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 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 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 via a micropump ; 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 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 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 via a micropump ; 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 via a micropump ; 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. <EOS>