## Appendix A Condensation Technology  
Condensing boilers are designed to extract the latent heat of condensation from the water vapor in combustion products. By extracting latent heat, condensing boilers can achieve a high level of efficiency. A critical factor for achieving maximum efficiency of condensing boilers is the return water temperature, which is generally kept below 57 \({}^{\circ}\)C. The return water temperature determines whether the boiler operates in condensing mode. Due to the need to extract as much latent heat as possible and the high corrosiveness of the condensate from combustion products, condensing boilers require special materials for construction. To withstand corrosive conditions, condensing boilers must be made of stainless steel and other corrosion-resistant (and therefore more expensive) materials. They require sophisticated management and careful installation to reach their potential capabilities. Additionally, other units (radiators, convectors, spiral heat exchangers) connected to the system with a condensing boiler are more expensive due to the larger heat exchange surface required for operation at low water temperatures. Condensation technology is an efficient method for converting natural gas and liquid fuels into useful energy through combustion [45; 46; 47]. While in low-temperature boilers, the condensation of hot gases is avoided due to the wetting of heating surfaces, condensation technology operates in a completely different manner: the condensation of gases is highly desirable and necessary to extract the latent energy contained in water vapor as well as additional sensible energy from flue gases into usable heat. At the same time, the residual heat of the flue gases is significantly reduced compared to low-temperature boilers. Acidic condensate can cause corrosion, leading to higher maintenance costs and reduced boiler lifespan [48]. Through a reaction with oxygen (O\({}_{2}\)), a component from the air, the combustion of liquid fuel or natural gas, which primarily contains carbon (C) and hydrogen (H\({}_{2}\)) compounds, produces carbon dioxide (CO\({}_{2}\)) and water (H\({}_{2}\)O). For natural gas (methane CH\({}_{4}\)), the following simplified combustion formula is applied (Equation (A1)):  
\[\text{CH}_{4}+2\text{O}_{2}\to 2\text{H}_{2}\text{O}+\text{CO}_{2}+\text{Energy}\] (A1)  
Condensate forms from steam in combustion products when the temperature of the walls on the hot gas side drops below the condensation point of water vapor. The different chemical compositions of natural gas and heating oil result in different evaporation temperatures at which water vapor from combustion gases condenses. As shown in Figure A1, the dew point of water vapor for natural gas is approximately 57 \({}^{\circ}\)C, while for extra-light liquid fuel, it is approximately 47 \({}^{\circ}\)C. Theoretically, energy increases by 11% compared to low-temperature technology. For heating oil, the application of condensation technology theoretically increases energy by about 6%. Condensation technology is closely linked to the fuel's heating value and the state of water in combustion products, whether in vapor or liquid form. The heating value is a measure of the energy released when a fuel is completely combusted. It indicates the amount of heat produced by the combustion of a specific quantity of fuel. Heating values are essential for calculating the efficiency of heating systems and comparing the energy content of different fuels. They are typically expressed in kWh/m\({}^{3}\) or MJ/m\({}^{3}\). There are two main types of heating values:* Lower heating value (LHV) is the energy released during complete combustion, with the water produced in the process, being separated as vapor;
* Higher heating value (HHV) determines the energy released during the complete process, including the heat of vaporization contained in the water vapor of hot gases. Table 14 provides an overview of fuel characteristics relevant to the use of condensation technology.