ason ry ▁is ▁used ▁to ▁retain ▁heat . ▁While ▁it ▁might ▁take ▁several ▁hours ▁of ▁he ating ▁to ▁reach ▁the ▁desired ▁surface ▁temperature , ▁a ▁properly ▁designed ▁bed ▁raised ▁to ▁sufficient ▁temperature ▁should ▁remain ▁warm ▁throughout ▁the ▁night ▁without ▁the ▁need ▁to ▁maintain ▁a ▁fire . ▁ ▁History ▁The ▁k ang ▁is ▁said ▁to ▁be ▁derived ▁from ▁the ▁concept ▁of ▁a ▁he ated ▁bed ▁floor ▁called ▁a ▁hu o qi ang ▁found ▁in ▁China ▁in ▁the ▁Ne ol ith ic ▁period , ▁according ▁to ▁analysis ▁of ▁ar che ological ▁exc av ations ▁of ▁building ▁remains ▁in ▁Ban po ▁X i ' an . ▁However , ▁ar che ological ▁sites ▁in ▁Sh eny ang , ▁L ia oning , ▁show ▁humans ▁using ▁the ▁he ated ▁bed ▁floor ▁as ▁early ▁as ▁ 7 , 2 0 0 ▁years ▁ago . ▁The ▁bed ▁at ▁this ▁exc av ation ▁is ▁made ▁of ▁ 1 0   cm ▁p ounded ▁cl ay ▁on ▁the ▁floor . ▁The ▁bed ▁was ▁he ated ▁by ▁z h idi ▁which ▁is ▁simply ▁the ▁process ▁of ▁placing ▁an ▁open ▁fire ▁on ▁the ▁bed ▁floor ▁and ▁clear ing ▁the ▁as hes ▁before ▁sleep ing . ▁It ▁is ▁mentioned ▁by ▁Tang ▁poet ▁M eng ▁J iao ▁in ▁his ▁poem ▁titled ▁Hand i ▁Ba ix ing ▁Y in . ▁' No ▁fuel ▁to ▁heat ▁the ▁floor ▁to ▁sleep , ▁standing ▁and ▁cry ing ▁with ▁cold ▁at ▁mid night ▁instead '. ▁In ▁the ▁exc av ated ▁example ▁the ▁repeated ▁burning ▁is ▁believed ▁to ▁have ▁turned ▁the ▁bed