Atomizing humidification sprays a fine mist of water directly into the warm air supply duct. The water is evaporated by the warm air from the furnace. The humidistat and built-in temperature switch automatically control the humidifier to ensure that the proper amount of humidity is achieved. The built-in temperature switch prevents operation if the temperature in the warm air duct is not high enough to ensure effective evaporation. The universal humidistat, installed either on the wall or in the cold air return duct, turns on the unit when the humidity level in the home is below the set-point. Air stream. Not recommended for heat pumps or systems where supply air is less than 120°f.