Many flats were heated with potbelly stoves (Kanonenofen) well into the 20th century. These were wrought-iron ovens with a flue that were fired with coals. Their cylindrical shape was similar to that of canons (hence canon ovens in German) which gave them their name. In those days there was no running hot water in the houses. Bathing ovens heated the water up to a suitable temperature for bathing or doing the washing up. These ovens were mostly fired with coal or wood, but rarely with heating oil. Later on boilers were used instead.
Central heating was really invented as far back as 1716 but for a long time only wealthy citizens could afford this luxury. When the first oil- and gas boilers appeared on the market at the beginning of the 20th century, the demand for hot water heating rose tremendously, as self-contained central heating systems became affordable for apartment buildings.
After the 2nd World War central heating systems for living accommodation were far out of reach. The required material was expensive and in short supply. That only changed in the seventies. Many flats were renovated during that time and central heating systems became standard in new buildings.
Since the oil crises in 1973 energy saving has become a serious demand for heating technology. This and our growing ecological conscience have since the 1970s and 80s led to the development of more environmentally friendly and efficient heating systems. Now solar heating systems, geo-thermal heat pumps or wooden pellets are alternatives to the more conventional gas- or oil heating systems.