I live in center of historical city in middle of Europe. All houses are from 15th to 19th century and most od them has chimneys. In our house there are two oven

I live in center of historical city in middle of Europe. All houses are from 15th to 19th century and most od them has chimneys. In our house there are two oven stoves and we use it during winter. We are able to keep 21°C in entire house even during -10°C outside. With rising prices of gas, it is incredibly handy and saves money. House has old wood windows, not ideal for keep constant temperature inside and yet, it works great.