## 2 Gas Boilers
### Calculation of Seasonal Efficiency  
Understanding the efficiency of gas boilers is essential for making informed decisions regarding heating systems, as efficiency directly influences both operational costs and environmental sustainability. One of the most significant indicators of boiler performance is seasonal efficiency, which reflects how a boiler performs over a typical year, accounting for variations in operating conditions. The Directive 92/42/EEC prescribes the efficiency levels that boilers must meet and under what conditions, which are the input data for calculating seasonal efficiency (see Table 1). Directive 92/42/EEC also specifies what is exactly meant by different types of boilers, such as low-temperature and condensing boilers. An algorithm for calculation of seasonal efficiency is presented, including a step-by-step approach for savings by switching an obsolete boiler with a more efficient one. The classification of gas boilers is easiest to perform based on the degree of seasonal efficiency obtained through a specially prescribed procedure. For the calculation of seasonal efficiency of household gas boilers or individual boiler models, it is necessary to have known data on the degree of utilization for the lower heating value (LHV) at full load (E\({}_{\text{full}}\)) and at 30% load (E\({}_{\text{part}}\)) as well as the type of boilers. Based on these input data, the calculation of seasonal efficiency can be approached according to the method prescribed by SAP (Standard Assessment Procedure) [22; 24]. SAP was first published in 1993 and has since been updated periodically, in 1998, 2001, 2005, 2009, 2012, and most recently in 2022. The highest efficiency values for condensing boilers at full load can be 101.0% and at 30% load 107.0%, while for non-condensing boilers, the highest values at full load can be 92.0% and at 30% load 91.0% if it is calculated for the lower heating value (LHV). The efficiency degree is calculated for the lower heating value (LHV) so that it can \begin{table}
\begin{tabular}{c c} \hline \hline
**SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) Efficiency Classes** & **Energy-Related Products (ErP) Efficiency Classes** \\ \hline -Provides a detailed percentage rating indicating the exact proportion of fuel converted into usable heat over a typical year & -Uses a simpler A+++ to G rating system to classify boiler efficiency \\ -Primarily used in the UK for new-build homes & -Applies across Europe to all residential and commercial heating products \\ -Was the standard from 1999 to 2015, still referenced for new buildings & -Introduced in 2015, replacing SEDBUK for existing properties \\ -Offers precise efficiency percentages, allowing detailed comparison & -Provides a more straightforward, consumer-friendly label system \\ -Focuses on detailed efficiency metrics & -Includes regulations for manufacturers to meet higher energy efficiency standards before products can be sold \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparisons between seasonal efficiency of gas boilers and ErP requirements. exceed 100% for condensing boilers because they are designed to utilize the thermal energy released by the condensation of water vapor. The following instructions should be followed to calculate seasonal efficiency:  
1. The degrees of utilization at full load (E\({}_{\rm full}\)) and partial load (E\({}_{\rm part}\)), which serve as input data and cannot exceed the specified values, should be converted to the higher heating value (HHV) by multiplying them by a coefficient of 0.901 for gas boilers and 0.921 for liquid petroleum gas (LPG) boilers (SAP [22], Table D2.2):
2. In the further course of calculation, it is necessary to determine the type of boiler in accordance with Section D1 and Table D2.3 in SAP [22]. Based on this, select the appropriate expression for the calculation of seasonal efficiency from Table D2.4. For example, for an on-off gas boiler (on-off regular), whether condensing or not, the Expression 101 applies (Equation (1)).