Abstract: A boiler is made up of a housing containing upper and lower manifolds. First and second sets of tubes connect the upper manifold with the lower manifold. One set of tubes joins the upper and lower manifold on the right and the other set joins the upper and lower manifold on the left. The first set rise from the lower manifold in crossing and recrossing pattern. The other set rise from the lower manifold and along the side wall of the housing to the upper manifold.