A humidifier comprising: a burner for burning a fuel within a water main tank for generating steam in response to a demand; a primary heat exchanger within the water main tank for transferring heat from products of combustion of the fuel to the water within the water main tank, characterized by a secondary heat exchanger (208, 450) comprising a combusted gas section coupled to primary heat exchanger for receiving the cooled products of combustion from the primary heat exchanger and a water section for transferring additional heat from the cooled products of combustion at an output of the primary heat exchanger to water flowing within the secondary heat exchanger (208, 450) from a fill box to the water main tank; the fill box comprising: a first water tank chamber coupled to a primary water fill from an external water source via a primary water fill valve in order to maintain cool water in the chamber; a first outlet coupled from the first water tank chamber to the secondary heat exchanger (208, 450) via a micropump (205, 4400); a second outlet coupled to a drain via a drain valve, wherein the drain also coupled the bottom of the water main tank; a second water tank chamber coupled to the bottom section of the water main tank that contains floats which are used to control water level in the water main tank; and a third outlet coupled to the upper section of the water main tank to maintain pressure balance between the water main tank and first water tank chamber. a burner for burning a fuel within a water main tank for generating steam in response to a demand; a primary heat exchanger within the water main tank for transferring heat from products of combustion of the fuel to the water within the water main tank, characterized by a secondary heat exchanger (208, 450) comprising a combusted gas section coupled to primary heat exchanger for receiving the cooled products of combustion from the primary heat exchanger and a water section for transferring additional heat from the cooled products of combustion at an output of the primary heat exchanger to water flowing within the secondary heat exchanger (208, 450) from a fill box to the water main tank; the fill box comprising: a first water tank chamber coupled to a primary water fill from an external water source via a primary water fill valve in order to maintain cool water in the chamber; a first outlet coupled from the first water tank chamber to the secondary heat exchanger (208, 450) via a micropump (205, 4400); a second outlet coupled to a drain via a drain valve, wherein the drain also coupled the bottom of the water main tank; a second water tank chamber coupled to the bottom section of the water main tank that contains floats which are used to control water level in the water main tank; and a third outlet coupled to the upper section of the water main tank to maintain pressure balance between the water main tank and first water tank chamber. a first water tank chamber coupled to a primary water fill from an external water source via a primary water fill valve in order to maintain cool water in the chamber; a first outlet coupled from the first water tank chamber to the secondary heat exchanger (208, 450) via a micropump (205, 4400); a second outlet coupled to a drain via a drain valve, wherein the drain also coupled the bottom of the water main tank; a second water tank chamber coupled to the bottom section of the water main tank that contains floats which are used to control water level in the water main tank; and a third outlet coupled to the upper section of the water main tank to maintain pressure balance between the water main tank and first water tank chamber.