A sectional heat exchanger for a heat cell, wherein - said sectional heat exchanger comprises two end segments (103, 104) and one or more intermediate segment(s) provided between said two end segments (103, 104); - said one or more intermediate segment(s) and said two end segments (103, 104) are assembled in said heat exchanger, wherein a combustion chamber is provided in said sectional heat exchanger, perpendicular to said one or more intermediate segment(s), - each of said one or more intermediate segments comprises at least one flow channel for a fluid to be heated, - in between each two consecutive segments (103, 110, 104) at least one flow channel (131, 133) for flue gas is present, and said flow channel extends from at said combustion chamber, characterized in that - in between two segments, in the plane parallel with said one or more intermediate segment, the total width (A, B) of the sectional heat exchanger decreases over at least part of the length in the direction away from said combustion chamber; - the depth (E, F) of said flow channels (131, 133) for flue gas, measured perpendicularly to said one or more intermediate segment(s) and between consecutive segments, decreases in the direction away from said combustion chamber, - in said one or more intermediate segment(s), the distance (I, J) between two walls (141, 143) delimiting said intermediate segment and which are in heat exchanging relation with flue gas channels between segments, increases in the direction away from said combustion chamber, thereby increasing the depth available for one or more fluid flow channels in said intermediate segment. - said sectional heat exchanger comprises two end segments (103, 104) and one or more intermediate segment(s) provided between said two end segments (103, 104); - said one or more intermediate segment(s) and said two end segments (103, 104) are assembled in said heat exchanger, wherein a combustion chamber is provided in said sectional heat exchanger, perpendicular to said one or more intermediate segment(s), - each of said one or more intermediate segments comprises at least one flow channel for a fluid to be heated, - in between each two consecutive segments (103, 110, 104) at least one flow channel (131, 133) for flue gas is present, and said flow channel extends from at said combustion chamber, - in between two segments, in the plane parallel with said one or more intermediate segment, the total width (A, B) of the sectional heat exchanger decreases over at least part of the length in the direction away from said combustion chamber; - the depth (E, F) of said flow channels (131, 133) for flue gas, measured perpendicularly to said one or more intermediate segment(s) and between consecutive segments, decreases in the direction away from said combustion chamber, - in said one or more intermediate segment(s), the distance (I, J) between two walls (141, 143) delimiting said intermediate segment and which are in heat exchanging relation with flue gas channels between segments, increases in the direction away from said combustion chamber, thereby increasing the depth available for one or more fluid flow channels in said intermediate segment.