## 3 Savings and Comparisons
### Estimates of Annual Gas Savings
If the most efficient boiler, which is no longer produced or is outdated, is taken for comparison, and the others from the corresponding group are compared, annual fuel savings of up to 39.76% are obtained if the least efficient, outdated boiler is replaced with the most efficient boiler that is no longer produced. In the class of current boilers, this saving goes up to 22.62%. Both can be seen in Figure 3.  
Figure 3: Savings of natural gas: summary of boilers from Serbian market; natural gas savings are in % of savings in gas by switching a less efficient with a more efficient boiler, where the presented boilers are listed according to their efficiency (No. 1 is the most efficient among more than 1100 listed boilers). The fuel savings achieved by replacing the boiler with the lowest estimated seasonal efficiency of 55% with a better boiler are shown in Figure 4. However, Figure 4 does not suggest that the more efficient boilers are more likely available on the market or opposite. As can be seen from Figure 4, upgrading from a G-rated boiler to a modern A-rated condensing boiler can result in substantial annual energy savings depending on the type of dwelling. For a house detached from other buildings, the expected savings range from approximately EUR 240 annually, while in the case of a mid-floor apartment, the annual savings are around EUR 110. In addition to traditional natural gas, the use of biogas [34] and gas derived from waste [35] can further enhance the sustainability of gas boilers. Biogas, produced from organic materials such as agricultural waste, manure, and food waste, is a renewable energy source that can significantly reduce greenhouse gas emissions. Similarly, gas from waste, generated through the anaerobic digestion of municipal solid waste, provides a sustainable alternative to fossil fuels. By incorporating these renewable gas sources, households can further reduce their carbon footprint. While high-efficiency boilers--particularly condensing models--offer clear energy and economic benefits, their real-world adoption depends on multiple factors, including cost, infrastructure readiness, and government support. In recent years, many European countries have introduced targeted measures to promote the uptake of such technologies as part of broader climate and energy strategies. Several European countries support the adoption of high-efficiency boilers through various measures. For example, Germany offers subsidies via the Federal Subsidy for Efficient Buildings (BEG) [36] and uses carbon pricing to discourage inefficient systems; France's MaPrimeRenov' [37] and Italy's Ecobonus [38] provide substantial financial aid for boiler replacements; Austria and the Netherlands promote condensing and hybrid systems through grants; and Central and Eastern European countries like Slovenia and Croatia utilize EU funds to co-finance boiler upgrades. Figure 4: Annual savings by replacing the least efficient boiler with a more efficient one in EUR (data and methodology [33]). In Serbia, although incentive mechanisms are not yet as extensive as those in the European Union, meaningful steps have been taken to align with EU energy efficiency goals. The country has adopted several national energy efficiency programs and participates in regional initiatives supported by EU funding, such as the Western Balkans Investment Framework (WBIF). These programs have contributed to financing building renovation projects that often include upgrades to heating systems. Additionally, local authorities and public utilities have occasionally implemented co-financing schemes for energy efficiency improvements--though these vary by municipality and are typically coordinated through Serbia's Ministry of Mining and Energy. Given Serbia's strong trade and regulatory ties with the European Union, the boiler models available on its market closely mirror those sold across Europe. This makes Serbia's experience and policy direction highly relevant to broader regional trends.