Abstract: A mobile or modular home hot air heating and humidifying system including a counter-flow furnace having a base pan and a housing resting on the base pan and containing a heat exchanger and a blower located above the heat exchanger for directing air downwardly within the housing in heat exchange relation to the heat exchanger and to and through an air outlet opening in the base pan. A humidifier unit resting on the floor below the furnace and structurally supporting the furnace defines a passageway communicating with the air outlet and with a feeder duct which extends downwardly through the floor and connects to a heat distribution duct system below the floor. The humidifier operates in response to operation of the blower. A method for installing a humidifier unit in an existing heating system of the aforedescribed general type is also disclosed.