A filling system for connecting a fluid supply to a pressurized fluid circuit, comprising: a filling device comprising a main fluid passageway having an inlet port adapted for connection to the fluid supply and an outlet port adapted for connection to the fluid circuit, a manual valve interposed in the main fluid passageway to open and close a first fluid path defined by the main fluid passageway, and a bypass fluid passageway connected between the inlet port and the outlet port to conduct fluid around the manual valve; and a thermal actuating device associated with the bypass fluid passageway to open and close a second fluid path defined by the bypass fluid passageway; said thermal actuating device comprising a casing, a thermal actuator received in the casing and having thermally expandable material contained therein, and a movable member cooperating with the thermal actuator to be actuated to move between a first and a second positions relative to the casing; wherein the movable member is disposed in the casing and has one end extending out of the casing for blocking the bypass fluid passageway when the movable member is located at a first position and unblocking the bypass fluid passageway when the movable member is located at a second position; a pressure sensor for monitoring pressure of fluid circulating in the fluid circuit; and a controller communicating with the pressure sensor and the thermal actuator; said controller being configured to activate the thermal actuator by heating the thermally expandable material to actuate the movable member to move from the first position to the second position for opening the second fluid path after receiving a signal representing that the fluid pressure falls below a first pressure threshold from the pressure sensor; characterised in that said controller is configured to estimate a remaining time when the fluid pressure reaches a second pressure threshold equal to or greater than the first pressure threshold; and the controller is further configured to deactivate the thermal actuator by stopping heating the thermally expandable material when the remaining time is equal to a fixed time interval, thereby actuating the movable member to move from the second position to the first position as the thermally expandable material cools down. a filling device comprising a main fluid passageway having an inlet port adapted for connection to the fluid supply and an outlet port adapted for connection to the fluid circuit, a manual valve interposed in the main fluid passageway to open and close a first fluid path defined by the main fluid passageway, and a bypass fluid passageway connected between the inlet port and the outlet port to conduct fluid around the manual valve; and a thermal actuating device associated with the bypass fluid passageway to open and close a second fluid path defined by the bypass fluid passageway; said thermal actuating device comprising a casing, a thermal actuator received in the casing and having thermally expandable material contained therein, and a movable member cooperating with the thermal actuator to be actuated to move between a first and a second positions relative to the casing; wherein the movable member is disposed in the casing and has one end extending out of the casing for blocking the bypass fluid passageway when the movable member is located at a first position and unblocking the bypass fluid passageway when the movable member is located at a second position; a pressure sensor for monitoring pressure of fluid circulating in the fluid circuit; and a controller communicating with the pressure sensor and the thermal actuator; said controller being configured to activate the thermal actuator by heating the thermally expandable material to actuate the movable member to move from the first position to the second position for opening the second fluid path after receiving a signal representing that the fluid pressure falls below a first pressure threshold from the pressure sensor; characterised in that said controller is configured to estimate a remaining time when the fluid pressure reaches a second pressure threshold equal to or greater than the first pressure threshold; and the controller is further configured to deactivate the thermal actuator by stopping heating the thermally expandable material when the remaining time is equal to a fixed time interval, thereby actuating the movable member to move from the second position to the first position as the thermally expandable material cools down.