Condensing technology: Where and how is it best used?

When it comes to getting the most out of a fuel and converting it into heat, condensing technology is a must. The term refers to the optimal use of the gross calorific value. This is contrasted with the term net calorific value, which merely describes the heat that can be measured when a fuel is burnt.

The term condensing technology explained

Right up until the 1970s, constant-temperature boilers were still regarded state of the art. They were later replaced by low temperature boilers, the latter still being used in many households today. Both types of boiler have one thing in common: They only use the heat that is produced as a measurable temperature (net calorific value) during the combustion of a fuel (natural gas or fuel oil). In contrast, the flue gases, which can reach temperatures of up to 200 degrees Celsius, are discharged from the chimney in a targeted manner so that condensation does not form within the heating system and the flue pipes. The flue gases contain valuable heat that is then lost.

The special feature of condensing technology is that it makes additional use of this condensation heat (gross calorific value) contained in the flue gas, thereby significantly increasing the standard seasonal efficiency [to DIN] of the boiler. The section on how gas condensing technology works explains in detail how the gross calorific value is obtained, using natural gas as the fuel.

Condensing technology for modernisation

Anyone who modernises their house or heating system today usually relies on condensing technology. This is a logical step, as there are many boilers in existing buildings that use outdated technology and operate inefficiently.

By replacing an outdated boiler with a new one, you will keep heating costs at a low level from day one. They also reduce the burden on the environment, as only as much fuel as necessary needs to be burned. Last but not least, a new condensing boiler makes a significant contribution to increasing the value of the property.

Savings through modernisation with a Vitodens 300-W gas condensing boiler*

Heating systemOld systemNew systemSavings
Consumption/year3400  m³2500  m³900  m³   /   1.8  t  CO₂
Costs/year€2510€1850€660  /   26  %

Savings through modernisation with a Vitoladens 300-C oil condensing boiler*

Heating systemOld systemNew systemSavings
Consumption/year3400  l2590  l810  l   /   2.1  t  CO₂
Costs/year€2380€1760€620   /   26  %

* Basis for comparison: House (year of construction 1985), 140  m² living space with old 27  kW oil or gas boiler. Rounded consumption costs applying standard values (EID) for 3400  l fuel oil or 3400  m³ natural gas. Average 2017 energy price

Photo: © Dariusz Jarzabek /

Condensing technology in new buildings

In addition to modernisation projects, condensing technology is of course also used in new buildings. Here, it not only provides long term and cost effective heat. Using of condensing technology enables homeowners to also meet the EU-wide, high requirements for new buildings, which would not be possible with conventional heating technology. Natural gas is usually the fuel of choice. Gas is still the most widely used raw material and is notable for its high energy content.

Condensing technology is considered to be proven, durable and cost effective. In addition, most Viessmann condensing boilers are designed for use with a solar system. This combination not only provides system owners with extremely economical heating, but also conserves resources and is clean. And finally, the state supports this duel mode operation with attractive subsidy programmes.

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