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How to Replace a Humidifier Solenoid

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How the Solenoid Works and How to Test It
How to Replace a Humidifier Solenoid

The solenoid is activated by a 24 volt current controlled by the humidistat

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The solenoid is an electrically actuated valve that allows water to flow to the humidifier and is controlled by a humidistat. A humidistat 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 which subsequently opens or closes water flow to the humidifier. The solenoid is usually operated by a 24 volt circuit.

In the photo above, the furnace thermostat is calling for heat and the humidistat is calling for more humidity (I will show you how to test for that next). The humidistat will only send voltage to the solenoid when the furnace is running and moving air.

You can see the multimeter is showing approximately 24 volts AC current to the solenoid. If you see this voltage being displayed and the solenoid is not allowing water to flow to the humidifier, then the solenoid is defective and needs to be replaced.

Now let’s review how to test the humidistat.

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