Some humidifiers disperse water vapor into the air the same way a tea kettle does: by boiling water. Steam units are manufactured as tabletop or wall-attached models that spray steam into a living space or into forced-air heating ductwork through a dispersion tube.

The most common furnace humidifier is the bypass humidifier. It makes use of the pressure differential between the return and supply plenums on a furnace to move air through it with minimal electrical consumption. Constructed of metal or plastic, it consists of a cabinet, reservoir, rotating drum with motor, and float. Preferably mounted on the supply plenum, warm air forced into the cabinet and flows over the rotating drum. The drum is a sponge or foam fabric stretched over a plastic frame. A small motor slowly spins the drum through a tray of water fed from the domestic water supply and regulated by a float valve. As the wet drum is exposed to the flow of warm air, water evaporates and humidifies the air. Ducted into the return air plenum, it is mixed with supply air and ducted to the house.