Boiler combustion figures by brand: CO₂, CO and ratio at commissioning

Readings are judged against the manufacturer's commissioning data for the appliance in front of you, and the tables below transcribe that data make by make, with the manual's document number and revision under each one. Use them to check a reading when the book is in the loft; where they differ from the current manual, the manual wins.

Why do the figures differ by make?

Because each maker sets its own air-to-gas balance for the appliance it built, and the CO₂ percentage is a direct read-out of that balance. The burner, the fan curve, the gas valve and the heat exchanger all differ, so a CO₂ figure that means correct combustion on one boiler means a lean or a rich mixture on another. Rate moves it as well, which is why every manual gives a maximum-rate figure and most give a separate minimum-rate one. Fuel moves it most of all: propane carries more carbon per unit of gas, so its CO₂ figures sit a percentage point or more above the natural gas figures for the same appliance.

TB 143 sits underneath all of it. Gas Safe Register's bulletin says the boiler manufacturer's guidance must be followed where it is available, and that makers may choose to give their own specific action levels for CO and combustion ratio in their instructions. Where none is given, manufacturers have agreed that CO at or below 350 ppm with a combustion ratio at or below 0.0040 will ensure the boiler can be considered safe. Every table below is one maker's layer sitting on top of that floor.

What are Worcester Bosch's combustion figures?

Worcester publishes a CO₂ window for each rate rather than a single value, and on the Greenstar 4000 it also asks the minimum-rate reading to step down by at least 0.2 from the maximum-rate reading you actually took. Both ranges below are read in chimney sweep mode with the combustion chamber cover on.

What are Vaillant's combustion figures?

Vaillant publishes its set values at full load only, and gives two CO₂ figures for the same appliance: one with the front casing closed and one with it removed. It also publishes its own CO ceiling and its own combustion ratio, both tighter than TB 143's, so on these boilers the maker's numbers are the ones that govern.

What are Baxi's combustion figures?

Baxi gives a nominal CO₂ with a tolerance range either side, an O₂ figure alongside it and a CO ceiling, all measured with the frontal panel closed. The natural gas numbers hold across every output size in these two ranges. The propane numbers do not, and the 36 kW 600 Combi 2 carries its own pair. CO appears twice in each manual: the service settings table allows under 400 ppm, but the commissioning flowchart passes the boiler only at under 350 ppm with a ratio below 0.004, so commission to the lower figure.

What are Ideal's combustion figures?

Ideal puts its CO₂ content in Table 1 of the General Data at the front of the manual, quoted at maximum and at minimum with a ± 0.7 tolerance, and the figure changes with the output size within a range. Both manuals below give Table 1 for natural gas only, at 2H, G20, 20 mbar.

What are Glow-worm's combustion figures?

Glow-worm gives one CO₂ check value per appliance, read in check programme P.01 with the setting value at 100, which is maximum heat input, and the same figure applies whether the front casing is removed or installed. The Essential manual adds its own full-load limits, CO at or below 250 ppm and a combustion ratio at or below 0.0027, both tighter than TB 143's. The Energy7 and Energy 2 manuals set no appliance limits of their own, so TB 143's levels are the ones to work to there. The Benchmark checklist still asks for CO, CO₂ and the ratio at maximum and, where possible, minimum rate.

How do I use these figures on site?

In TB 143's order: maximum rate first, then minimum, judging each settled reading against the manual for that appliance and recording both sets on the Benchmark checklist. Set the boiler to maximum in whatever test mode it has, leave the probe out of the flue while the combustion settles, take the air inlet reading to prove flue integrity, then move to the flue gas point and let the figures stabilise before you write anything down. Drop to minimum and repeat.

The tables are for checking a reading, not for setting one. On every appliance here the air/gas ratio valve is factory set, and TB 143 and the manuals agree on what to do about that during commissioning, which is nothing, unless the maker's technical line has told you otherwise. Where a maker's figure is tighter than TB 143's, as Vaillant's 250 ppm and 0.0031 and the Glow-worm Essential's 250 ppm and 0.0027 are, the maker's figure is the one to work to.

What if my reading is outside the maker's figure?

Work the flowchart in that appliance's instructions rather than reaching for the gas valve. Our guide on analyser readings walks the common patterns and what each one points at, and the flue gas analysis guide carries the wider procedure. What the manuals here have in common is the order they send you in: check the seals around the burner and the internal flue, door and case seals, confirm flue integrity, confirm the inlet pressure and the gas rate, then retest. If the readings are still outside the figures, the appliance goes off and that maker's technical helpline gets the call before it goes back into service.

Once the readings are inside the maker's figures, they still have to survive the trip to the record. Manifold's FlueScan reads CO, CO₂ and the ratio straight off the analyser screen with the phone camera for you to confirm at the appliance, so the numbers that reach the commissioning record are the numbers the analyser showed, and next year's service sits next to this year's when the appliance's history matters.