In flue and water tube boilers provided with water tubes arranged in a combustion reaction zone or a convective heating zone, the values of L/D or H/D satisfy the following relationships: 1.8</=L/D</=2.5 and 1.2</=H/D</=1.7 and the water tubes are disposed in an (aligned) in-line arrangement. L(mm) is the longitudinal pitch of the water tubes taken in the direction of gas flow, D(mm) is the outer diameter of the water tubes and H(mm) is the transverse pitch of the water tubes taken at right angles to the direction of gas flow.
