A horizontal connection system for subsea connection of a first hub arranged at an end of a first tubular member to a second hub arranged at an end of a second tubular member, the horizontal connection system comprising: a first holding unit (11; 11'), wherein the first hub is fixed to this first holding unit (11; 11'); a second holding unit (21; 21'), wherein the second hub is fixed to this second holding unit; a clamp connector for connecting and securing the first hub to the second hub, the clamp connector being secured to the first holding unit (11; 11'); and a heat bank for thermally insulating the clamp connector from ambient cold sea water, the heat bank comprising a casing which encloses an internal fluid chamber, wherein this fluid chamber is configured to accommodate a fluid having heat-storing capacity, preferably sea water, and wherein the clamp connector is to be received in the fluid chamber with the fluid surrounding the clamp connector, said casing comprising a rear part which is secured to the first holding unit (11; 11') and a front part which is releasably connectable to the rear part, characterized in: that the front part of the casing is configured to be temporarily fixed to the rear part of the casing by means of connecting members (42; 42') extending between the front part and the rear part on the outside thereof; and that the front part of the casing is provided with fastening members (46; 46') which are configured to come into engagement with associated fastening members (26; 26') on the second holding unit (21; 21'), to thereby allow the front part to be secured to the second holding unit (21; 21'), when the front part and the second holding unit (21; 21') are brought into contact with each other by an axial movement of the front part towards the second holding unit together with the rear part of the casing and the first holding unit or by an axial movement of the second holding unit towards the front part of the casing. a first holding unit (11; 11'), wherein the first hub is fixed to this first holding unit (11; 11'); a second holding unit (21; 21'), wherein the second hub is fixed to this second holding unit; a clamp connector for connecting and securing the first hub to the second hub, the clamp connector being secured to the first holding unit (11; 11'); and a heat bank for thermally insulating the clamp connector from ambient cold sea water, the heat bank comprising a casing which encloses an internal fluid chamber, wherein this fluid chamber is configured to accommodate a fluid having heat-storing capacity, preferably sea water, and wherein the clamp connector is to be received in the fluid chamber with the fluid surrounding the clamp connector, said casing comprising a rear part which is secured to the first holding unit (11; 11') and a front part which is releasably connectable to the rear part, characterized in: that the front part of the casing is configured to be temporarily fixed to the rear part of the casing by means of connecting members (42; 42') extending between the front part and the rear part on the outside thereof; and that the front part of the casing is provided with fastening members (46; 46') which are configured to come into engagement with associated fastening members (26; 26') on the second holding unit (21; 21'), to thereby allow the front part to be secured to the second holding unit (21; 21'), when the front part and the second holding unit (21; 21') are brought into contact with each other by an axial movement of the front part towards the second holding unit together with the rear part of the casing and the first holding unit or by an axial movement of the second holding unit towards the front part of the casing. that the front part of the casing is configured to be temporarily fixed to the rear part of the casing by means of connecting members (42; 42') extending between the front part and the rear part on the outside thereof; and that the front part of the casing is provided with fastening members (46; 46') which are configured to come into engagement with associated fastening members (26; 26') on the second holding unit (21; 21'), to thereby allow the front part to be secured to the second holding unit (21; 21'), when the front part and the second holding unit (21; 21') are brought into contact with each other by an axial movement of the front part towards the second holding unit together with the rear part of the casing and the first holding unit or by an axial movement of the second holding unit towards the front part of the casing.