How oil condensing technology works

A boiler generates heat by burning a fuel and using the resulting hot exhaust gases to heat the heating water. The heat generated in the flue gas is dissipated directly into the atmosphere in conventional boilers. This is necessary because the water vapour contained in the exhaust gases would otherwise condense as soon as the exhaust gases have cooled below their dew point temperature. However, components of this condensate promote corrosion, which is why the hot exhaust gases were considered unusable for a long time.


Exhaust gases contain useful condensation heat

The hot exhaust gases - to be more precise, the hot steam - still contains heat that the boiler can use. You can feel this warmth in a simple way: if you hold your hand over a pot of boiling water, you will eventually have to pull it back. The hot steam rises from the boiling water and condenses on the palm of the hand. During condensation, it then releases the hidden heat, also known as condensation heat.

In order to increase the energy efficiency of a condensing oil boiler and to develop combustion technology that conserves resources in the long term, this heat must be obtained in a controlled manner and added to the heating system without the condensate damaging the system or the flue gas system. Oil condensing technology includes the planned condensation of the hot rejected heat.

The heat exchanger is an essential part of the oil condensing technology, as shown in the illustration.

Heat exchangers as an essential component of an oil condensing boiler

In practice, the hot exhaust gases are passed through a heat exchanger and condensed by means of the cool return water before they reach the outside. Due to the risk of corrosion from some components in the condensate, the heat exchanger itself must be insensitive to acid and moisture. Both properties apply to stainless steel, which is why condensing heat exchangers are manufactured using it.

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

Heat exchanger and return temperature

In simple terms, the stainless steel heat exchanger is the connecting piece between the combustion chamber and the radiators. The heating water flowing through the heat exchanger is heated by the exhaust heat and transported to the radiators by means of a pump. Once inside the radiators, it heats the radiators and gradually cools them down. It then flows back to the heat exchanger as "cooled" return water and the process starts again.

Since the function of the oil combustion technology lies in the condensation of the exhaust gases, it is essential that the return water flows back as cool as possible. In oil condensing boilers, the dew point temperature is around 49 degrees Celsius. If the return temperature is higher, the condensing technology fails to work. If it is lower, the exhaust gases release the hidden heat to the return water and the condensing oil heating system has to perform less, since the heating water is already preheated. Surface heating systems such as underfloor or wall heating are a particularly efficient way of cooling down the return water.


Condensate drainage as part of the functionality of oil condensing technology

Condensate is created by the condensing of the water vapour contained in the exhaust gases. Although this has acidic components, it can normally be discharged into the sewage system (not small sewage treatment plants) - provided that the drainage system itself is acid-resistant. The pH value is also important here. Water is assigned a pH of about seven. This value is considered neutral. Anything below that is called acidic. The pH value of the condensate for natural gas is 3.5 to 5.5, for crude oil 1.5 to 3.5. 


The flue must also match the oil condensing technology

Not only the discharge system must be designed for the operation of a condensing boiler. The flue gas system must not be damaged by condensate either. In addition, it must be able to withstand a certain amount of excess pressure without showing any leakage. This is necessary because during operation it is essential to use a fan that actively blows out the already cooled exhaust gases. Exhaust pipes made of stainless steel or special plastic have proven themselves in practice.