When　an　air　source　heat　pump　(ASHP)　is　operated　at　frosting　conditions,　controlling　its　defrosting　initiation　is　very　important,　which　affects　its　operating　efficiency　and　output　heating　capacity.　Therefore,　to　improve　the　accuracy　of　defrosting　initiation　of　ASHPs　under　frosting-defrosting　operations,　an　investigation　on　determination　of　an　optimal　defrosting　initiating　time　point　(t(opt))　for　an　ASHP　unit　based　on　the　minimum　loss　coefficient　in　the　nominal　output　heating　energy　(epsilon(NLmin))　has　been　carried　out　and　the　investigation　results　are　presented　in　this　paper.　Firstly,　the　existence　of　t(opt)　was　verified　through　a　field　study.　Secondly,　the　development　of　multiple-variable-nonlinear　models　for　predicting　epsilon(NLmin)　and　toot　is　detailed.　Finally,　the　results　of　a　related　modeling　study　using　the　developed　multiple-variable-nonlinear　model　for　t(opt)　is　presented.　The　results　from　the　investigation　suggested　that　initiating　defrosting　for　an　ASHP　unit　at　t(opt)　could　result　in　the　best　possible　operating　performances　under　its　frosting-defrosting　operations.　It　was　further　demonstrated　through　the　modeling　study　that　at　a　fixed　ambient　relative　humidity,　an　increase　in　ambient　air　temperature　would　lead　to　a　decreasing-increasing　variation　trend　in　t(opt),　and　at　a　fixed　ambient　air　temperature,　t(opt)　was　decreased　with　an　increase　in　relative　humidity.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.