It　has　been　known　that　for　space　heating　ASHPs,　their　different　configurations　and　operation　may　influence　their　frosting　performances.　However,　there　has　been　not　a　single　index　that　can　be　used　to　comprehensively　and　realistically　evaluate　the　frosting　suppression　performances　for　a　space　heating　ASHP　unit　by　considering　all　the　influencing　configurational　and　operational　factors.　Therefore,　in　this　paper,　a　novel　characteristic　index　for　frosting　suppression　based　on　the　configuration　and　operation　of　ASHPs　has　been　proposed　and　its　development　is　reported.　Firstly,　the　characteristic　index,　CICO,　was　defined,　a　detailed　analysis　on　the　heat　transfer　taking　place　in　the　outdoor　coil　of　an　ASHP　was　carried　out,　and　the　relationship　between　CICO　and　temperature　difference　in　the　heat　transfer　between　outdoor　coil　and　ambient　air　(Delta　T)　was　established.　Secondly,　the　description　of　an　experimental　setup　with　four　experimental　ASHP　units　is　given.　The　set　was　used　to　establish　all　empirical　numerical　relationship　between　CICO　and　Delta　T.　Thirdly,　using　the　experimental　results,　the　empirical　numerical　relationship　between　CICO　and　Delta　T　was　established,　which　suggested　that　a　higher　CICO　would　lead　to　a　smaller　Delta　T,　thus　a　better　frosting　suppression　performance.　The　development　results　suggested　that　the　index　of　CICO　could　be　used　to　evaluate　the　level　of　frosting　suppression　performance　for　ASHPs,　and　to　help　improve　the　frosting　suppression　performances　for　an　ASHP　during　its　design　and　operation,　to　ensure　its　best　possible　operation　performance.　(C)　2019　Elsevier　Ltd　and　IIR.　All　rights　reserved.