How to Replace a Humidifier Solenoid

A water heater and furnace
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  • 01 of 10


    Diagram of the important parts of a humidifier
    The humidifier solenoid is controlled by the humidistat. 2013

    A furnace humidifier is a piece of HVAC equipment that adds water vapor to the heated air being distributed in your home. The tutorial How a Furnace-Mounted Home Humidifier Works describes the various components of a flow-through, furnace-mounted humidifier. One of those components is called solenoid valve assembly, which controls water flow and is also found on most types of humidifiers.

    If your humidifier is not getting water from the solenoid, assuming you have the water tap open, it's likely because either the solenoid or the humidistat is bad. It’s much more common for the solenoid valve to fail over time, causing it to get stuck shut and not open, even when the humidistat is still allowing voltage to the solenoid. When this happens, your humidifier will stop working, since it course requires water to operate.

    In this tutorial, you will learn how to test to see if the solenoid has failed, with step-by-step instructions on how to replace the solenoid valve.

    The subject of this tutorial is the General 1099 flow-through bypass humidifier, as manufactured by General Filters, Inc. The solenoid replacement process described here is representative of many humidifiers.

    Tools and Materials Needed

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  • 02 of 10

    How the Solenoid Works and How to Test It

    Checking a solenoid's status
    The solenoid is activated by a 24-volt current controlled by the humidistat. © 2013

    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.

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

    You can see that the multimeter is showing approximately 24 volts AC 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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  • 03 of 10

    How the Humidistat Works and How to Test It

    Checking a solenoid's voltage
    The humidistat controls electrical current to the solenoid. 2013

    As mentioned earlier, the humidistat is like a thermostat; only instead of being a switch activated by temperature, it is activated by the humidity level of the air returning to your furnace. 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.

    To check the operation of the humidistat using 24 volts proceeds as follows:

    1. Temporarily set the thermostat higher than its current setting, so that it calls for heat.
    2. With the furnace running, attach one end of the multimeter probe to one of the wiring connections that go from the multimeter to the solenoid, then attach the other probe to the other wiring connection that links the multimeter to the solenoid.
    3. Turn the humidistat to a very low setting or to “off.” The multimeter should register no voltage.
    4. Turn the humidistat to a very high setting, such as 80% relative humidity, or until you hear a “click” sound. The multimeter should register about 24 volts.
    5. If you show 24 volts at the connection from the humidistat to the solenoid, then the humidistat is operating correctly.

    If the humidistat is operating correctly (if you see 24 volts being displayed) and the solenoid is not allowing water to flow to the humidifier, then the solenoid is defective and needs to be replaced.

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  • 04 of 10

    Turn off the Water Supply

    Turning off the water tap on a humidifier
    The water supply tap feeds the humidifier's solenoid water valve. © 2013

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

    1. Trace the water supply line from the bottom of the solenoid to where it runs into a shut-off tap (usually a saddle valve) on a larger water supply line, such as that bringing cold water into your water heater.
    2. Turn this small valve clockwise until it is fully seated and in the closed position.
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  • 05 of 10

    Remove Electrical and Water Connections to Solenoid

    Detaching water tubes from the solenoid
    Remove plumbing connections to the solenoid. 2013

    The solenoid will have two water lines attached to it. One is a water supply line, and the other is a water inlet line that runs to the top of the humidifier. There are also two low voltage electrical connections to the solenoid. These need to be removed as follows:

    1. Turn off power to the furnace.
    2. Disconnect the wire connections from the solenoid to the humidistat.
    3. Using an adjustable wrench, loosen and remove the female threaded compression nut on the compression union that secures the water supply line to the solenoid. (Note: you may have to hold the union in place with another wrench while loosening the female threaded compression nut.)
    4. Next, repeat the process for the compression nut that attaches the water inlet tube to the solenoid.
    5. All water lines and electrical connections should now be removed from the solenoid.
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  • 06 of 10

    Remove the Defective Solenoid

    Solenoid attached by screws to the humidifier
    The solenoid may be attached with screws or screw posts. 2013

    Remove the Defective Solenoid: The solenoid will be held in place with screws or posts. Remove the solenoid from the humidifier—in this case by unscrewing the knurled knobs from the screw posts.

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  • 07 of 10

    Attach the New Solenoid to the Humidifier

    A new solenoid being attached to a humidifier
    Attach new solenoid to humidifier body. © 2013

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

    A great source of humidifier solenoids is at, for great pricing and fast shipping.

    Next, we need to plumb the solenoid back up.

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  • 08 of 10

    Attach Water Lines to the Solenoid

    Plumbing the solenoid with the water connection
    Connect water supply and water inlet tubes to solenoid. 2013

    To plumb the solenoid, proceed as follows:

    1. 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).
    2. Carefully seat the copper water inlet tube (bottom tube in photo) from the water shut-off valve into the solenoid, and tighten the compression nut snugly.
    3. 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.
    4. Turn the water tap back on and provide water flow to the solenoid by rotating the shut-off valve counter-clockwise until fully open.
    5. Check for leaks.
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  • 09 of 10

    Reattach the Electrical Connections

    Low voltage wires connecting the humidistat to the solenoid
    Connect low voltage wires from humidistat to the solenoid. 2013

    Next, connect the new 24-volt solenoid wiring to the humidistat wiring by twisting the wires together and attaching wire nuts.

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  • 10 of 10

    Complete and Test the Installation

    A newly installed solenoid
    Complete the installation by checking for leaks. © 2013

    Your installation is now complete. While the furnace is running, temporarily turn the humidistat up to about 80%, 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 is successful.