Gas condensing technology – function, benefits and potential savings

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A boiler requires heating energy to heat a room. The more heating energy it consumes, the higher the subsequent heating costs. Moreover, burning fossil fuels such as natural gas or oil as the energy source also has an impact on the environment. This is precisely where condensing technology makes a difference: it increases energy efficiency and reduces CO₂ emissions to a minimum.

Many reasons to use gas condensing technology

In Germany alone, there are still around two million heating systems in use today that are more than 25  years old. Operators are often completely unaware of how much money they are wasting unnecessarily on energy which is pointlessly burned and goes straight up and out of the chimney as unused heat. Furthermore, these old systems have an impact on our climate through unnecessarily high CO₂ emissions and thus contribute more to global warming.

By promptly replacing these systems with highly efficient gas condensing boilers combined with solar technology, you can shave your energy consumption by up to 35  percent. This would translate into ten  percent of Germany's total energy needs, with annual CO₂ emissions being reduced by 54  million tonnes at the same time.

Large selection of gas condensing boilers

Changing from an old and inefficient boiler to an energy saving gas condensing boiler is very quick, whether you are opting for a wall mounted and extremely compact gas boiler or a floorstanding unit with integrated solar cylinder. Owners of detached homes, two-family houses and small apartment buildings in particular will have no problem finding the right Viessmann boiler that meets all their needs in terms of output (up to 80 kW) and price. And you'll be able to count on the technology in the future too. This is because, in addition to natural gas, liquid gas and a biogas mixture, the gas condensing boilers are also suitable for use with gas blends containing up to 20 percent hydrogen. 

Gas condensing boiler product range

How gas condensing technology works

In contrast to constant temperature and low temperature boilers, the way gas condensing technology works cannot be simply divided into combustion, gasification and release of heat. Between the last two steps, the flue gases are also condensed.

When a fuel, in this case gas, is burned, flue gases are produced in addition to the measurable heating energy. These consist largely of hot water vapour, the components of which can be acidic. To protect the boiler itself and the flue system from this acidic water vapour, older boilers discharge the flue gases directly into the open air. Depending on the heating technology, the flue gas temperature can be up to 200  degrees Celsius. Condensation of the water vapour in the flue gases is thus intentionally prevented.

The fact that hot water vapour still contains energy can be proven with a simple trick: if you hold your hand over a pan of boiling water, you will soon notice this energy on your palm. This is because the steam condenses on the surface of the hand and at that moment releases the latent heat, also called the heat of condensation. Gas condensing technology involves recovering this heat of condensation and adding it to the heating water. For this, the hot flue gases are passed through a heat exchanger before they are released into the open air.

The heat exchanger is made of moisture and acid resistant stainless steel and is very compact due to its special design. As the hot flue gases flow through its coil, they are cooled down by the heating water that is flowing back to the boiler from the radiators (return water). The flue gases gradually condense. The heat from the resulting water vapour is then transferred to the heating system. To ensure that condensation occurs, the temperature of the return water must not exceed the limit of 57  degrees Celsius. Otherwise, the hot flue gases cannot condense. Area heating systems such as underfloor heating are suitable for cooling this return water.

The heat exchanger is therefore an important component of a gas condensing boiler and indispensable to this technology. It is able to transfer the heat of condensation obtained to the heating system with virtually no losses. In this way, it converts up to 98  percent of the energy contained in the hot gas into heat. An indestructible component for which system owners have a guarantee of up to ten years.


The deliberate condensation of the water vapour contained in the flue gases produces condensate. This can normally be discharged into the waste water (not small sewage treatment plants), provided the pH value is not too high. In addition, the drainage system must be acid-resistant. Code of Practice "DWA-A 251" of the German Association for Water, Wastewater and Waste (DWA) is used as the basis for the calculation. The size of the gas condensing boiler is also an important factor. If the rated heating output is less than 25  kilowatts (kW), the condensate does not need to be neutralised. If it is between 25  kW and 200  kW, direct discharge into waste water is only permitted in some circumstances by diluting with waste water. For gas condensing heating systems with an output of more than 200  kW, neutralisation is generally mandatory.

In addition to condensate drainage, the flue system must also be designed for condensing technology. Here it's important that the amount of acid contained in the condensate does not damage the system. The positive pressure generated by the use of a fan (e.g. a burner fan) must also not affect the new pipe system. Flue pipes made of stainless steel or special plastic are tried and tested. If a chimney is already in place, the new pipe can usually be inserted into the existing chimney. Routing to the outside without using the existing chimney is also possible. However, it is essential that the advice of a flue gas inspector is sought in this case.

You can also find more information on how gas condensing technology works in the Guide to gas heating at


How to save with gas condensing technology

Thanks to the additional heat from the condensed flue gases, condensing boilers achieve a standard seasonal efficiency [to DIN] of up to 98  percent and so are particularly energy efficient. This function not only saves valuable energy, but also protects the environment through significantly lower CO₂ emissions. The following table shows exactly what sort of savings can be achieved:

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  %

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

Central heating backup with solar energy

The gas heating system is one of the most commonly installed heating systems in this country. It is particularly noted for its compact design and high energy efficiency. Its efficiency and environmental credentials can be increased even further if system owners opt for a hybrid solution of gas condensing technology and solar thermal. The free energy delivered by the sun is still not used enough for central heating backup and DHW heating. even though it is the cleanest heat source of all. And an infinite supply of it is available free of charge. When modernising, you should consider combining your new heating system or boiler with a solar thermal system.

Viessmann gas condensing boilers can be operated in mono or dual mode. For dual mode operation with a solar thermal system, a suitable cylinder is essential. The Vitocell range from Viessmann is matched perfectly to the solar thermal system. Whether it be DHW cylinders for DHW heating, combi cylinders or heating water buffer cylinders, we have the perfect cylinder solution to suit every need.

Tip: The free energy delivered by the sun is still not used enough for central heating backup and DHW heating. When modernising, you should consider combining your new heating system or boiler with a solar thermal system.

Straightforward renting solutions for advanced gas condensing technology

Did you know that you can simply rent your new gas condensing boiler? This alternative option to purchasing offers you a flexible choice of different heat packages. But no matter which one you choose, you'll benefit from

  • €0 purchasing costs
  • an all-inclusive guarantee for up to 15 years

To start savouring the warmth, select a contract model now at Viessmann Wärme.