Abstract: Retrofitted boiler 10 includes a coupling flange 60 and a door 68 mounted in a boiler chamber 14. A high efficiency burner 80 is coupled in the door 68. Door panels 82 are provided to cover door openings 72. This allows access to the inside of the boiler 10 and provides maximum efficiency. The boiler retrofitting method requires removal of doors 18 and 34. A gas manifold 40 is modified and insulated with insulation 54. A coupling flange 60 is positioned in the boiler 10 and door 68 is coupled thereto. A high efficiency burner 80 is coupled in a boiler door opening 74 adjacent the modified gas manifold 48. Door panels 82 are then coupled to the door 68 to cover openings 72. With the boiler 10 retrofitted with a single door 68, a much more efficient boiler is provided.