Early central heating seems to have developed along several lines that included hearths and fireplaces, stoves, and underfloor systems. Fixed central hearths date as far back as 2500 B.C. They were excavated in Greece.Crude fireplace heating was used as early as the 800s A.D., and was widespread in Europe by the 13th century. Castles built at that time had fireplaces that had a short flue to the outside, a crude form of chimney. All of the early fireplaces were constructed entirely of stone, but casting technology improvements led to the introduction of cast iron firebacks designed to protect the stone from direct fire heat.