The French order of 15 September 2009 requires, as part of the annual service of boilers from 4 to 400 kW, an assessment of the boiler's efficiency, an assessment of whether it is correctly sized, and an assessment of its air pollutant emissions. Its article 3 sets out how to read the carbon monoxide measured in the ambient air: a normal situation below 10 ppm, a situation deemed abnormal between 10 and 50 ppm, and a serious and immediate danger at 50 ppm and above, with an instruction to keep the boiler shut down. The efficiency the analyser displays is quoted on a net calorific value basis, which the 3CL-DPE 2021 method caps at 111% for natural gas.
What the analysis has to prove, and what the rules expect of it
What the annual service has to cover
The annual service is not a cleaning package. The order of 15 September 2009 sets out its content, and every line below carries your liability if it is skipped.
| Required operation | What that means on site |
|---|---|
| Inspection of the boiler and of the heat distribution and control installations, and cleaning and adjustment where necessary | Checking the appliance but also the circuit and the controls, not just the heat exchanger |
| Assessment of the boiler's efficiency | Analyser measurement, under stabilised conditions, with an instrument whose make and reference are recorded |
| Assessment of whether the boiler is correctly sized | Comparing the installed output against the real demand, and documenting it |
| Assessment of the boiler's air pollutant emissions | Carbon monoxide measurement, read against article 3 |
| Provision of the necessary advice on correct use of the installation | Temperature, ventilation and programming guidance, in writing |
The text covers boilers fired by gaseous, liquid and solid fuels, in the 4 to 400 kW band. It was last amended by the order of 21 November 2022. Assessing correct sizing means setting the installed output against the home's real demand, which Argile recalculates room by room to NF EN 12831-1.
When to run an analysis outside the annual service
- At commissioning, or after replacing the burner or the gas valve block.
- After any work touching the air supply, the gas, the nozzle or the flue.
- On a complaint about smells, soot, spillage or repeated safety lockouts.
- Before and after descaling or power flushing, so you hold a comparable pair of readings.
Carbon monoxide thresholds and the action each one requires
The measurement is taken in the ambient air of the room, with the appliance running at its rated output, after at least three minutes of operation. The three bands below are those of article 3 of the order, and they are not open to negotiation with the occupant.
| Ambient CO concentration | Classification | Action required |
|---|---|---|
| Below 10 ppm | Normal situation | Continue the service |
| From 10 to under 50 ppm | Situation deemed abnormal | Investigate the flue draught and the ventilation of the room |
| 50 ppm and above | Serious and immediate danger | Instruction to the user to keep the boiler shut down |
Preparing the measurement: safety, instrument and conditions
Putting the dwelling into its real operating configuration first
Open the air inlets and check the grilles are not obstructed, then confirm the room is not being put under depression by mechanical extract, a cooker hood or a tumble dryer running. A measurement taken with a window open or the extracts switched off describes a situation that never occurs in use. If you suspect a flue fault, make the site safe before measuring, not afterwards.
Checking the measuring chain before touching the burner
Check the calibration date against the manufacturer's interval, zero the instrument in fresh air, then inspect the probes, seals and hoses. Replace blocked filters and empty the condensate trap. The make and reference of the instrument will have to appear on the certificate, so it is worth confirming it is in order before you start rather than discovering a drift as you write up.
Sampling point, stabilised operation and air supply
Take the sampling point on a straight section, after the appliance and before any dilution. Wait for genuinely stable operation, then record O₂, CO, CO₂, flue gas temperature and draught. On a modulating appliance, measure at several steps: a burner can be faultless at full load and produce CO at the bottom of its range, which is precisely where it spends the season.
Recording and adjusting on site
The quantities to record and the order to take them in
Record ambient CO first, before doing anything else: that is the measurement that decides whether you carry on. Then take O₂, CO and CO₂ in the flue gases, the flue gas temperature, the combustion air temperature and the flue draught. Note the excess air and the calculated efficiency the instrument displays, along with the settings in place. Without the measuring conditions, the values cannot be compared from one visit to the next.
Adjusting combustion without leaving the manufacturer's instructions
Set the air first, then the gas or oil rate, following the manufacturer's instructions strictly: they carry the target values, and they are what counts. Work in small increments, let the appliance settle between adjustments, and recheck modulation at several steps. A sound delta T limits short cycling and helps a condensing boiler stay in its efficient band, which also depends on balancing the emitters.
The usual causes behind an out-of-range reading
Fouling of the heat exchanger or burner: clean before adjusting, or you are compensating one fault with another. Air starvation: obstructed grilles, an over-sealed room, an air supply to redo. Flue: insufficient draught, poor sealing, badly made joints. Condensation on an appliance not designed for it: flue gas temperature too low or return too cold, to be addressed by adjustment or a hydraulic change. Diagnosis comes before the screwdriver.
Reading the efficiency: net and gross calorific value, and flue losses
The basis on which the displayed efficiency is calculated
Almost every analyser displays a combustion efficiency quoted on a net calorific value basis, a reference that excludes the heat of condensation of the water vapour in the flue gases. That is why a figure above 100% appears on a condensing appliance without anything being wrong. The 3CL-DPE 2021 method sets the conversion factor to the gross basis at 1.11 for natural gas, 1.09 for LPG and 1.07 for heating oil, which mechanically caps the net-basis efficiency at 111%, 109% and 107%. The full mechanism is set out in our article on the condensing gas boiler.
What the flue losses actually tell you
Combustion efficiency depends first on flue losses, themselves a function of flue gas temperature and excess air. Cooler flue gases and controlled excess air improve the figure. But the aim is not the lowest possible value: on an appliance not designed to condense, going too low makes it condense inside the flue and destroys the flue system. The adjustment targets a band, not a record.
Spotting a drift rather than reading an isolated value
A single measurement says almost nothing. What speaks is the gap between two visits under the same conditions: CO climbing, excess air drifting, flue gas temperature rising for the same service. Archive the readings year on year and compare against the conventional efficiency of the 3CL method matching the appliance's type and age to place the gap.
Recording the job and keeping the file
The analysis report: what it has to carry
A useful report fits on one page and carries the values measured before and after adjustment, the measuring conditions (output, setpoints, operating mode, settling time), the adjustments made and the recommendations. Add the time-stamped printout from the analyser and two photographs. This format ties directly into the service contract and what it has to contain.
The service certificate: deadline, retention, required statements
Article R. 224-41-8 of the code de l'environnement sets fifteen days after the visit to issue the certificate. The customer keeps it for at least two years and makes it available to inspectors. The order of 15 September 2009 requires the make and reference of the measuring instrument used to be stated on it, and requires that instrument to be suited to the measurement and checked so the value it gives can be relied on.
What you commit to when you sign the record
A signed record commits you to the state of the installation on that date and under the conditions described, and no further. Write the reservations explicitly: ventilation to be redone, flue to be checked, appliance out of range at one modulation step. Then propose consumption monitoring to show what the adjustment actually did over the season, rather than announcing a percentage saving that nothing will ever confirm.
