How to Wiring Humidifier to Furnace Control Board?

Every home could use more moisture in the air during the winter months.

But here’s something that might surprise you – your furnace isn’t the cause of dry winter indoor air. It’s your leaky house. More on that at the end.

But first, here are steps for, as the search phrase goes, how to wire humidifier to furnace control board, aka, how to wire a humidifier to a furnace control board.

Not a Job for Everyone – But You Can Do It

Installing a humidifier to your furnace control board is a challenging task, but it can be done if you pay attention to details, take your time, and follow the instructions perfectly.

When you wire a humidifier to your furnace, you will also have to connect a humidistat.

Term tip: A humidistat is a necessary component to the whole furnace/humidifier system. The humidistat is also called a hygrostat. It is a small electrical device that measures the humidity levels and responds as needed, like a thermostat responds to temperature. The humidistat is often included in the package when you purchase a furnace humidifier.

Step by Step Directions on How to Wire a Humidifier to a Furnace Control Board

Here is how to wire a humidifier to the furnace control board in 6 easy steps.

  • Step 1:  Begin by turning the power off to your heating and cooling system. This is best done by simply turning the power off at the circuit breaker. There is also a switch on your furnace, but the breaker box is your best option.
  • Step 2:  Once you connect the humidistat to the return plenum ductwork, you will have to connect it to the control board on your furnace. Use 18-2 low voltage wire as directed in the humidistat or humidifier instructions. Everything should be labeled on the humidistat. Two terminals will connect to the humidistat and two will go to the control board. A wiring diagram should be included and will look something like this.
  • Step 3: Wire the outside / outdoor sensor bulb to the humidistat. This bulb will run from the humidistat to the outside of your home and connect to your siding. Use the 18-2 low voltage wire and connect the wire to the humidistat and the outside sensing bulb. There will be two connections on the bulb and two on the humidistat for the process. Everything should be labeled for easy identification and installation.
  • Step 4: Connect the 24-volt wire from the humidistat to the furnace. On the control, there will be a terminal labeled HUM (humidifier) for the wire connection. Every furnace brand and model is different, but the overall procedure is similar regardless of furnace board. Clear directions should be provided with your humidifier. The key to success is following them “perfectly."
  • Step 5: Use the included 24-volt wire to connect the solenoid sensor to the humidistat controller. The terminals on the humidistat will be labeled AA for easy identification and connection.
  • Step 6: If your humidifier comes with a 110-volt power cord, simply plug it in and test to make sure that everything is working properly. If there is not an outlet available, you will need to install one or hire an electrician to handle the task.

See our review of Aprilaire humidifiers - one of the top-selling brands in furnace humidifiers. 

Possible Issues, Things to Avoid and Helpful Hints

  • You can run on the wiring inside the ductwork to keep it safer and for a cleaner look.
  • If you have any wires that are not protected by ductwork, you should protect them by using some type of flexible conduit.
  • Be careful that the wires never run on any sharp surfaces.
  • Double-check that all your connections are done correctly. Do not over tighten, but make sure that the wire is securely connected to the terminals.
  • Modern furnaces have terminals on the circuit board marked HUM (humidifier) and NEUTRAL for easier humidifier hook up.
  • Be sure you don't tie into the 24V common by mistake. The NEUTRAL is for 110V use.
  • Use pliers to squeeze your spade connectors onto the transformer's black and white wires.
  • Then, just securely press the black connector onto the HUM terminal and the white connector onto the NEUTRAL.
  • If you don't have these terminals, you'll have to tap into another 110V source using wire nuts.

Pro Tip: Keep your humidifier clean for everyone’s health and wellbeing! See our Guide to Cleaning a Humidifier for details.

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OK – Why is Winter Air So Dry?

It’s not the furnace running, or the fireplace or wood stove.

It is the exchange of air. Warm air can hold more moisture than cool/air. If your home has air gaps around the windows and doors or isn’t wrapped in Tyvek or similar vapor barrier, air will be exchanged. Warm, moist air leaves. The air from outside is cooler, and so it holds less moisture. As air continues to be exchanged, the air in your home gets increasingly drier. And that’s a good reason to use a humidifier.

Pro Tip: You can stop this outflow of warm, moist air and inflow of cool, dry air by using weather-stripping around doors and windows. That’s a start. More costly fixes that will also help with energy efficiency include adding house wrap the next time you replace siding and replacing old doors and windows with energy efficient units.

Make your home’s envelope tighter, and your indoor air in winter will have more moisture – but here’s the downside. It will also be more polluted, so you might also want to add a high-quality, effective air purifier to your home. 

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