To　investigate　the　impact　of　defrosting-water　retention　on　the　surface　of　heat　exchangers　of　the　air　source　heat　pump　(ASHP)　performance　during　frosting-defrosting　cycles,　an　experimental　study　has　been　conducted.　Testing　was　carried　out　in　a　controlled-environment　chamber　with　ASHP　operational　data　collected　over　16　periodic　frosting-defrosting　cycles　spanning　a　360-min　period.　It　was　found　that　the　defrosting-water　retention　undergoes　three　stages:　single-phase　water,　a　mixture　of　water　and　ice,　followed　by　a　predominance　of　ice.　In　this　study,　the　third　stage　appears　after　the　unit　has　been　in　operation　for　256　mm.　A　＂permafrost　area＂　forms　on　the　lower　quadrant　of　the　heat　exchanger,　which　occupies　over　20%　of　the　total　heat　exchanger　surface.　This　area　remained　continuously　occupied　regardless　of　whether　the　unit　was　in　a　frosting　or　defrosting　cycle.　The　appearance　of　this　＂permafrost　area＂　not　only　impacts　the　performance　of　the　ASHP,　it　also　indicates　the　beginning　of　repetitive　＂shutdown　accidents＂　for　the　ASHP　units.　It　is　the　first　time　that　these　phenomena,　originally　caused　by　the　defrosting-water　retention　have　been　revealed.　And　a　quantitative　analysis　is　presented　for　the　resulting　performance　degradation　of　the　ASHP　related　to　the　＂permafrost　area＂　conditions　and　the　related　＂repetitive　shutdown　accidents＂.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.