How to Replace a Furnace Humidifier Solenoid Valve

Test the Solenoid

You can use a multimeter to determine if the solenoid is faulty.

Begin by temporarily setting the thermostat higher than its current setting, so that it calls for heat. With the furnace running, attach one multimeter probe to one of the wiring connections that goes from the humidistat to the solenoid, then attach the other probe to the other wiring connection that links the humidistat to the solenoid.

In our example, the humidistat is installed on the outside of the furnace’s return duct, and its humidity measuring sensor is in the return air stream. To check the operation of the humidistat and solenoid:

Turn the humidistat to a very low setting or to “off.” The multimeter should register no voltage. Then, turn the humidistat to a very high setting, such as 80 percent relative humidity, or until you hear a “click” sound. The multimeter should register about 24 volts if the humidistat is functioning correctly. If this test does not register voltage, then you have a problem with the humidistat.

If you show 24 volts at the connection from the humidistat to the solenoid, then the humidistat is operating correctly and sending the proper voltage to the solenoid. But if the humidistat is operating correctly (24 volts is displayed) and the solenoid still is not allowing water to flow to the humidifier, then this means that the solenoid is defective and needs to be replaced.

Turn Off the Water Supply

The first step in replacing the humidifier solenoid is to turn off the water supply to the solenoid valve.

Trace the water supply line from the bottom of the solenoid to its shut-off point. This is often a saddle valve on a larger cold water supply pipe. In other instances, it may be a standard fixture shutoff valve. Turn this small valve clockwise until it is fully seated and in the closed position.

Disconnect Electrical and Water Connections

The solenoid will have two water tubes attached to it. One is a water supply tube and the other is a water inlet tube that runs to the top of the humidifier. There are also two low-voltage electrical connections attached to the solenoid.

Turn off the power to the furnace. This can be done with the shutoff switch, usually mounted on or near the furnace, or by flipping off the circuit breaker controlling the furnace at the main service panel. Disconnect the wire connections from the solenoid to the humidistat.

Using an adjustable wrench, loosen and remove the female threaded compression nut on the compression union that secures the water supply tube to the solenoid. Repeat the process for the compression nut that attaches the water inlet tube to the solenoid.

Remove the Defective Solenoid

The solenoid is held in place with screws or posts. Remove the solenoid from the humidifier by unscrewing the knurled knobs from the screw posts.

Attach the New Solenoid to the Humidifier

Once you obtain a replacement solenoid for your make and model of humidifier, the next step is to attach it to the humidifier body by threading the knurled knobs onto the mounting posts. Make sure you have the solenoid properly positioned by aligning the threaded union male ends of the solenoid with their matching union nuts on the water lines to which they attach.

Attach Water Lines to the Solenoid

Wrap the male threads of the union body with plumber's tape by tightly wrapping two to three complete wraps of the tape around the threads in the same “tightening” direction as the threads (clockwise as you look at the opening).

Carefully seat the copper water inlet tube (bottom tube in photo) from the water shutoff valve into the solenoid, and tighten the compression nut snugly.

Make sure one end of the water feed tube (top tube in photo) is in position at the top of the humidifier trough, then carefully set the copper water inlet feed line from the humidifier into the solenoid, and again tighten the compression nut snugly.

Turn the water supply back on by turning the valve handle counter-clockwise until fully open. Check for leaks.

Reattach the Electrical Connections

Connect the wire leads on the new 24-volt solenoid to the humidistat wiring by twisting the wires together and securing them with wire nuts.

Complete and Test the Installation

Your installation is now complete. While the furnace is running, temporarily turn the humidistat up to about 80 percent, or until you hear it click on. Water should now be flowing into the humidifier. Wait a minute or two and then check the ​humidifier drain to make sure water is flowing through the humidifier. If you see drips coming out of the drain or the end of the humidifier drain pipe, your installation has been successful.