A recent condensing gas boiler, properly sized, can lose several efficiency points over two seasons if the combustion analysis reveals an uncorrected excess of air. In Brussels, where the gas boiler stock remains dense and often aging, mastering combustion parameters is crucial for both safety and energy bills. Here, we address the technical points that general public guides often overlook.
Combustion Analysis and Excess Air: The Adjustment That Changes Everything
The efficiency of a gas boiler primarily depends on the air/fuel ratio at the burner. An excess of air that is too high dilutes the flue gases and lowers the condensation temperature, which reduces the recovery of latent heat in condensing models. Conversely, a deficit of air leads to incomplete combustion, resulting in carbon monoxide production.
During a PEB maintenance check, the certified technician measures the O₂ and CO content in the flue gases, the temperature of the combustion gases, and calculates the instantaneous efficiency. We regularly observe installations where the burner has never been recalibrated since commissioning, even though the quality of the distributed gas and the gradual fouling of the heat exchanger change the combustion conditions over time.
A point often neglected in articles about maintenance and safety of gas boilers concerns the adjustment of excess air on modulating burners. These burners adjust their power to demand, but the air/gas ratio can drift at partial loads if the modulation curve is not checked at several levels.
We recommend asking the technician for measurements at both minimum and maximum loads, not just at nominal load. This is the only way to validate that combustion remains clean across the entire modulation range.
Digital PEB Certificate and Traceability of Non-Conformities in Brussels
Since the decree of the Government of the Brussels-Capital Region dated June 21, 2018, each PEB maintenance of a gas boiler results in a standardized certificate recorded in a regional database. This system is not limited to simple administrative archiving.
The centralized database allows Brussels Environment to cross-reference performance data of installations with reports of non-conformity. Specifically, if a technician notes an abnormally low efficiency or a combustion anomaly, this information is tracked and can trigger follow-up actions.
What the PEB Certificate Actually Contains
The certificate is not just a stamp of approval. It records the measured values (O₂, CO, flue gas temperature, calculated efficiency), the condition of safety devices, and any recommendations or formal notices. A property owner who receives a certificate with a non-conformity mention has a deadline to rectify the situation, failing which the issue is reported.
Keeping a history of PEB certificates allows for documenting the gradual drift of an installation and justifying a replacement to a property manager or landlord. This point is particularly relevant in Brussels condominiums where investment decisions are slow.
CO Prevention: Combining PEB Maintenance, Ventilation, and Detection
Brussels Environment now directly links the obligation for PEB maintenance every two years to the regional strategy for combating carbon monoxide. The 2024-2025 campaigns emphasize a combined approach that goes beyond just the technician’s visit.
The official recommendation includes four measures:
- PEB maintenance by a certified heating technician, including combustion and safety device checks
- Maintenance of the chimney or flue gas evacuation duct, often forgotten in apartment buildings
- Permanent ventilation of the room where the boiler is located, ensuring that the ventilation grilles are not obstructed
- Installation of a CO detector in the dwelling, placed near the combustion appliance
The risk of CO poisoning is higher in older Brussels homes where the atmospheric boiler (type B) draws combustion air from the room. In these devices, insufficient draft in the flue can cause burnt gases to backflow into the room. Checking the draft with a manometer is part of PEB maintenance but does not replace properly sized ventilation.
Sealed Boilers and False Sense of Security
Sealed combustion circuit boilers (type C) eliminate the risk of backflow into the room. However, this does not mean that the CO risk disappears entirely. A damaged concentric terminal, a degraded combustion chamber seal, or a crack in the heat exchanger can cause an internal leak.
We observe that some occupants deactivate CO detectors after switching to a sealed boiler, believing the risk to be nonexistent. This is a mistake.
Commissioning Check: The Critical Month After Installation
The PEB regulations in Brussels require a mandatory unique check no later than one month after the delivery of a new gas boiler, whether it is a first installation or a replacement. This commissioning check, performed by a certified PEB heating technician distinct from the installer, results in a specific certificate.
This check verifies that the sizing, hydraulic connection, flue gas evacuation, and combustion adjustments meet the manufacturer’s specifications and PEB requirements. A discrepancy noted at this stage is much easier to correct than after several months of operation, when fouling masks the initial parameters.
Five-Year Diagnostics for High-Power Installations
Heating systems exceeding a certain power threshold, or consisting of multiple generators in cascade, are subject to an in-depth PEB diagnostic every five years. This diagnostic assesses the overall efficiency of heat production, regulation, distribution, and the adequacy between installed power and the actual needs of the building.
In office buildings or condominiums with a collective boiler room, this diagnostic frequently reveals oversizing. A boiler that operates continuously at low load loses efficiency and accelerates wear on its components, particularly the heat exchanger and modulating valves.
The Brussels regulatory framework, with its digital certificates and various levels of control, forms a traceability system that few European regions can match. For a property owner or building manager, leveraging this data rather than merely enduring it remains the best lever for anticipating replacements and maintaining combustion efficiency in accordance with PEB requirements.