I do not understand how this is possible in general. For example, I use ~3,600 litres of oil to heat through the winter, and the same boiler consumes less than 300 litres to produce domestic hot water for the other six months of the year.

That places my central heating on the order of 5-10x higher energy expenditure than my domestic hot water - this does not seem atypical in my area, and apparently data for the entire EU is similar (60% heating, 15% hot water): https://ec.europa.eu/eurostat/web/products-eurostat-news/w/d...