A boiler comprising a support assembly for supporting a furnace of the boiler to a support frame of the boiler, - wherein the furnace comprises four vertical, planar water tube walls (72, 74, 76, 78) which are joined together and which, in a horizontal plane, define a rectangular cross section with four corner sections, two of the water tube walls being joined in each corner section, the four corner sections including a first corner section at which a first water tube wall and a second tube wall that are transverse to each other are joined, and - wherein the boiler further comprises at least one vertically extending pipe that is for the transport of water and/or steam and situated outside the furnace, the pipe being close to the first corner section, characterized in that: - the support frame further comprises at least two horizontal supporting beams (26, 28, 30, 32) which are separated from the water tube walls and include a first supporting beam and a second supporting beam that are transverse to each other, - wherein the support assembly close to the first corner section comprises - a first assembly part that attaches the pipe to the first supporting beam or to a third supporting beam supported to the first or second supporting beam (30, 32), wherein the first assembly part defines, at the first or third supporting beam (32, 88), a first point of support where loads incurred by the weight of the pipe and the furnace attached to the pipe are transmitted to the first or third supporting beam (32, 88), and - a second assembly part that attaches the same pipe to the second supporting beam or to a fourth supporting beam supported to the second supporting beam, wherein the second assembly part defines, at the second or fourth supporting beam (30, 86), a second point of support where loads incurred by the weight of the pipe and the furnace attached to the pipe are transmitted to the second or fourth supporting beam (30, 86). - wherein the furnace comprises four vertical, planar water tube walls (72, 74, 76, 78) which are joined together and which, in a horizontal plane, define a rectangular cross section with four corner sections, two of the water tube walls being joined in each corner section, the four corner sections including a first corner section at which a first water tube wall and a second tube wall that are transverse to each other are joined, and - wherein the boiler further comprises at least one vertically extending pipe that is for the transport of water and/or steam and situated outside the furnace, the pipe being close to the first corner section, characterized in that: - the support frame further comprises at least two horizontal supporting beams (26, 28, 30, 32) which are separated from the water tube walls and include a first supporting beam and a second supporting beam that are transverse to each other, - wherein the support assembly close to the first corner section comprises - a first assembly part that attaches the pipe to the first supporting beam or to a third supporting beam supported to the first or second supporting beam (30, 32), wherein the first assembly part defines, at the first or third supporting beam (32, 88), a first point of support where loads incurred by the weight of the pipe and the furnace attached to the pipe are transmitted to the first or third supporting beam (32, 88), and - a second assembly part that attaches the same pipe to the second supporting beam or to a fourth supporting beam supported to the second supporting beam, wherein the second assembly part defines, at the second or fourth supporting beam (30, 86), a second point of support where loads incurred by the weight of the pipe and the furnace attached to the pipe are transmitted to the second or fourth supporting beam (30, 86). - a first assembly part that attaches the pipe to the first supporting beam or to a third supporting beam supported to the first or second supporting beam (30, 32), wherein the first assembly part defines, at the first or third supporting beam (32, 88), a first point of support where loads incurred by the weight of the pipe and the furnace attached to the pipe are transmitted to the first or third supporting beam (32, 88), and - a second assembly part that attaches the same pipe to the second supporting beam or to a fourth supporting beam supported to the second supporting beam, wherein the second assembly part defines, at the second or fourth supporting beam (30, 86), a second point of support where loads incurred by the weight of the pipe and the furnace attached to the pipe are transmitted to the second or fourth supporting beam (30, 86).