A boiler system (10), comprising a support construction (42) and a furnace (12) supported to the support construction at a vertically middle section of the furnace, the furnace being enclosed by water tube walls comprising two side walls (18) and two end walls (20), a roof (16) and a bottom (14), the side walls having a total height (H) from the bottom to the roof, wherein each of the two side walls comprises a vertical upper portion (30) that extends from the roof to a level of 30-70 % of the height (H), a lower portion (32) that extends from the bottom to a level of 30-70 % of the height (H) and has a vertical upper portion (36), and an in downward direction outwards bent intermediate portion (38) at a level between the upper portion of the side wall and the vertical upper portion of the lower portion of the side wall, wherein the support construction (42) comprises horizontal wall supporting beams (48) that are arranged parallel to the side walls (18) at a level below the roof (16) of the furnace and directly above the vertical upper portions (36) of the lower portions (32) of the two side walls (18), and the furnace (12) is supported to the support construction (42) by having the intermediate portions (38) of the side walls connected to adjacent horizontal wall supporting beams (48) so as to balance vertical loads of the furnace.