Full condensing boiler
A full condensing boiler is a boiler that enables condensing operation even at system temperatures (temperature of the heating water or hot water return ) that are above the flue gas dew point of the fuel used (heating oil: approx. 45 ° C, natural gas approx. 56 ° C) . This is achieved through further heat exchange between flue gas and supply air (similar to an air-exhaust system , only that the heat exchange does not take place in the chimney pipe, but in the boiler room and the supply air can also come from the boiler room). Thus, which is latent heat of the exhaust gas used, the efficiency of the boiler can thereby against low temperature heating systems by up to 8% for heating oil boilers and 11% for natural gas boilers increased (natural gas has a higher hydrogen content, so it creates more water vapor, thus more heat is latent bound, which can be recovered during condensation).
Cousin furnace
The full condensing boiler and basically the application of condensing technology to small heating systems are an invention of Richard Vetter , which is why the boiler is also known as the Vetter furnace . In the Vetter furnace, the exhaust gas is cooled to 20 to 30 ° C in two stages using two heat exchangers . This eliminates the restriction to low temperature heating.
In the first heat exchanger, the heating water is heated in counterflow to up to 80 ° C, so there are practically no restrictions on the flow and return temperatures. The flue gases are cooled down to 60 to 70 ° C, so there is no condensation of acidic water vapor here, the materials do not have to be made of corrosion-resistant material, which means that manufacturing costs can be kept lower.
In the second heat exchanger, the combustion air for the burner is heated and at the same time the flue gas is cooled even further, with the water vapor contained in the flue gas condensing.
The degree of condensation always depends on the seasonally different coldest heat exchanger medium (cold water, heating return, outside air). In winter - the time when the combustion system has been in operation for the longest - the outside air is colder than the return or fresh water. Then the most condensation can take place and the system functions most effectively.
In heating oil systems, the harmful sulfur dioxide is bound with the help of a special water injection process . A calcareous granulate is used to dispose of the condensate , which binds the sulfuric acids formed ( sulfuric acid and sulfuric acid ).
Comparison with "normal" condensing boilers
In order to achieve the same effect as a full condensing boiler, other condensing boilers are equipped with an air-exhaust system (LAS) in the chimney that takes on the part of air preheating in conjunction with a "room air-independent supply air duct" . The supply air flows in countercurrent to the flue gas and can therefore absorb its heat content. Even then, full condensing operation is often used .
With a room air-independent air supply , the necessary combustion air is not sucked out of the boiler room, but either drawn in through a free chimney shaft, a specially built air supply line or, as with the aforementioned LAS system, through the gap between the pipe and the inside wall of the chimney from the chimney head on the roof.
In order to avoid corrosion of the burner chamber, some condensing boilers have a return increase (to increase the temperature) for the warmest part of the heat exchanger. To do this, heated water is fed back into the circuit and added to the “cold” return flow. This ensures that the heat exchanger does not cool below the dew point there. In principle, a warm water flow circulates constantly in the warmest part of the heat exchanger, from which hot water is released and replaced by cooler return water.
