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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.