▁vent ing ▁in ▁which ▁very ▁hot ▁fl ue ▁g ass es ▁are ▁exha usted ▁through ▁a ▁( us ually ▁P VC ) ▁air ▁int ake ▁pipe , ▁which ▁could ▁lead ▁to ▁a ▁structure ▁fire ▁and / or ▁hot ▁fl ue ▁g ass es ▁being ▁released ▁into ▁the ▁structure . ▁ ▁F uel ▁ ▁Hard wood ▁or ▁soft wood ▁ ▁Fire wood ▁is ▁usually ▁measured ▁in ▁English - spe aking ▁countries ▁in ▁a ▁quantity ▁called ▁a ▁cord , ▁meas uring ▁ ▁( an ▁order ly ▁' t ight ly ▁pack ed ' ▁stack ▁ 4 ' ▁high ▁x ▁ 4 ' ▁deep ▁x ▁ 8 ' ▁wide ). ▁Fire wood ▁may ▁be ▁purchased ▁by ▁the ▁cord , ▁or ▁by ▁a ▁fraction ▁of ▁a ▁cord . ▁The ▁term ▁" face ▁cord " ▁is ▁commonly ▁used ▁to ▁describe ▁varying ▁volumes ▁of ▁wood . ▁N omin ally ▁it ▁means ▁ 4 ' ▁x ▁ 8 ' ▁x ▁an ▁un spec ified ▁third ▁measurement . ▁When ▁purch asing , ▁cutting , ▁or ▁collect ing ▁fire wood , ▁it ▁is ▁good ▁to ▁be ▁aware ▁of ▁the ▁difference ▁between ▁hard wood ▁and ▁soft wood . ▁ ▁Both ▁hard wood ▁and ▁soft wood ▁have ▁similar ▁energy ▁contents ▁by ▁mass , ▁but ▁not ▁by ▁volume . ▁In ▁other ▁words , ▁a ▁piece ▁of ▁hard wood ▁would ▁usually ▁be ▁heav ier ▁and ▁have ▁more ▁available ▁energy ▁than ▁the ▁same ▁s ized ▁piece ▁of ▁soft wood . ▁Hard wood s , ▁derived ▁from ▁trees ▁such ▁as ▁o ak ▁and ▁ash , ▁may ▁burn ▁at