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How To Test And Replace The Solenoid On Your Furnace's Humidifier

by Wade Black

The humidifier on your furnace directs water vapor into the duct system to spread moisture in the air throughout your home during the winter. The moisture in the air helps keep the house warmer on cold days (due to the increased humidity level), and helps keeps your skin from drying out. The solenoid open and closes the valve on the humidifier to let the water vapors into the furnace ducts. If the solenoid won't open, the humidifier won't work and your house can get very dry. Here is how you can test your solenoid and replace it if it no longer works.

You Will Need:

  • New Solenoid
  • Voltmeter
  • Wrench
  • Plumber's Tape

Test Solenoid

Most solenoids are connected to the side of the humidifier. The humidifier is roughly the size of a bread box and is attached to the side of furnace at about eye level.

Make sure the furnace and humidifier are turned on and running.

Take a voltmeter and connect the prongs to the positive and negative wires going to the solenoid. The positive wire is colored red and the negative one is white. Connect the red probe on the voltmeter to the red wire on the humidifier, and place the black probe on the white wire. If electricity is running through the humidifier, but the valve won't open, you'll need to replace the solenoid.

Replacing Solenoid

Turn the water supply off to the solenoid.

Next, unscrew the water lines connected to the solenoid. One water line is connected at the bottom of the solenoid and the other line is connected to the top of the solenoid.

Disconnect the electrical wires going to the solenoid. The bare ends of the wires from the solenoid are typically twisted together with the bare ends of the wires coming from the electrical supply. The bare ends of the wires are covered by a wire cap. Remove the cap and untwist the wires so they separate from each other.

Unscrew the screws holding the solenoid to the side of the furnace and remove the solenoid.

Replacement Solenoid

You need to make sure you use an exact replacement solenoid for your humidifier. If you are not sure which solenoid to buy, take the one you removed with you to the hardware store so the store's employees can match it up with the solenoids in stock (or so they can order you an exact replacement).

Replace the solenoid and reconnect it to the humidifier in the reverse order in which you took it out. 

If you don't feel comfortable doing this on your own, contact a company like Appliance Service Co Inc.