustion readings mean anything at all.

CO should be low and it should be steady. A figure that climbs while you watch, or swings about, is telling you something is changing in front of you, whether that is a seal, the condensate route or the heat exchanger, and it wants finding before the number goes on paper. Give the appliance time to stabilise before the probe goes in, then let the reading settle before you write it down.

What should you do if the ratio is above 0.0040?

Work through the flue and the seals, and if the reading does not come back inside the limits, turn the appliance off and ring the manufacturer’s technical helpline before you touch anything else. TB 143 and the manufacturers’ own versions of its flowchart set out the same order:

Take and record both readings first. Check CO and the combustion ratio at maximum rate and at minimum rate before you contact anyone, so you have the full picture to hand.
Check flue integrity at the air inlet sample point. Readings outside the limits there mean products of combustion and inlet air are mixing.
Check that the flue components are assembled, fixed and supported the way the boiler and flue manufacturer’s instructions require, and that neither the flue nor the terminal is obstructed.
Check the seals around the burner, the internal flue seals, and the door and case seals. Rectify what needs rectifying and re-test.
Run down the manufacturer’s own list. Worcester, for one, sends you to the gas inlet pressure, the gas rate, the fan test pressure, and the flue and air intake including any blockage in the condensate disposal when CO₂ is out of tolerance.
Leave the air/gas ratio valve alone. TB 143 says no adjustment should be attempted without first ringing the manufacturer’s technical helpline for advice, and the manuals repeat it: the valve is factory set.
If it still fails, turn the appliance off and call the helpline. The appliance must not be commissioned until the problem is found and resolved, and if commissioning cannot be completed it must be disconnected from the gas supply in line with the Gas Safety (Installation and Use) Regulations.
Once the readings are good, record the CO level and the combustion ratio on the Benchmark commissioning checklist. HHIC manufacturers treat completing it as a condition of warranty, and Gas Safe will defect an installation where the figures are not recorded.

None of that is a substitute for the manufacturer’s instructions for the appliance in front of you, and the whole procedure assumes a calibrated analyser of the type BS 7967 specifies, in the hands of someone assessed to use it.

Questions

Is there a CO/CO₂ ratio chart?

Yes, on this page. It gives the ratio for CO readings from 10 to 400 ppm against CO₂ from 7 to 11%, worked out with the same sum the calculator uses, and every cell above 0.0040 is marked as over. Read down the column for your CO₂ reading and across from your CO reading.

How is the combustion ratio calculated?

It's the measured carbon monoxide divided by the measured carbon dioxide, on the same basis. With CO in ppm and CO₂ as a percentage, the ratio is CO ÷ (10000 × CO₂). This tool does that for you.

What is an acceptable CO/CO₂ ratio?

Gas Safe Technical Bulletin 143 sets the action level at 0.0040. At or below that, combustion is within the accepted limit for a correctly operating appliance; above it, the appliance needs investigation following the manufacturer's instructions and the standard. TB 143 also says the boiler manufacturer's guidance must be followed where it exists, and makers often set a tighter figure than the generic one.

What is a normal CO reading?

Low, and steady. In the absence of a figure from the manufacturer, TB 143 treats CO at or below 350 ppm as acceptable alongside a ratio at or below 0.0040. Manufacturers often ask for less: Worcester Bosch's Greenstar 4000 instructions give a CO reading below 250 ppm at both maximum and minimum rate. The manual for the appliance in front of you is the reference, not a general figure.

What does BS 7967 cover?

BS 7967:2015 is the British Standard guide for using electronic portable combustion gas analysers to measure carbon monoxide in dwellings and to judge the combustion performance of domestic gas-fired appliances. It covers flueless, open-flue and room-sealed appliances and appliances with a purpose-designed sampling point, and it is the standard your analyser and your method are held to. It is a paid BSI document, so read it there rather than relying on anyone's summary.

Is this a substitute for the standard?

No. It's a quick screening and refresher figure for qualified engineers. Always work to the appliance manufacturer's commissioning data and the current Gas Safe / BS 7967 procedure, and use a calibrated analyser.

Is my data stored?

No. The calculation runs entirely in your browser. The values you enter are never sent to a server or saved.

Sources

Stop copying readings into the van.

Manifold’s FlueScan reads CO, CO₂ and the ratio straight off your analyser into the appliance check for you to confirm. Try it free for 30 days, no card required. In early access for UK gas engineers.

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