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Cool to Dry and Dehumidifier (using Heat Pump) setting

Community Member

An older single stage Carrier heat pump system was just replaced with a FV4C indoor unit and 38MURA outdoor unit. These have a variable speed motor and compressor and were advertised as being able to pull more than a typical amount of humidity out of the air. The FV4C has a control board with a DH (dehumidify) terminal that can put the unit in dehumidify mode where it slows the fan speed down to 80% of nominal. That combined with the ENH (enhanced) setting delays the fan for 30 seconds, then runs the fan at 70% for 150 seconds, and has no off delay really does seem to do a good job at dehumidifying the air. However, the Nest's Dehumidifier setting is a bit lacking as the adjustment slider goes from one water drop to three water drops... So what are the actual relative humidity targets for one to three water drops? This information isn't available anywhere...

When Cool to Dry is enabled it uses the dehumidifier setting as the second setpoint when controlling the cooling. When the humidity is higher than the 1-3 drop setpoint (just tell us what %RH it is!!!) but the temp is below the temp setpoint it will call for cooling in the dehumidify mode. If you have it set to one drop it seems to run the AC 5 minutes on and 10 off continuously to try and bring the RH down as much as possible while not let the temperature drop about 3F below the temperature setpoint. Obviously running the AC in this mode cools the air off quite a bit, which is why it has to run frequently for short periods of time to pull more moisture out of the air. Not knowing the relative humidity setpoint sucks though. Is there any way to find out what RH target it is trying to achieve!?


Community Specialist
Community Specialist

Hi bww129, 


Thanks for posting, and I'm sorry for the delayed response. 


An external humidity sensor can be used to monitor and control humidity levels more accurately. Some standalone humidity sensors are compatible with Nest Thermostats and can provide real-time RH readings. Integrating an external humidity sensor with the thermostat requires compatibility and following the manufacturer's instructions. The sensor can provide specific RH values, allowing you to fine-tune the dehumidification setpoint to achieve desired indoor humidity levels. However, in some cases, you’ll need professional installation for your Nest thermostat. Choosing the wrong options for a dehumidifier can result in system damage, so it’s important for a trained professional to assess the requirements for your system. Learn more about this here on the Cool to dry and Humidifier and dehumidifier compatibility guide. 


Let us know if you have any other questions.




Community Member

Thanks for the reply, Zoe. Unfortunately that didn't answer my question. I have read through the Cool to Dry information plenty of times, but nothing mentions the actual %RH number the thermostat is using. Why bother giving a degree F, why not just have a slider that goes from ice cube to flame? I don't care if the internal humidity sensor isn't terribly accurate, just want to know what %RH it is trying to hit...

I have arrived at a number that correlates to the single water droplet based on an external temp/humidity sensor I placed near the thermostat, but it makes no sense that you can't choose a Cool to Dry %RH (even 5% increments would be better than 1-3 drops) when it already gives you the inside humidity reading.