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!