Combustion system with high turn down ratio

A low NOx combustion system for firing a Scotch Marine boiler, the combination comprising:

• a boiler having sidewalls defining a closed interior containing a fluid medium to be heated;
• a burner assembly having a firing head with an outlet end, the outlet end being mounted on a selected sidewall of the boiler;
• fuel supply means, fluidly coupled to the burner assembly, for conveying a combustible fuel to the burner assembly;
• a combustion air fan, fluidly coupled to the burner assembly, for conveying combustion supporting air to the burner assembly;
• a pilot, positioned adjacent said outlet end of the burner assembly, for igniting the combustible fuel to thereby heat the fluid medium contained in the boiler;
• and
• a windbox connecting the combustion air fan and the firing head of the burner assembly, the windbox having an exterior comprised of opposing sidewalls connected by a mid wall and having an interior including a scroll-shaped interior passageway, an inlet and an outlet openings, and wherein air from the combustion air fan travels at a right angle to the windbox opposing sidewalls as it enters the inlet opening and as it passes out the outlet opening in passing from the windbox to the burner assembly, and wherein the scroll-shaped passageway is defined by lateral edges which fit at right angles to the windbox opposing sidewalls, the lateral edges being flush with the outlet opening without forming a lip region with respect to the opposing sidewalls, thereby providing more uniform air flow though the windbox to the burner assembly.