SYNOPSIS:　A　heat　pump　system　with　a　flash　tank　is　suitable　to　use　as　the　thermodynamic　cycle　in　the　compact　air-source　heat　pump　for　cold　climate　regions.　This　paper　presents　the　characteristics　of　a　novel　heat　pump　system　with　a　flash　tank　coupled　with　a　scroll　compressor　and　its　improved　system;　the　proposed　apparatus　has　been　named　the　heat　pump　system　with　flash　reservoir.　With　the　aid　of　an　analytical　model　based　on　the　first　law　of　thermodynamics　and　experimental　data,　the　performance　of　the　two　systems　are　predicted　in　detail.　The　calculated　results　show　that　the　two　systems　can　improve　the　heating　capacity　efficiently　when　run　under　the　conditions　of　low　climate　temperature,　and　the　intermediate　pressure　evaluated　between　8　bar　and　10　bar　is　favourable　for　the　two　systems.　©　2010　Taylor　&　Francis　Group,　LLC.