Air　source　heat　pumps　(ASHPs)　have　been　widely　used　for　space　cooling　in　recent　years.　When　a　space　cooling　ASHP　unit　is　operated　in　an　ambience　with　poor　air　quality,　fouling　of　its　outdoor　coil　can　be　easily　built　up,　leading　to　a　significant　drop　in　the　operating　efficiency　and　output　cooling　capacity　of　the　ASHP　unit.　Therefore,　as　a　follow-up　to　a　previous　related　study,　a　study　to　develop　a　novel　defouling　initiation　method　for　the　fouled　outdoor　coil　in　a　space　cooling　ASHP　unit　based　on　its　measurable　characteristic　ambient　and　operating　temperatures　has　been　carried　out　and　the　study　results　are　reported　in　this　paper.　Firstly,　a　characteristic　parameter　of　TTT,　which　can　be　evaluated　based　on　ambient　air　Temperature,　outdoor　coil　metal　surface　Temperature　and　outdoor　coil　outlet　air　Temperature,　is　defined.　Secondly,　the　development　of　the　novel　defouling　initiation　method　is　detailed.　Finally,　further　field　comparative　studies　using　the　developed　novel　defouling　initiation　method　have　been　carried　out,　and　the　study　results　demonstrated　that　the　novel　defouling　initiation　method　can　provide　an　accurate　and　convenient　way　to　initiate　defouling　of　the　fouled　outdoor　coil　in　an　ASHP　unit　at　a　reasonable　defouling　frequency　to　ensure　its　best　possible　operating　performances.　(C)　2019　Elsevier　Ltd　and　IIR.　All　rights　reserved.