Decentral　booster　heat　pumps　(BHPs)　integrated　in　ultra-low　temperature　district　heating　(ULTDH)　systems　are　deemed　a　sustainable　solution　for　low-carbon　building　systems.　The　performance　of　BHPs　is　critical　for　the　implementation　of　ULTDH　systems.　In　this　study,　we　experimentally　investigate　the　effect　of　refrigerant　charge　on　temperature,　pressure,　system　coefficient　of　performance　(COP),　and　heating　capacity　to　evaluate　the　operability　and　sensitivity　of　ULTDH.　A　performance-guaranteed　range　of　refrigerant　charge　is　proposed　to　ensure　the　feasibility　of　the　system.　We　also　analyse　the　component-level　exergy　destruction　and　the　system-level　exergetic　efficiency　of　a　BHP　prototype.　The　exergetic　efficiencies　range　from　24.9　%　to　33.4　%　under　operational　conditions　for　ULTDH.　The　condenser　and　evaporator＇s　exergy　destruction　is　influenced　by　the　temperature　profiles　on　both　sides,　suggesting　the　potential　to　improve　BHP　performance　by　tailoring　operating　parameters.　Additionally,　the　return　water　temperature　significantly　affects　the　system　COP　when　the　domestic　hot　water　supply　temperature　is　fixed.　Within　the　allowable　range　of　return　water　temperature,　BHP　operation　with　high　evaporator　outlet　water　temperature　can　improve　the　overall　efficiency　of　the　ULTDH　system.