The humidistat is a critical control device used in humidification systems for furnaces. It controls water flow to the humidifier and operates like a temperature thermostat, only for humidity. For example, when the humidity level hits a certain point the humidistat switch will open or close an electrical circuit that provides current to the solenoid water valve which subsequently opens or closes water flow to the humidifier. The solenoid is usually operated by a 24 volt circuit.
The humidistat may be a remote device mounted in the room where humidity is being monitored, or in the photo above, the humidistat is installed on the outside of the furnace’s return duct and its humidity measuring sensor is in the return air stream.
Also Known As: humidistat