EcoBee Thermostats and Dehumidification


Dehumidification features have been common on residential systems ever since the introduction of variable speed blower motors. The system is set up so that the blower can produce less CFM per ton when the latent load (humidity) in the space is higher than the setpoint relative humidity. Slowing the blower increases moisture removal by reducing the sensible load on the evaporator coil, therefore dropping the coil temperature and surface dewpoint.

Most variable speed fan coils and furnaces have a terminal designated for Dehumidification, and it can be called D, dehum, DH, or something else, depending on the manufacturer. In all cases I am aware of, this dehumidification terminal must be energized for the blower to go to full speed. When that terminal is de-energized, the blower speed (usually) drops to 80% of full speed.

For years, we have seen thermostats with designated dehumidification terminals to match up with the fan coil/furnace terminal. It was just a matter of disconnecting a jumper from the dehumidification terminal to the R terminal in the unit and connecting a wire from the designated thermostat terminal to that dehumidification terminal in the unit. The diagram below shows this on an old Carrier Thermidistat with a variable-speed Carrier fan coil.

We now have 24v control smart thermostats like Ecobee, Cor, Nest, and Lyric. These have a lot more flexibility in how they can be set up rather than having a single, designated dehumidification terminal.

I am a big fan of EcoBee for many reasons, including their Alexa integration, remote wireless sensors, and application flexibility. However, you need to be really careful with how you set them up, ESPECIALLY when setting up dehumidification.

The image above is a GIF and should show you the first part of the dehumidification setup. I am setting it up for a single-speed compressor heat pump with a variable-speed fan coil. EcoBee has contacts labeled acc+ and acc- that can be set up to do a wide variety of functions. For this typical dehumidification function using the system, you would select Menu > Installation Settings > Dehumidifier > 1 Wire ACC+ > Open contact state to activate the dehumidifier.

This setup uses 24V power from the R terminal to energize the acc+ terminal. Therefore, the dehumidification terminal in the fan coil/furnace when there is NO call for dehumidification.

Now, here's the controversial part:

Go to the equipment menu and select Dehumidifier to “dehumidify with fan”= no. We have seen several occasions where the blower continues running with no cooling call if this setting is set to yes when there is a dehumidification demand and no cooling demand. According to the EcoBee website HERE, it appears to say the opposite, but we have confirmed on a few occasions that this occurs. There appear to be no adverse effects from setting it to OFF because the blower is still controlled by the thermostat for cooling operation, and dehumidification without cooling is not possible without an external dehumidifier.

For the system to over-cool below the temperature setpoint to dehumidify, you need to go into the thresholds menu and set up AC Overcool Max to the maximum temperature below the setpoint allowed during dehumidification by the equipment.




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4 responses to “EcoBee Thermostats and Dehumidification”

  1. I’m getting a new Bryant/Carrier system installed soon with variable speed FV4C fan coil. It has DH terminal just as pictured here which uses NEGATIVE logic (i.e. energized DH is normal cooling mode and de-energized DH is dehumidification mode).

    In normal dehumidification mode, the fan speed is reduced to 80% of nominal. However, there is a 3rd even lower speed mode meant to be used when there is NO call for cooling but still a call for dehumidification. This is activated when the G terminal on the FV4C is de-energized while the Y1/Y2 terminal(s) are energized and DH is de-energized. They call it “super” dehumidifcation:

    1.) Temp>Setpoint and HumiditySetpoint and Humidity>Setpoint : Y1/Y2 closed, G closed, DH open : 80% nominal fan speed : dehumidification
    3.) TempSetpoint : Y1/Y2 closed, G open, DH open : 50% nominal fan speed : super dehumidifcation

    The bottom line is, I’m trying to figure out if I can use the ecobee4 instead of the Bryant/Carrier/Cor thermostat without losing any of this functionality. The Bryant/Carrier/Cor smart thermostats have lots of bad reviews and complaints. From your article here, it looks like the ecobee can be configured to use one of the ACC terminals to correctly control the DH terminal on the fan coil. Regarding the “Dehumidify with fan” setting, when you set this to off, will it:

    1.) ALWAYS de-energize the G terminal when there is a call for dehumidifcation, regardless of whether or not there is also a call for cooling
    2.) will it only de-energize G when there is a dehumidifcation call WITHOUT a call for cooling?

    If 1.), then I’ll lose the regular dehumidifcation mode and always use the super mode (i.e. 50% nominal fan speed). If 2.), then I think I’m golden.

    • fixing cut-and-paste error

      1.) Temp>Setpoint and HumiditySetpoint and Humidity>Setpoint : Y1/Y2 closed, G closed, DH open : 80% nominal fan speed : dehumidification
      3.) TempSetpoint : Y1/Y2 closed, G open, DH open : 50% nominal fan speed : super dehumidifcation

    • Did you ever get this figured out? I just had a that carrier system installed and they can’t get mine into super dehumid. So frustrated because i spent the extra money for that.

  2. I know it’s a long shot to get a reply on this since this tip is from 2017; however, my question is regarding a 2 wire dehumidifier. Would you still set the “dehumidify with fan” option to no with an aprilaire 1830 whole home dehumidifier? The dehumidifier is wired directly to the ecobee in acc-/acc+.

    Thank you.

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