Abstract: An improved boiler for generating steam includes a housing that has a proximal end and a distal end, with a water-entry port at the proximal end. A plurality of firetubes are disposed lengthwise in the housing, and the firetubes extend substantially the length of the housing between the proximal end and the distal end. The firetubes are used to pass the heated flue gas through the housing. A pair of side baffles are affixed to the inner surface of the housing that extend at least partially down the length of the housing to separate a portion of the last pass of firetubes near the housing from the remaining firetubes in the housing. At least two cross-sectional baffles are positioned adjacent the side baffles to direct the flow of water opposite to the flue gas flow in the separated part of firetubes.