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How oil condensing technology works briefly explained

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As a reliable and energy-rich fuel, crude oil has been used for heating for a long time. However, it is a fossil fuel and is therefore only available in finite quantities. In addition, the price of oil is sometimes subject to strong fluctuations and can consequently have a negative impact on heating costs. In order to keep costs as low as possible, Viessmann relies on proven and futureproof oil condensing technology.

Viessmann oil condensing boilers reflect the latest engineering standards. With a wide range of different outputs up to 80 kW, they are also ideally prepared for use in detached and two-family houses, as well as small apartment buildings. You can find out more about the features and benefits in the section on Oil heating.

Energy consumption reduced by up to 35  percent

In Germany alone, there are still around two million heating systems in use today that are more than 25  years old. Their owners are often completely unaware of how much money they are wasting unnecessarily on energy, which is pointlessly burned up and exits the chimney as unused heat. In addition, these old systems pollute the climate through unnecessary CO₂ emissions and contribute to global warming. By promptly replacing these systems with highly efficient oil condensing systems combined with solar technology, end users could reduce their 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.

How does an oil condensing boiler work?

A boiler generates heat by burning fuel and using the resulting hot flue gases to heat the heating water. With conventional boilers, this flue gas heat is discharged directly into the open air. This is necessary because the water vapour contained in the flue gases would otherwise condense as soon as the flue gases had cooled below their dew point temperature. However, components of this condensate are corrosive, which is why the hot flue gases were considered unusable for a long time.

Yet there is still enough heat in the hot flue gases –– to be more precise, in the hot water vapour –– to be used for heating. You can feel this heat in a simple way: if you hold your hand over a pot of boiling water, sooner or later you will have to withdraw it. This is because the hot steam rises from the boiling water and condenses on the palm of your hand. As it condenses, the steam then releases its latent heat, also called the heat of condensation.

In order to increase the energy efficiency of an oil heating system and to develop combustion technology that is more resource-efficient in the long term, it is important to recover this heat in a controlled manner and feed it back to the heating system without the condensate damaging the system or the flue. The operating principle of oil condensing technology therefore also includes deliberate condensation of the hot flue gases.

In practice, the hot flue gases are passed through a heat exchanger and condensed with the help of the cool return water before being discharged into the open air. Due to the risk of corrosion from some components in the condensate, the heat exchanger itself must be resistant to acid and moisture. Both properties apply to stainless steel, which is why condensing heat exchangers are made of this material.

What also distinguishes them is their very space saving design and their robustness. System owners receive a guarantee for up to ten years on stainless steel heat exchangers from Viessmann.

Simply put, the stainless steel heat exchanger is the connection piece between the combustion chamber and the radiators. The heating water flowing through the heat exchanger is heated by the flue gas heat and transported to the radiators with the help of a pump. Once in the radiators, it heats them up and gradually cools down. It then flows back to the heat exchanger as "cooled" return water and the process starts all over again.

Since oil condensing technology works by condensing the flue gases, it is essential that the return water is as cool as possible when it flows back. For oil condensing boilers, the dew point temperature is around 49  degrees Celsius. If the return temperature is above this, the condensing technology fails to work. If it is below this, the flue gases give off the latent heat to the return water and the oil condensing heating system has to work less hard because the heating water is already preheated. A particularly efficient way to cool down the return water is to use area heating systems such as underfloor or wall heating.

Important information for the use of oil condensing technology

The deliberate condensation of the water vapour contained in the flue gases produces condensate. Although this has acidic components, it can normally still be discharged into the waste water system (not small sewerage treatment plants) – provided that the drainage system itself is acid-resistant. The pH value is important here. Water has a pH value of about seven. This value is considered neutral. Everything below this is acidic. The pH value of the condensate in crude oil is 1.5 to 3.5. A basis for calculation is provided in Code of Practice "DWA-A 251" of the German Association for Water, Wastewater and Waste (DWA). It is also advisable to enquire with the responsible municipal waste water association.

Other important factors for discharge into the normal waste water are the size of the oil condensing boiler and the type of fuel oil. As a rule, systems that use oil condensing technology and have a rated heating output of less than 200 kW are exempt from the obligation to neutralise. In some cases, the condensate must be diluted before discharge. Those who still use standard fuel oil must always neutralise the condensate before discharging it.

The chimney must also be suitable for oil condensing technology

Not only must the drainage system be designed for operating a condensing boiler, but it is also important to ensure that the flue system cannot be damaged by condensate. In addition, it must withstand a certain overpressure without leaking. This is necessary because during operation it is essential to use a fan, which actively blows out the cooled flue gases. Flue pipes made of stainless steel or special plastic are tried and tested.

Combine oil condensing technology and a solar thermal system

Viessmann solar thermal collectors

A further point in favour of oil condensing technology is its variety of combinations. Combination with solar technology makes sense, both economically and ecologically. The free energy delivered by the sun is still not used enough for central heating backup and DHW heating. When modernising, homeowners should consider combining their new heating system or boiler with a solar thermal system.

With the help of solar technology, solar energy can be used for central heating backup or DHW heating. This is extremely economical, as it means that the oil condensing boiler does not have to work at all on some days. Since less oil is burned, system owners also reduce the burden on the environment and thus actively contribute to climate protection. Last but not least, this combination makes them a little less dependent on their fuel supplier.

Contrary to the assumption of many consumers, a hybrid heating system consisting of oil condensing technology and solar thermal can be used not only in older buildings, but also in new ones. Provided that a few factors are taken into account, the requirements of the German Buildings Energy Act (GEG) can be met.

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