Dehumidification features are 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 latent (humidity) load 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 and 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 and 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, so 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 is an example of 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 with 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… but 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>Dehumidifer >1 Wire ACC+>Open contact state to activate dehumidifier.
This setup uses 24V power from the R terminal to energize the acc+ terminal and therefore the dehumidification terminal in the fan coil/furnace when there is NO call for dehumidification.
Now for a 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 and there appears to be no adverse effects from setting it to off becasue the blower is still controlled by the thermostat for cooling operation and dehumidification without cooling is not possible without an external dehumidifier.
In order for the system to over cool below the temperature setpoint to dehumidify you need to go into the thresholds menu and set up AC over cool Max to the maximum temperature below setpoint that would be allowed during dehumidification by the equipment.