How a Condensing Gas Boiler Works
A condensing hot water boiler is equipped with a flue gas heat exchange system that lowers exhaust gas temperature and recovers latent heat from condensed water vapor in the flue gas. The higher the water vapor content in the flue gas, the more latent heat is recycled — natural gas is rich in hydrogen, generating substantial water vapor after combustion.
Main component of natural gas: Methane (CH₄)
Combustion reaction: CH₄ + 2O₂ → CO₂ + 2H₂O(g) + 8600 kcal
When 1m³ of natural gas fully combusts, it produces 2m³ of gaseous water vapor and releases 8600 kcal of heat. Total heat released reaches approximately 880 kcal if all water vapor in the flue gas is condensed. The condensate liquid produced during this process is safely discharged via the integrated drainage system, requiring only standard plumbing connections.
Thermal Efficiency: Why It Can Exceed 100%
Condensing boilers recover heat that conventional boilers waste. Two mechanisms drive the efficiency gain:
- Reduced flue gas exhaust temperature cuts sensible heat loss, boosting efficiency by 3%~5%. By lowering the exhaust temperature to as low as 40–50°C, the boiler minimizes sensible heat loss and ensures stable performance even under variable load conditions.
- Latent heat recovery from condensed flue-gas steam adds another 5%~10%.
Total comprehensive efficiency improvement: 8%~15%, greatly reducing energy consumption.
The key: conventional boilers calculate efficiency against Net Calorific Value (NCV = 8600 kcal/Nm³), which excludes latent heat. Condensing boilers recover that extra latent heat (880 kcal/m³), so efficiency is measured against Gross Calorific Value (GCV = 9480 kcal/Nm³).
Maximum efficiency: η = Recovered Total Heat / NCV × 100% = up to 110%
This is not "free energy" — it is simply counting heat that was always there but previously vented to the atmosphere.
Calorific Value Reference: Sensible heat per 1m³ natural gas: 31669 kJ/m³ | Latent heat: 3427 kJ/m³
Operating Conditions & System Design
Condensing boilers achieve maximum efficiency when the return water temperature is below 50°C, allowing flue gas to condense within the heat exchanger. In typical space heating applications, system design should account for this to ensure the boiler operates in condensing mode for the majority of its runtime.
Applications include domestic hot water preparation and space heating. Unlike a heat only boiler which requires a separate hot water cylinder, our combo boiler configuration integrates both functions in a single unit, saving space and installation cost. Both heat only and combo configurations are available depending on project requirements.
Belitto Condensing Boiler Series
TBLN Series (60–2800kW): Compact footprint, suitable for residential and light commercial applications. Equipped with high-efficiency Al-Mg-Si heat exchanger. All units deliver high efficiency condensing boiler performance with SEDBUK seasonal efficiency ratings among the highest in class.
TBYG Series (3500–7000kW): Designed for large-scale commercial and district heating applications. Compatible with cascade control systems for modular capacity scaling. Ultra-low NOx emission burners comply with stringent environmental standards.
