A humidifier is disclosed for use with a forced air furnace in which water flowing in a coiled tube disposed in the exhaust path for waste gas from the combustion chamber is vaporized by heat transfer from the waste gas and transferred in admixture with unvaporized water into a container in the warm air duct of the furnace. The steam humidifies the air while the unvaporized water is separated and returned to the exhaust path of the combustion chamber where it in turn is vaporized and vented to the atmosphere. The heat transfer to the water in the coiled tube is enhanced by a convoluted configuration of the tubing which induces turbulence in the waste gas and/or water. The humidifier is regulated by controls which are response to the flow of air through the furnace as well as to the humidity.