## 4 Discussion
In fact, what was targeted was an overall thermal demand for a standardized heating season, expressed as hours of nominal operation during the heating season. The boiler's domestic hot water production was estimated based on the delivery schedules presented in the European Standards for boiler hot water production energy classification. The total daily hot water production was converted into hours of daily functioning and, by considering the number of days in a year, a total functioning time for the boiler was found for domestic hot water production. Summing the yearly functioning of the boiler for heating purposes with the yearly functioning of the boiler for hot water production purpose, a total yearly functioning time, at nominal thermal output, was determined. As an example, considering these premises, and based on an estimation of 700 K boilers functioning in the U.K. in 2020 [17] having thermal power lower than 100 kW, and assuming the working conditions previously presented, a total estimated reduction of: 0.9 (to/year/boiler) \(\times\) 700 K (boilers) = 630,000-ton CO\({}_{2}\)/year can be deducted. In Romania, the Eco-Design boilers minimum performance standards became mandatory starting in September 2015 [18], and has led to a shift towards condensing boilers starting in 2016. Nonetheless, a lot of domestic water heating devices are still noncondensing boilers fed by natural gas or solid-type fuel. If, for example, in 2014 before the Eco-Design Directive implementation, 90% of the wall-hung condensing boilers sold were of the non-condensing type, a 180-degree change took place leading to 97.5% of condensing type boilers being sold in 2021. According to [19], the Romanian condensing boiler market is expected to have a slight increase from one year to the next, roughly 2%, starting with the end of the COVID-19 pandemic. Considering the current market reports and future predictions we can deduct the number of gas wall-hung condensing boilers that are going to be sold between the years 2019 and 2024. Thus, 1.415 million units resulted, of which 81% have a thermal power of just about 25 kW and 19% of 70 kW, respectively. By assuming the same value considered above: 0.9 (to/year/boiler) \(\times\) 283 K boilers (average value obtained by dividing 1.415 million to 5 years) = a 254,700-ton CO\({}_{2}\)/year reduction resulted. It should be noted that this value has only been calculated taking into account new appliances, but it should be borne in mind that the use of hydrogen-enriched gaseous fuel in existing appliances will also contribute to Romania's decarbonization. For further expanding the area of knowledge, the authors will test in the next period certain condensing boilers available on the Romanian market, both with natural gas from the national grid and with G222 gas, which is a mixture of 23% hydrogen and 77% methane. The experimental tests are going to be conducted in similar working conditions and will be further compared with the outcome of the numerical modelling program presented in this article. In addition, experimental measurements are going to be conducted on a fire-tube boiler with a separate condensing stage and, subsequently, a particular modelling program for this type will be conducted.