A fusion welding socket (30, 130, 230) for coupling together double walled plastic conduits (10, 20, 110, 120), which double walled conduits comprise an inner pipe (12, 22, 112, 122, 212, 222), an outer pipe (11, 21, 111, 121, 211, 221) surrounding the inner pipe and an annular space (13, 23, 113, 123, 213, 223) arranged between the inner and outer pipes. The fusion welding socket comprises a main body (31, 131, 231) comprising a first end portion (32, 132, 232), a second end portion (33, 133, 233) and an intermediate portion (34, 134, 234) arranged longitudinally between the first and second end portions. The intermediate portion has at least one inner diameter which is smaller than the inner diameters of the first and second end portions. First (35a), second (35b) and third (35c-d) heating coils embedded in the first end portion, the second end portion and the intermediate portion respectively for fusing together the first and second end portions with a respective outer pipe of a first and second conduit and the intermediate portion with a respective inner pipe of the first and second conduit. A passageway (39, 41-43, 139, 141-144, 239, 241-242) comprising an annular space (39, 139, 239) is arranged to connect the annular spaces of the first and second conduit after fusion.