An adapter (2, 3) capable of being attached to a gas water heater and connecting to external radiators to achieve an additional central heating function, comprising: a first fluid pipelines, for being in fluid communication with the gas water heater; and a second fluid pipelines and a third fluid pipelines, wherein one of said second and third fluid pipelines is adapted for connecting to external radiators, and the other of said second and third fluid pipelines is adapted for connecting external plumbling to supply hot water for domestic usage; a heat exchanger (201, 301) comprising a first fluid passageway (2011, 3011) and a second fluid passageway (2012, 3012); wherein said first fluid passageway forms one portion of the first fluid pipelines, and said second fluid passageway forms one portion of the second fluid pipelines; wherein said third fluid pipelines comprises a fluid incoming line (2131, 3131) and a fluid outgoing line (2132, 3132); wherein the adapter further comprises a first diverter valve (203, 302), a second inlet tube (22, 32) and a second outlet tube (21, 31) both extending from said side of the housing and connecting to the first fluid pipelines for respectively being input fluid from and outputting fluid to the gas water heater, and said first diverter valve is operable to permit the fluid outgoing line and the first fluid passageway to alternatively intercommunicate with the second inlet tube, characterised in that the adapter comprises a housing independent of the gas water heater, said housing accommodating the first fluid pipelines, the second fluid pipelines, the third fluid pipelines, the heat exchanger and the first diverter valve; and a connection interface disposed at one side of the housing facing the gas water heater for connecting the adapter to the gas water heater being capable of supplying hot water to achieve an additional central heating function. a first fluid pipelines, for being in fluid communication with the gas water heater; and a second fluid pipelines and a third fluid pipelines, wherein one of said second and third fluid pipelines is adapted for connecting to external radiators, and the other of said second and third fluid pipelines is adapted for connecting external plumbling to supply hot water for domestic usage; a heat exchanger (201, 301) comprising a first fluid passageway (2011, 3011) and a second fluid passageway (2012, 3012); wherein said first fluid passageway forms one portion of the first fluid pipelines, and said second fluid passageway forms one portion of the second fluid pipelines; wherein said third fluid pipelines comprises a fluid incoming line (2131, 3131) and a fluid outgoing line (2132, 3132); wherein the adapter further comprises a first diverter valve (203, 302), a second inlet tube (22, 32) and a second outlet tube (21, 31) both extending from said side of the housing and connecting to the first fluid pipelines for respectively being input fluid from and outputting fluid to the gas water heater, and said first diverter valve is operable to permit the fluid outgoing line and the first fluid passageway to alternatively intercommunicate with the second inlet tube, characterised in that the adapter comprises a housing independent of the gas water heater, said housing accommodating the first fluid pipelines, the second fluid pipelines, the third fluid pipelines, the heat exchanger and the first diverter valve; and a connection interface disposed at one side of the housing facing the gas water heater for connecting the adapter to the gas water heater being capable of supplying hot water to achieve an additional central heating function.