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How to Keep Your Whole House Humidifier From Running Continuously

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Tom Lohr is an avid home improvement enthusiast. He prefers to spend the money he saves on new tools and gardening supplies.

Typical humidifier wiring and plumbing

Typical humidifier wiring and plumbing

Humidifiers Demystified

Wintertime can bring some seriously dry indoor conditions. Your furnace may be good at heating your home, but it can also dry out the air in the process. Waking up with five pounds of nastiness in your nose and alligator skin are only part of the problem. Your hardwood floors don't like the extremes in humidity either. Keeping at least an acceptable amount of moisture in the air during the winter is vital for comfortable living.

Many homes solve this issue by installing a whole house humidifier in their forced air furnace. It is an easy and affordable way to keep indoor humidity at acceptable levels without breaking the bank. A whole house humidifier is essentially an add on unit that is attached to a hole cut into the ductwork, just above where the hot air is forced out of the furnace for distribution.

The air blowing through the ductwork will suck out water vapor that is produced by the humidifier. This is usually accomplished one of two ways. Either water is poured over a paper filter, making the paper damp, and the forced air sucks vapor out of it as it passes by, or a wheel made of similar material turns and the bottom part remains submerged in a small tank of water, keeping the wheel wet as it turns.

Both of these methods require that a small water line is run from the normal home water distribution lines to a solenoid (a small valve that is operated electrically) that will open and allow that water to pass and keep the paper elements damp. Each time the humidistat drops below the humidity level it is set at, it sends a signal to the solenoid to open and allow water to flow.

This system is simple and works well as long as the humidity levels are below the setting on the humidistat. Ideally, and as designed, water should only flow when the furnace fan is running; either heating the home or in the “on” or “circulate” modes. Without the fan running, there is no air flow to suck the water vapor out of the paper element, and that water is wasted.

For the straight paper element type, that continuously flowing water goes down the drain and ends up on your water bill. The tank type with the rotating wheel will only ask for water when the tank drops below a certain level, so not near as much water is wasted. But it still takes electricity to keep the wheel turning, and that wasted juice ends up on your electric bill. Both are wasteful if they are running when the furnace fan is not.

If you had a whole house humidifier installed, or you installed one yourself, and it runs while the furnace fan is not, something in the installation was done incorrectly. More often than not, it was wired wrong when installed. This tutorial addresses how to check if your humidifier was wired correctly, and how to correct it if it wasn't.

Basic humidifier wiring goes in this sequence: the main power comes into your furnace. The wire that powers the fan has a branch wire tapped into it that feeds to a small transformer (usually attached to the side of your furnace). That transformer converts the 115 volts AC that powers the fan into 24 volts DC, which is the power used by the humidifier's humidistat.

That 24 volts DC goes into the humidistat and waits for the humidistat to tell it that the air is too dry. When humidity drops below the set level, the 24 volts DC is sent via another wire the comes out of the humidistat to a small solenoid. When the solenoid gets 24 volts, it opens the valve and allows water to flow to your humidifier.

When the furnace fan shuts off, or the humidity reaches the desired level, the 24 volts ceases to flow to either the solenoid (when the humidity reaches the set level), or to the humidistat (when the furnace fan stops running). Both conditions will stop the water flow to the humidifier and/or to the motor that turns the humidifier wheel.

Either to save a step, or due to lack of knowledge about a how a humidifier works, many installers will supply power to the transformer that makes the 24 volts DC directly from the main power line that enters the furnace. The result is the humidifier will run as long as the furnace is turned on, which is normally always. The waste in water and/or electricity is enormous. It's an easy problem to correct if you discover that your humidifier is wired wrong. Here's how.

Typical furnace component configuration

Typical furnace component configuration

1. Secure Power to Furnace

Getting electrocuted can ruin your day. Pick a day when the outside temperature will not be extremely cold as your furnace may be off for a while. Set the furnace to “off” on the thermostat. After the furnace automatically shuts off, turn off the power to the furnace. There is usually a light switch type switch mounted near the furnace that will shut off the power. If you cannot find or do not have such a switch, turn off the power at the circuit breaker panel. Remove the furnace access panels.

Another view of typical furnace configuration

Another view of typical furnace configuration

2. Explore Your Furnace

You can't rewire your furnace correctly unless you know where all of the important parts are. You need to be able to identify and find the main power input, the power input to the fan, the DC transformer, the humidistat and the solenoid. Take a look at the photo of my furnace to get an idea of where these items are usually located. Finding the main power in is easy, and you can trace the wires with your finger to determine where it goes.

The bulk of furnace wiring

The bulk of furnace wiring

3. Find the Important Wires

Take a look at the photo of power going to my furnace fan. This is a very typical setup. Then find the wires that feed the 24VDC transformer. Finding those should be even easier as the transformer is usually mounted on the side of the furnace and is easily identifiable. Note that you will only be concerned with the black “hot” wires. The white wires are neutral returns and do not need to be altered.

Where main power meets transformer power line in a wrong configuration

Where main power meets transformer power line in a wrong configuration

4. Disconnect Fan Power Wire

The black power wire that goes to the furnace fan motor will likely have a quick disconnect to it somewhere near the fan. See photo for what one looks like (for those wondering, that's drywall mud on my hand). Ensure that the black wire feeding the transformer is long enough to meet the black wire feeding the fan if not, you will need to add a section of wire to the transformer black wire). Disconnect the power to the fan at the quick disconnect. If you don't have a quick disconnect, you will have to cut the wire and strip back the insulation to expose about ½ inch of bare wire.

Close up of how the power wires run to the transformer

Close up of how the power wires run to the transformer

5. Disconnect 24VDC Transformer Power Wire

Follow the black power wire that feeds the transformer to where is is tied into the main power to the furnace. That connection is likely near the transformer. Disconnect that wire and then reconnect the two remaining wires. Your furnace will now have power, but the transformer will not.

Close up of power wires feeing the fan motor

Close up of power wires feeing the fan motor

6. Merge Fan and Transformer Power Feed

You should now have three disconnected black wires. One will be the black power wire to the transformer. The other two will be the wire from the fan and the one that fed it. Since you disconnected or cut it, it is now two separate wires. Use a wire nut or a lever nut to connect those three wires. You will be reconnecting the two power wires to the fan that was previously cut and the transformer wire. When competed, the 24VDC transformer that feeds the humidifier controller will only have power when the fan is running.

Quick disconnect for fan power wires

Quick disconnect for fan power wires

7. Tidy Up the Interior of the Furnace

Ensure you did not move any wires or cables to a point where they will rub against any moving parts (the fan). Now is good time to vacuum out the interior of the furnace if it needs it. Close up the access panels.

8. Test Your Work

Restore power to your furnace by turning on the switch near the furnace or its circuit breaker. You must reinstall the access panels for the furnace to work. There is a interlock switch that disconnects power to the furnace if the panels are off. Give your furnace a few minutes before testing. If you have a smart thermostat, turning off the power to the furnace will probably turned off power to the thermostat as well and it will need a time to reconnect to your Wi-Fi.

Once the thermostat is operational, turn your furnace on and either set it to a temperature that will cause it to turn on or just turn on the fan mode if your model has that selection available. One the fan begins running, check that the humidifier is running as well. You will probably hear the water being fed to it. If it isn't running, try turning the humidistat to its maximum setting. If it still doesn't turn on, check your home's internal humidity level. Most humidifiers have a 45% humidity maximum. If your interior humidity is above that maximum, it will not turn on.

Enjoy the Dampness

You can now savor the energy/water savings that comes with your humidifier only activating when the furnace fan is running. It's better for you, your wallet and the environment. You can also now add a new skill to your DIY quiver.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.


Pamela Oglesby from Sunny Florida on January 17, 2021:

I would love to have a "whole house humidifier" a it is too dy for me. I , and Iam going to show this article to my husband, and I appreciate this information.