Combustion Analysis: Checking Boiler Efficiency

Understanding the value of a combustion analysis for your boiler

What the analysis actually measures: flue gas, excess air, flame stability

On a gas or oil boiler, the analysis reads the flue gas at the outlet. It mainly measures O2, CO2, CO, flue gas temperature and sometimes draught. These values are used to estimate excess air, spot incomplete combustion and check that the flame stays stable, even when the burner modulates.

When to run it: commissioning, servicing, customer complaints, after an adjustment

• At commissioning, or after replacing the burner.
• During the annual service, to validate the correct settings.
• In the event of a customer complaint. Odors, soot, smoke, safety shutdown.
• After any work on air, gas, nozzle or flue.

On-site benefits: efficiency, CO safety, lower consumption

On site, a properly run analysis helps with fine-tuning. You gain real efficiency, limit overconsumption and strengthen CO safety with a measured check. It also leaves a useful record in the service report.

Preparing your analysis: safety, equipment and measurement conditions

Safety check above all: ventilation, airtightness, carbon monoxide risk

Before the analysis, put the home in its real operating configuration. Open the air inlets, check that the grilles are not blocked, and that the room is not being depressurized by an extractor, a range hood or a dryer. If you suspect a flue defect, stop, ventilate, and secure the job with a suitable detector. CO risk — no shortcuts.

Choosing and checking your analyzer: calibration, probes, filters, battery

A poorly maintained analyzer gives false readings. Check the calibration date, run the fresh-air zero, then inspect probes, seals and hoses. Replace clogged filters, drain the condensate trap if fitted, and start with a full battery. Calibration OK before you touch the burner.

Key setup points: flue tap, operating temperature, draught and air intake

Take the tap on a straight section, downstream of the appliance and before any dilution. Wait for a stable firing rate, then record O2, CO, flue gas temperature, and draught. Also check the air intake, especially in a home made more airtight after insulation work. Stable firing rate, otherwise the analysis has to be redone.

Running the combustion analysis on site: steps and adjustments

Recording the useful values: O₂, CO, CO₂, flue gas temperature, draught, dry flue gas

Stabilize the boiler at its nominal firing rate, then start the analysis with a calibrated analyzer. Record O₂, CO, CO₂, flue gas temperature, flue draught and the "dry flue gas" indicator (corrected CO). Also note the excess air and calculated efficiency. Absolute priority to safety. If CO climbs, cut the burner and ventilate.

Adjusting combustion: air/gas, pressure, nozzle, valve, modulation

Adjust the air first, then the gas or oil flow rate. Adjust pressure, nozzle and gas valve following the manufacturer's instructions. Work in small steps, allow 2 to 3 minutes of stability between two adjustments, and check modulation across several stages to avoid high CO at low firing rate.

Common cases and fixes: fouling, air defect, flue, condensation

Fouling. Clean the heat exchanger and burner before any adjustment. Air defect. Blocked grilles, a room that is too airtight, an air intake to correct. Flue. Insufficient draught, sealing issues, poor joints. Condensation. Flue gas temperature too low or return temperature too cold, to be handled through adjustment, insulation or hydraulic changes. A good diagnosis stops you from "adjusting blind."

Interpreting the results: improving efficiency without degrading combustion

Reading the calculated efficiency: LHV, flue losses, influence of flue gas temperature

The efficiency shown is often calculated on the LHV (lower heating value). It mainly depends on flue losses. The higher the flue gas temperature climbs, the more energy escapes up the flue. The analysis also cross-checks O2 or CO2 to estimate excess air. The goal: cooler flue gas, but a stable draught, without risking condensation on a boiler not designed for it.

Spotting drift: high CO, excess air, instability, under-combustion

High CO, or CO that swings sharply, points to incomplete combustion, fouling or a poor air/fuel setting. Too much air is safer but less efficient. Too little means under-combustion and instability. On site, also keep an eye on ambient CO and the condition of the flue.

Optimizing overall: burner setting, heat exchanger cleaning, hydraulic balancing

You often gain quickly with a clean burner setting and a cleaned heat exchanger. Next, look at the hydraulics. Consistent flow rates, properly sized pumps, balancing the emitters. A good delta T limits short cycling and helps a condensing boiler stay in its efficient zone.

Traceability and 2026 requirements: reports, tolerances and customer advice

Writing a clear analysis report: before/after values, adjustments, recommendations

A useful report fits on one page. Note the measured values before and after. Specify the conditions of the analysis (temperature, flow rates, setpoints, heating mode). Add the adjustments made, the accepted tolerances, and 3 concrete recommendations. Attach 2 photos and, if possible, a time-stamped record (app or device).

Meeting servicing and safety obligations: certificates, CO prevention (2026)

In 2026, your clients mainly expect proof. Hand over the service certificate where it applies (boilers, heat pumps, air conditioning depending on power). On safety, remind them of the CO basics: clear ventilation, controlled flue evacuation, scheduled chimney sweeping. A CO detector remains a good option to suggest, without being alarmist.

Showcasing your work: proof of efficiency, savings arguments, annual follow-up

Showcase your work with simple proof: estimated efficiency, consumption gap, comfort gained. Translate it into kWh and then into euros using the price assumption you used. Offer consumption monitoring after a retrofit. Even 15 minutes remotely to review the settings and adjust prevents drift and builds loyalty.