<P> After the collapse of the Roman Empire, overwhelmingly across Europe, heating reverted to more primitive fireplaces for almost a thousand years . </P> <P> In the early medieval Alpine upland, a simpler central heating system where heat travelled through underfloor channels from the furnace room replaced the Roman hypocaust at some places . In Reichenau Abbey a network of interconnected underfloor channels heated the 300 m2 large assembly room of the monks during the winter months . The degree of efficiency of the system has been calculated at 90% . </P> <P> In the 13th century, the Cistercian monks revived central heating in Christian Europe using river diversions combined with indoor wood - fired furnaces . The well - preserved Royal Monastery of Our Lady of the Wheel (founded 1202) on the Ebro River in the Aragon region of Spain provides an excellent example of such an application . </P> <P> The three main methods of central heating were developed in the late 18th to mid-19th centuries . </P>

When did central warm air systems for residential and commercial applications became well​ defined