A　heat　pump　system　with　a　flash-tank　coupled　with　a　scroll　compressor　is　compared　with　a　system　with　a　sub-cooler.　The　heat　pump　performance　was　measured　experimentally.　The　heating　capacity　of　the　prototype　decreased　as　the　evaporation　temperature　decreased,　but　the　decrease　was　much　slower　than　that　of　a　conventional　air-source　heat　pump.　The　power　input　varied　slightly　with　the　evaporation　temperature.　The　heat　pump　system　with　a　flash-tank　is　more　efficient　than　the　system　with　a　sub-cooler　at　low　ambient　temperatures,　so　it　will　be　very　useful　as　a　small　capacity　air-source　heat　pump.　(C)　2007　Published　by　Elsevier　B.V.