Abstract: An existing single-pass sectional boiler is retrofitted with one or two replacement sections to become a multi-pass boiler. One intermediate section of the original boiler is replaced by a new section having a downwardly extending water-filled target wall portion that divides the original combustion chamber into a smaller combustion chamber on the front side and a heat exchange chamber on the rear side of the target wall portion. The target wall portion also forces at least most of the combustion gas to flow from the combustion chamber upwardly through a first flue passage of the boiler's heat exchanger, into the upper flue collector chamber. Another intermediate section of the original boiler may be replaced by a new section having an upwardly extending draft diverter portion, or a draft diverter is installed in the upper flue collector chamber, to divert the flue gas back downwardly through a second flue passage of the heat exchanger to the heat exchange chamber.

Abstract: Modifications convert an existing single-pass boiler to operate with a multi-pass combustion gas flow through the flue passages of the heat exchanger of the boiler. A diverting target wall is arranged partway along the length of the original combustion chamber, and an upper draft diverter is arranged in a flue collector chamber above the flue passages, to divert some or all of the combustion gas into a multi-pass flow pattern through several flue passages of the heat exchanger in series. The modifications may further include changing the nozzle oil delivery rate and cone angle, the oil supply pressure, the draft conditions, and others. Heat extraction from the hot combustion gas to the boiler water is increased, exhaust gas temperature is decreased, and overall efficiency is increased, to result in a fuel and cost savings.

Abstract: Modifications convert an existing single-pass boiler to operate with a multi-pass combustion gas flow through the flue passages of the heat exchanger of the boiler. A diverting target wall is arranged partway along the length of the original combustion chamber, and an upper draft diverter is arranged in a flue collector chamber above the flue passages, to divert some or all of the combustion gas into a multi-pass flow pattern through several flue passages of the heat exchanger in series. The modifications may further include changing the nozzle oil delivery rate and cone angle, the oil supply pressure, the draft conditions, and others. Heat extraction from the hot combustion gas to the boiler water is increased, exhaust gas temperature is decreased, and overall efficiency is increased, to result in a fuel and cost savings.