This setting lets your Ecobee run the humidifier during heat, cool, or with the fan only.

How to Wire a Humidifier to an Ecobee Smart Thermostat 12

In my original review of my very first Ecobee Smart Thermostat, I described how I had wired my Ecobee’s equipment interface to my whole-house evaporative humidifier at the Utah house. Because my furnace’s control board had a 24V HUM connector, I used that connector to help activate the humidifier. The problem, however, is that the furnace would only energize the 24V HUM terminal when the system called for heat, meaning I couldn’t run the humidifier with A/C or with the furnace fan alone.

So my updated method wiring a whole house humidifier is as now follows:

1. Bridge the R/H terminal on the Ecobee Equipment Interface to the ACC1r terminal on the Ecobee Equipment Interface

Wiring for a humidifier on the Ecobee Equipment Interface

Wiring for a humidifier on the Ecobee Equipment Interface

The short green wire above connects the R/H terminal on the Ecobee’s EI (the lettering is hidden by the wire) down to the ACC1r terminal. This provides 24V power from the R/H terminal to the ACC1r terminal.

2. Connect the ACC1 terminal on the Ecobee Equipment Interface to one of the humidifier leads

The slightly darker red wire (on the right) in the above photo connects the Ecobee EI’s ACC1 terminal to one of the low voltage trigger leads on the humidifier (it doesn’t matter which one). I just used a wire nut to connect, and then insulated with electrical tape:

Wiring to the humidifier's low voltage trigger leads

Wiring to the humidifier’s low voltage trigger leads

3. Connect the 24V/COM terminal on the furnace’s control board to the other humidifier lead

The other low voltage trigger lead on the humidifier connects to the COM 24V terminal on the furnace’s control board. That’s a very popular terminal in my furnace, so to ensure a solid connection for all the devices that need it, I attached a single wire to the terminal, and then wire-nutted that wire to the other COM wires. The brown wire in the following photo connects to the remaining lead on the humidifier:

The humidifier's other lead connects to the COM 24V terminal on the furnace control board.

The humidifier’s other lead connects to the COM 24V terminal on the furnace control board.

4. Configure settings on the Ecobee thermostat

In addition to running the humidifier while heating, if you want your humidifier to also be allowed to run while cooling,  or simply with the fan, you need to set the “Humidify only while Heating” setting in the thermostat’s Installation Settings / Equipment section to “No.”

This setting lets your Ecobee run the humidifier during heat, cool, or with the fan only.

This setting lets your Ecobee run the humidifier during heat, cool, or with the fan only.

And that’s how I connect a whole house humidifier to my Ecobee Smart thermostat! My humidifer is powered by a standard 120V wall plug, and is triggered when the two low voltage leads are energized.

When the Ecobee wants to turn on the humidifier, it “closes” the circuit between ACC1 and ACC1r. Because the ACC1r terminal gets a constant 24V from the equipment interface’s R/H terminal, closing the circuit between ACC1 and ACC1r completes a 24V circuit on the humidifier trigger wires, which then turns on the humidifier.

Your wiring needs may be slightly different, depending on the triggering needs of your humidifier and/or whether or not your furnace board runs on 24V. If you have a 120V control board, you may need to use a 120V to 24V transformer and wire it between the control board, the Ecobee EI, and your 24V triggered humidifier.

As always, I welcome your feedback, questions, and comments below!

  • Pingback: Product Review: ecobee Smart Thermostat | Steve Jenkins' Blog()

  • kjake

    Thanks for posting this. Installed the same humidifier today without any issues – worked the first time. One change that I made was: my Common/C terminal is already connected to the Ecobee EI’s 24V terminal block, so I just kept the wiring inside the EI from the humidifier’s solenoid and connected one lead to the ACC1 and one of the 24V part that lead to Common/C.

    • Hey, Kevin. Congrats on the successful install! And yes – connecting to the COM terminal inside the Ecobee EI is just as good as connecting to the COM on the furnace’s control board! The COM is “common” to a lot of connections! 🙂

  • Craig Hairrell

    I’m not sure I’d bother with this. Looking at the statistics of how efficiently my bypass humidifier adds moisture without the furnace supplying heat, it doesn’t look too good. In about 90 minutes with the furnace and humidifier running nearly constantly, it raised RH by 6%. That same day, in a 75 minute period, with just the fan and humifier running, it raised humidity by only 1%. I can even find periods where the humidity drops with the fan and humidifier on. I’m turning on the option to humidify only when heating. It looks like a waste of water and electricity to do otherwise.

    • Glenview Jeff

      Yea, I too would only run an evaporative humidifier when the heat is running. Especially if your humidifier uses wastewater. I bought the recirculating Aprilaire 400, which doesn’t have the wastewater drain. It recirculates the water. However, the amount of electricity used by the furnace fan vs. the amount of added moisture contributed to the air is probably not terribly efficient either.

  • Going to do this today. I picked up a Trion Mister-Mini due to space constraints. We’ll see how well it works.

  • Michael

    Not sure what happened to my set up. I have 2 stage heat and single cool. I hooked everything up as shown in the install manual. After registering etc, everything worked great. Then I tried to hook my dehumidifier. I again followed the install in the manual. Hit the power and all looked good. First time furnace called for heat, humidifier kicked on, but seconds later, smoke was emanating from the interface board, I took off the cover and saw that little green thing next to ACC3 bright orange, then it popped and caught fire and I hit the breaker off! I assume that port is fried. But what the heck happened??? Do I dare hook it up to one of the other slots? I don’t have plans to hook air to air up as you can’t turn it on from mobile only at thermostat. May want to hook up try switch for warning of sump pump. But for sure want humidifier hooked up….help,please

  • Mark

    I was looking to purchase the older model (Smart Si), becuase I could get a good deal on one, and I wanted it to control my humidifer, which this covers. However, the Ecobee web-page implies the thermostat already has humidity contoro: where it states it has “Optimal Humidity: Smart Si regulates indoor humidity to prevent frost buildup on windows when it’s cold outside and high humidity indoors.” I’m confused? Can the Smart Si thermostat control humidity out of the box, or not? Has anyone come across it’s logic for controling humidity levels in cold weather? I live in Minnesota, and when the temperature drops below 0F, I need the logic of the thermostat to bring the humidity down otherwise you face condensation. Hmm… maybe I’ll post this to the Ecobee Knowledge Center – just curious if there were any thoughts. -Cheers!

    • Mark

      Sorry – confusion between the “Smart” and “Smart Si” abounds – I get it now, this blog past was for the “Smart.” Although, the Ecobee link remains elusive by stating the “Smart Si” regulates humidity – but even the Ecobee pages point out the Smart Si does NOT have humidity control. I think there’s room for confusion on how the Smart Si can prevent frost buildup yet not have any control over a humidstat. Humpf.

      • Hi, Mark. The Smart, Smart Si, and the ecobee 3 will all control a humidifier without need for any additional hardware. The Frost Control feature (also on all units) will allow the thermostat to adjust the target humidity based on two variables: the efficiency of your windows (set by you) and the external temperature that it reads off the Internet. A little tinkering with the window efficiency setting is required, but once you find the right setting, you’re good to go!

  • Pingback: Using a Whole-Room Dehumidifier in a Gun Vault / Workshop()

  • Glenview Jeff

    Just wanted to leave a trail for Google; if anyone else measures 12V or 14V across the Ecobee3 control wires when the humidifier is supposed to be off, apparently this is a common situation. According to support, it shouldn’t cause any issues with the humidifier. They said it’s a normal voltage leak. I wonder if this has something to do with the fact that they use FETs in the Ecobee3 instead of solenoids; they couldn’t answer that.