1. A humidifier for a central hot air system having a plenum chamber, said humidifier comprising, an evaporator pan mounted in the plenum chamber, a reservoir amounted at the same level as said evaporator pan, a conduit connecting sAid reservoir to said evaporator pan, means including a solenoid valve connecting said reservoir to a source of water, a float mounted to respond to the level of water in said reservoir, a mercury switch mounted on said float and electrically connected to said solenoid valve to energize and open said solenoid valve when the water level in the reservoir drops below a predetermined low water level and to deenergize and close said solenoid valve when the water in said reservoir reaches a predetermined high level.

2. A humidifier adapted to be connected to an electric power source and mounted in a plenum chamber for a hot air heating system, said humidifier comprising an evaporator pan, means for mounting said pan in the plenum of the furnace, means for evaporating water in said pan, a reservoir mounted at a point remote from said pan and at substantially the same level, a conduit having a restricted orifice connecting said reservoir to said evaporating pan, means including an electrically actuated solenoid valve for supplying water to said reservoir, and means responsive to the level of water in said reservoir for controlling said supplying means, said controlling means including mercury switch having a first set of contacts connected to an electric power source and to said solenoid valve to energize said solenoid valve whenever the water in the reservoir drops below a predetermined minimum and to deenergize said solenoid valve when the water reaches a predetermined high water level and a second set of contacts connected to the electric power source and to said evaporating means to disable said evaporating means when the water level in the reservoir drops below a predetermined low water level.

6. A humidifier for a central hot air heating system having a plenum chamber, said humidifier comprising an evaporator pan positioned in said plenum chamber, electric means for sensing high and low water levels in said pan, an electric heater positioned to evaporate water in said pan, means including a solenoid valve for supplying water to said evaporator pan, a switch having a first set of contacts connected to power source for controlling said solenoid valve and a second set of contacts for controlling said heater, linkage means for actuating said switch, means including an armature operatively connected to said linkage means and a coil electrically connected to said sensing means for actuating said switch, a transformer having a primary winding connected to the electrical power source independent of said first set of contacts, and a secondary winding, an electrically actuated relay switch connected to control said heating element and a low current circuit connected to said secondary winding, relay switch and second set of contacts whereby when said water reaches a predetermined low level, said coil will be de-energized and said armature will move said switch to a position to close said first set of contacts and energize said solenoid valve to supply water to said pan and open said second set of contacts to de-energize said heater, and when said water in said pan reaches a predetermined high level, said coil will be energized and said armature will move said switch to a position to open said first set of contacts and deenergize said solenoid valve and close said second set of contacts to energize said relay switch.