In contrast to the functionality of constant and low-temperature boilers, the functionality of gas condensing technology does not only consist in the combustion, gasification and release of heat. Between the last two steps, the flue gases also condense.
During the combustion of a fuel, in this case gas, flue gases are produced in addition to the measurable heating output. The latter consist mainly of hot steam, the components of which can be acidic. To protect the boiler itself and the flue gas system from this acidic steam, older boilers direct the flue gases outside. Depending on the heating technology, the exhaust gas temperature can be up to 200 degrees Celsius. Condensation of the water vapour in the exhaust gases is thus deliberately prevented.
A simple demonstration can be made that hot steam still contains energy: Whoever holds their hand over a pot of boiling water will quickly notice this energy on the palm of their hand. This is because the steam condenses on the surface of the hand and releases the hidden heat, also called condensation heat, at this point. One of the functions of gas condensing technology is to recover this condensation heat and add it to the heating water. For this purpose, the hot flue gases are passed through a heat exchanger before they reach the outside.
The heat exchanger is made of moisture and acid resistant stainless steel and is very compact due to its special design. When the hot flue gases flow through its coil, they are cooled by the heating water that flows back from the radiators to the boiler (return water). The exhaust gases gradually condense. What follows is the heat dissipation of the resulting water vapour to the heating system.
To ensure that condensation also occurs, the temperature of the return water must not exceed 57 degrees Celsius. Otherwise it cannot cause the hot flue gases to condense. Surface heating systems such as underfloor heating systems are suitable for cooling this return water.
The heat exchanger is therefore an important component of a gas condensing boiler or condensing gas heating system and is indispensable for the operation of this technology. It is able to transfer the condensation heat obtained to the heating system with virtually no loss. In this way, it converts up to 98 percent of the energy contained in the fuel into heat. An indestructible component for which system owners have a guarantee of up to ten years.
Condensate is created by the planned condensation of the water vapour contained in the flue gases. This can normally be discharged into the waste water, provided the pH value is not too high.
In addition to condensate drainage, the flue gas system must also be designed for condensing technology. In this case, the amount of acid contained in the condensate must not damage the system. The excess pressure generated by the use of a fan (e.g. by a burner fan) must not affect the new pipe system in any way. Flue pipes made of stainless steel or special plastic have proven themselves in practical use. If a flue already exists, the new pipe can usually be inserted into the existing flue. It is also possible to guide the flue externally without the existing flue stack.