▁surface ▁hard ▁and ▁mo ist ure ▁resist ant . ▁ ▁The ▁first ▁known ▁type ▁of ▁he ated ▁platform ▁appeared ▁in ▁modern - day ▁N ortheast ▁of ▁China ▁and ▁used ▁a ▁single ▁fl ue ▁systems ▁found ▁in ▁the ▁hyp oca ust ▁of ▁An cient ▁Rome ▁and ▁the ▁on d ol ▁of ▁Korean ▁origin . ▁An ▁example ▁of ▁this ▁type ▁of ▁he ated ▁platform ▁was ▁un ear th ed ▁in ▁ 1 st - century ▁building ▁remains ▁in ▁the ▁Heil ong ji ang ▁Province . ▁Its ▁single ▁fl ue ▁is ▁' L ' ▁sh aped , ▁built ▁from ▁ad obe ▁and ▁c ob bl est ones ▁and ▁covered ▁with ▁stone ▁sl abs . ▁ ▁He ated ▁walls ▁with ▁a ▁double ▁fl ue ▁system ▁was ▁found ▁in ▁a ▁ 4 th - century ▁ancient ▁palace ▁building ▁in ▁the ▁J il in ▁Province . ▁It ▁has ▁an ▁' L ' ▁sh aped ▁ad obe ▁ben ch ▁with ▁a ▁double ▁fl ue ▁system . ▁It ▁is ▁struct ur ally ▁more ▁complex ▁than ▁a ▁single ▁fl ue ▁system ▁and ▁has ▁functionality ▁similar ▁to ▁a ▁k ang . ▁ ▁The ▁word ▁k ang ▁means ▁" to ▁dry ", ▁first ▁documented ▁in ▁the ▁Chinese ▁dictionary ▁in ▁AD 1 2 1 . ▁The ▁earliest ▁k ang ▁remains ▁have ▁been ▁discovered ▁at ▁N ing hai , ▁Heil ong ji ang ▁Province , ▁in ▁the ▁Long qu an fu ▁Palace ▁( 6 9 9 - 9 2 6 ) ▁of ▁Bal ha e ▁origin . ▁ ▁The ▁k ang ▁may ▁have ▁evol ved ▁to