Condensing boilers: how they work

In short: A condensing boiler is a gas or oil heating system that also uses the heat contained in the water vapour of the flue gases. As a result it extracts more heat from the same amount of fuel than older boilers.

In more detail: Burning gas or oil produces water vapour. If the flue gas cools down inside the boiler far enough for the vapour to condense, the heat released in the process is passed on to the heating water.

In detail

• Net and gross calorific value: The net calorific value describes the heat without condensation, the gross calorific value the heat including the heat of condensation. Efficiencies above 100 % in older figures only arise because they were based on the smaller net value.
• A cool return is crucial: Condensation only works if the heating water flowing back is cool enough; for natural gas the dew point of the flue gases is roughly around 55 °C. Low flow temperatures and large heating surfaces help.
• Condensate: The water produced is slightly acidic and is drained into the sewer via a connection. Depending on the system and fuel, a neutralisation unit is required.
• Flue: Because the flue gases are relatively cool and moist, the boiler needs a moisture-resistant flue. In old chimneys a liner is often installed.
• Modulating operation: Modern units adjust their output to demand. Frequent switching on and off, known as short cycling, suggests a system that is too large or poorly adjusted.
• Correct settings: The heating curve, pump and hydraulic balancing determine how often the boiler actually operates in condensing mode.
• Maintenance: Regular servicing by a heating engineer keeps combustion, the heat exchanger and the condensate drain in good order.
• Legal framework: The requirements for newly installed heating systems have changed in recent years, especially regarding the share of renewable energy. Before buying a new system, check the rules currently in force.
• Context: Condensing boilers benefit greatly from hydraulic balancing; depending on the building, a heat pump, for example, may be an alternative (hydraulischer-abgleich-der-heizung, waermepumpe, brennwert-und-zustandszahl-bei-gas).