Fouling　and　frosting　of　the　outdoor　coil　in　an　air　source　heat　pump　(ASHP)　unit　are　common　but　undesirable　operating　problems.　At　an　increased　level　of　air　pollution,　fouling　of　an　outdoor　coil　becomes　increasingly　severe　during　its　space　cooling　operation　in　summer　and　may　potentially　impact　its　frosting　operation　in　winter.　This　paper　reports　on　a　field　study　on　the　operating　performances　of　a　space　heating　ASHP　unit　with　a　foul　outdoor　coil　as　a　result　of　a　prolonged　cooling　operation.　Firstly,　a　detailed　description　of　the　field　ASHP　unit　with　the　foul　outdoor　coil　and　test　cases　is　given.　Secondly,　field　study　results　for　the　test　cases　and　their　analyses　are　presented.　The　results　demonstrate　that　fouling　of　an　outdoor　coil　during　its　space　cooling　operation　in　summer　can　significantly　impact　its　frosting　performance　during　its　space　heating　operation　in　winter.　(C)　2018　Elsevier　Ltd　and　IIR.　All　rights　reserved.